Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space

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1 TIONAL ACADEMIES PRESS THE NA This PDF is available at http://nap.edu/24938 SHARE     Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space DET AILS 700 pages | 8.5 x 11 | PAPERBACK ISBN 978-0-309-46757-5 | DOI 10.17226/24938 CONTRIBUTORS Committee on the Decadal Survey for Earth Science and Applications from Space; THIS BOOK GET Space Studies Board; Division on Engineering and Physical Sciences; National Academies of Sciences, Engineering, and Medicine FIND RELA TED TITLES .edu Visit the National Academies Press at NAP and login or register to get: – Access to free PDF downloads of thousands of scientific reports  – 10% of f the price of print titles  – Email or social media notifications of new titles related to your interests  – Special o ffers and discounts Distribution, posting, or copying of this PDF is strictly prohibited wit hout written permission of the National Academies Press. is PDF are copyrighted by the National Unless otherwise indicated, all materials in th (Request Permission) Academy of Sciences. All rights reserved. Academy of Sciences. Copyright © National

2 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space PREPUBLICATION—SUBJECT TO FURTHER EDITORIAL UNEDITED CORRECTION Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Co mmittee on the Decadal Survey for Earth Science and Applications from Space Space Studies Board Division on Engineering and Physical Sciences A Consensus Study Report of UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION Copyright National Academy of Sciences. All rights reserved.

3 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space THE NATIONAL ACADEMIES PRESS 500 Fifth Street, NW Washington, DC 20001 ract ... between the National Academy of Sciences This study is based on work supported by the Cont tion. Any opinions, findings, conclusions, or and the National Aeronautics and Space Administra recommendations expressed in this publication and do not necessarily reflect the views of any agency or that provided support for the project. organization International Standard Book Number-13: 978-0-309-XXXXX-X International Standard Book Number-10: 0-309-XXXXX-X Digital Object Identifier: https://doi.org.10.17226/24938 Copies of this publication are available free of charge from: Space Studies Board The National Academies of Sciences, Engineering, and Medicine 500 Fifth Street, N.W. Washington, DC 20001 Additional copies of this publication are availabl e from the National Academies Press, 500 Fifth Street, N.W., Keck 360, Washington, DC 20001; (800) 313; http://www.nap.edu. 624-6242 or (202) 334-3 8 by the National Academy of Sciences. All rights reserved. 01 Copyright 2 Printed in the United States of America Suggested Citation 8 . Thriving on Our : National Academies of Sciences, Engineering, and Medicine. 201 g Planet: A Decadal Strategy for Earth Observa Changin Space. Washington, DC: The National tion from ttps://doi. org.10 .17226/24938. Academies Press. h UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION Copyright National Academy of Sciences. All rights reserved.

4 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space The National Academy of Sciences was established in 1863 by an Act of Congress, signed by President Lincoln, as a private, nongovernmenta l institution to advise the nation on issues related to science and technology. Members are elected by their peers for outstanding contributions to research. Dr. Marcia McNutt is president. National Academy of Engineering was established in 1964 under the charter of the The National Academy of Sciences to bring the prac tices of engineering to advising the nation. Members are elected by their peers for extraordin ary contributions to en gineering. Dr. C. D. Mote, Jr., is president. The National Academy of Medicine (formerly the Institute of Medicine) was established in 1970 under the charter of the National Academy of Sciences to advise the nation on medical and health issues. Members are elected by th eir peers for distinguished contributions to medicine and health. Dr. Victor J. Dzau is president. The three Academies work together as the Sciences, Engineering, National Academies of and Medicine to provide independent, objective an alysis and advice to the nation and conduct other activities to solve complex proble ms and inform public policy decisions. The National Academies also encourage education and research, recognize outstanding contributions to knowledge, and increase public understanding in matters of science, engineering, and medicine. Learn more about the National Academies of Sciences, Engineering, and Medicine at www.nationalacademies.org . UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION Copyright National Academy of Sciences. All rights reserved.

5 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Consensus Study Reports published by the National Academie s of Sciences, Engineering, and Medicine document the evidence-based consensu s on the study’s statement of task by an authoring committee of experts. Reports typi cally include findings, conclusions, and recommendations based on information gath ered by the committee and the committee’s deliberations. Each report has been subjecte d to a rigorous and independent peer-review process and it represents the position of the National Academies on the statement of task. published by the National Academies of Sciences, Engineering, and Medicine Proceedings chronicle the presentations and discussions at a workshop, symposium, or other event convened by the National Academies. The stat ements and opinions contained in proceedings are those of the participants and are not endorsed by other participants, the planning committee, or the National Academies. For information about other products and activi ties of the National Academies, please visit www.nationalacademies.org/about/whatwedo. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION Copyright National Academy of Sciences. All rights reserved.

6 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space COMMITTEE ON THE DECADAL SURVEY FOR EARTH SCIENCE AND APPLICATIONS FROM SPACE Co-Chair WALEED ABDALATI, University of Colorado Boulder, Co-Chair WILLIAM B. GAIL, Global Weather Corporation, 1 2 ANTONIO J. BUSALACCHI, JR., NAE, University Corporation for Atmospheric Research, Co-Chair STEVEN J. BATTEL, NAE, Battel Engineering, Inc. STACEY W. BOLAND, Jet Propulsion Laboratory ROBERT D. BRAUN, NAE, University of Colorado SHUYI S. CHEN, University of Washington 3 WILLIAM E. DIETRICH, NAS, University of California, Berkeley SCOTT C. DONEY, University of Virginia CHRISTOPHER B. FIELD, NAS, Stanford University HELEN A. FRICKER, Scripps Institution of Oceanography SARAH T. GILLE, Scripps Institution of Oceanography DENNIS L. HARTMANN, NAS, University of Washington DANIEL J. JACOB, Harvard University ANTHONY C. JANETOS, Boston University EVERETTE JOSEPH, University of Albany, SUNY 4 MOLLY K. MACAULEY, Resources for the Future JOYCE E. PENNER, University of Michigan SOROOSH SOROOSHIAN, NAE, University of California, Irvine GRAEME L. STEPHENS, NAE, Jet Propulsion Laboratory/Caltech BYRON D. TAPLEY, NAE, University of Texas, Austin W. STANLEY WILSON, National Oceanic and Atmospheric Administration Staff ARTHUR CHARO, Senior Program Officer, Study Director LAUREN EVERETT, Program Officer CHARLES HARRIS, Research Asso ciate (through August 2016) MARCHEL HOLLE, Research Associate (from November 2016) ANDREA REBHOLZ, Program Coordinator MICHAEL H. MOLONEY, Director, Space Studies Board and Aeronautics and Space Engineering Board 1 Member, National Academy of Engineering. 2 Resigned from the committee on May 5, 2016. 3 Member, National Academy of Sciences. 4 Dr. Macauley passed away on July 8, 2016. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION v Copyright National Academy of Sciences. All rights reserved.

7 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Panel on Global Hydrological Cycles and Water Resources ANA P. BARROS, Duke University, Co-Chair JEFF DOZIER, University of California, Santa Barbara, Co-Chair NEWSHA AJAMI, Stanford University JOHN D. BOLTEN, NASA Goddard Space Flight Center 1 DARA ENTEKHABI, NAE, Massachusetts Institute of Technology GRAHAM E. FOGG, University of California, Davis EFI FOUFOULA-GEORGIOU, University of California, Irvine DAVID C. GOODRICH, U.S. Department of Agriculture TERRIS S. HOGUE, Colorado School of Mines JEFFREY S. KARGEL, University of Arizona CHRISTIAN D. KUMMEROW, Colorado State University VENKAT LAKSHMI, University of South Carolina 2 3 ANDREA RINALDO, /NAE, Ecole Polytechnique Federale de Lausanne NAS EDWIN WELLES, Deltares ERIC F. WOOD, NAE, Princeton University ARTHUR CHARO, Senior Program Officer, Study Director ED DUNNE, Program Officer (through July 2017) LAUREN EVERETT, Program Officer (from July 2017) TAMARA DAWSON, Program Coordinator Panel on Weather and Air Quality: Minutes to Subseasonal STEVEN A. ACKERMAN, University of Wisconsin-Madison, Co-Chair Co-Chair NANCY L. BAKER, Naval Research Laboratory, PHILIP E. ARDANUY, INNOVIM, LLC ELIZABETH A. BARNES, Colorado State University STANLEY G. BENJAMIN, National Oceanic and Atmospheric Administration MARK A. BOURASSA, Florida State University BRYAN N. DUNCAN, NASA Goddard Space Flight Center 4 YING-HWA KUO, University Corporation for Atmospheric Research CHARLES E. KOLB, NAE, Aerodyne Research, Inc. W. PAUL MENZEL, University of Wisconsin-Madison MARIA A. PIRONE, Harris Corporation ARMISTEAD G. RUSSELL, Georgia Institute of Technology JULIE O. THOMAS, Scripps Institution of Oceanogr aphy, University of California, San Diego DUANE W. WALISER, Jet Propulsion Laborator y, California Institute of Technology XUBIN ZENG, University of Arizona ARTHUR CHARO, Senior Program Officer, Study Director SANDRA GRAHAM, Senior Program Officer ANDREA REBHOLZ, Program Associate 1 Member, National Academy of Engineering. 2 Resigned from the panel on June 6, 2017. 3 Member, National Academy of Sciences. 4 Resigned from the panel on September 14, 2016. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION vi Copyright National Academy of Sciences. All rights reserved.

8 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Panel on Marine and Terrestrial Ecosystems and Natural Resource Management COMPTON J. TUCKER, NASA Goddard Space Flight Center, Co-Chair JAMES A. YODER, Woods Hole Oceanographic Institution, Co-Chair GREGORY P. ASNER, NAS, Carn egie Institution for Science FRANCISCO CHAVEZ, Monterey Bay Aquarium Research Institute INEZ Y. FUNG, NAS, University of California, Berkeley SCOTT GOETZ, Woods Hole Research Center PATRICK N. HALPIN, Duke University ERIC HOCHBERG, Bermuda Institute of Ocean Sciences CHRISTIAN J. JOHANNSEN, Purdue University RAPHAEL M. KUDELA, University of California, Santa Cruz GREGORY W. MCCARTY, U.S. Department of Agriculture LINDA O. MEARNS, National Center for Atmospheric Research LESLEY E. OTT, NASA Goddard Space Flight Center MARY JANE PERRY, University of Maine DAVID A. SIEGEL, University of California, Santa Barbara DAVID L. SKOLE, Michigan State University SUSAN L. USTIN, University of California, Davis CARA WILSON, National Oceanic and Atmospheric Administration ARTHUR CHARO, Senior Program Officer, Study Director CONSTANCE KARRAS, Program Officer PAYTON KULINA, Senior Program Assistant JAMES HEISS, Postdoctoral Fellow Panel on Climate Variability and Change: Seasonal to Centennial Co-Chair CAROL ANNE CLAYSON, Woods Hole Oceanographic Institution, VENKATACHALAM RAMASWAMY, NOAA GFDL, Co-Chair NOAA Earth System Research Laboratory ARLYN E. ANDREWS, ENRIQUE CURCHITSER, Rutgers University LEE-LUENG FU, NAE, Jet Propulsion Laboratory GUIDO GROSSE, Alfred-Wegener-Ins titute for Polar and Marine RANDAL D. KOSTER, NASA Goddard Space Flight Center SONIA KREIDENWEIS, Colorado State University EMILIO F. MORAN, NAS, Michigan State University CORA E. RANDALL, University of Colorado PHILIP J. RASCH, Pacific Northwest National Laboratory ERIC J. RIGNOT, University of California, Irvine CHRISTOPHER RUF, University of Michigan ROSS J. SALAWITCH, University of Maryland AMY K. SNOVER, University of Washington JULIENNE C. STROEVE, University of Colorado Boulder BRUCE A. WIELICKI, NASA Langley Research Center GARY W. YOHE, Wesleyan University ARTHUR CHARO, Senior Program Officer, Study Director LAUREN EVERETT, Program Officer ERIN MARKOVICH, Senior Program Assistant UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION vii Copyright National Academy of Sciences. All rights reserved.

9 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Panel on Earth Surface and Interior: Dynamics and Hazards Co-Chair DOUGLAS W. BURBANK, NAS, University of California, Santa Barbara, DAVID T. SANDWELL, NAS, Scripps Institution of Oceanography, Co-Chair ROBIN E. BELL, Columbia University EMILY E. BRODSKY, University of California, Santa Cruz DONALD P. CHAMBERS, University of South Florida, St. Petersburg LUCY FLESCH, Purdue University GEORGE E. HILLEY, Stanford University KRISTINE M. LARSON, University of Colorado Boulder STEFAN MAUS, University of Colorado Boulder MICHAEL S. RAMSEY, University of Pittsburgh JEANNE SAUBER, NASA Goddard Space Flight Center KHALID A. SOOFI, ConocoPhillips HOWARD A. ZEBKER, Stanford University ARTHUR CHARO, Senior Program Officer, Study Director ANNE LINNE, Scholar ERIC EDKIN, Senior Program Assistant UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION viii Copyright National Academy of Sciences. All rights reserved.

10 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space SPACE STUDIES BOARD 1 FIONA HARRISON, NAS, California Institute of Technology, Chair 2 ROBERT D. BRAUN, NAE, University of Colorado, Boulder, Vice Chair DAVID N. SPERGEL, NAS, Princeton University and Center for Computational Astrophysics at the Vice Chair Simons Foundation, JAMES G. ANDERSON, NAS, Harvard University JEFF M. BINGHAM, Consultant JAY C. BUCKEY, Geisel School of Medicine at Dartmouth College MARY LYNNE DITTMAR, Dittmar Associates JOSEPH FULLER, JR., Futron Corporation THOMAS R. GAVIN, California Institute of Technology SARAH GIBSON, National Center for Atmospheric Research WESLEY T. HUNTRESS, Carnegie Institution of Washington ANTHONY C. JANETOS, Boston University CHRYSSA KOUVELIOTOU, NAS, George Washington University DENNIS P. LETTENMAIER, NAE, University of California, Los Angeles ROSALY M. LOPES, Jet Propulsion Laboratory DAVID J. M COMAS, Princeton University C LARRY PAXTON, Johns Hopkins University, Applied Physics Laboratory SAUL PERLMUTTER, NAS, Lawrence Berkeley National Laboratory ELIOT QUATAERT, University of California, Berkeley BARBARA SHERWOOD LOLLAR, University of Toronto HARLAN E. SPENCE, University of New Hampshire MARK H. THIEMENS, NAS, University of California, San Diego MEENAKSHI WADHWA, Arizona State University Staff MICHAEL H. MOLONEY, Director CARMELA J. CHAMBERLAIN, Administrative Coordinator TANJA PILZAK, Manager, Program Operations CELESTE A. NAYLOR, Information Management Associate MARGARET KNEMEYER, Financial Officer SU LIU, Financial Assistant 1 Member, National Academy of Sciences. 2 Member, National Academy of Engineering. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION ix Copyright National Academy of Sciences. All rights reserved.

11 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Copyright National Academy of Sciences. All rights reserved.

12 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Preface 1 This report is the final product of the 2017-2027 decadal survey for Earth science and al survey in Earth science and applications applications from space (“ESAS 2017”), the second decad from space carried out by the National Academies of Sciences, Engineering, and Medicine. The survey effort began in earnest in late 2015 with the appoi ntment of the steering committee to conduct the study and the appointment of its supporting study panels. As shown in the statement of task (reprinted in Appendix E), the study’s overarching task is to generate “recommendations for the environmental es for an integrated and sustainable approach to monitoring and Earth science and applications communiti the conduct of the U.S. government’s civilian space- based Earth-system science programs.” As discussed is charge resulted in recommendations that would, in Chapter 1 of this report, the interpretation of th within known constraints such as anticipated budgets , advance Earth system science and deliver critical information to support a broad range of national economic and societal needs. 2 The inaugural decadal survey in this scientific domain, published in 2007, organized its work around the overarching theme of Earth system science for societal benefit. Perhaps its most notable achievement was that the various co mmunities that constitute Earth science, which span a set of diverse disciplinary boundaries and had no tradition of coming toge ther, were able to reac h consensus on decadal research priorities. The resulting integrated program proved highly beneficial to both the sponsoring agencies and to a nation whose needs for the info rmation and data products derived from agency 3 programs were accelerating rapidly. ESAS 2017 was sponsored by the National Aeronautics and Space Administration (NASA), the National Oceanic and Atmospheric Administration ( NOAA), and the U.S. Geological Survey (USGS)— ng and execution of civilian programs of Earth federal agencies with responsibilities for the planni observations from space. Internally, the survey effo rt at the National Academies was led by the Space 1 Decadal surveys are notable in their ability to sample thoroughly the research interest, aspirations, and needs of a scientific community. Through a rigorous process, a primary survey committee and thematic panels of community members construct a prioritized program of sc ience goals and objectives and define an executable strategy for achieving them. These report nation’s agenda in that science area for s play a critical role in defining the the following 10 years—and often beyond. See National Academies of Sciences, Engineering, and Medicine, 2015, The Space Science Decadal Surveys: Lessons Learned and Best Practices , Washington, D.C.: The National Academies Press, https://doi.org/10.17226/21788. 2 See National Research Council, 2007, Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond . Washington, D.C.: The National Academies Press, https://doi.org/10.17226/11820. For a review of the successes and shortcomings of the survey, see National Research Council, 2012, Earth Science and Applications from Space: A Midterm Assessment of NASA’s Implementation of the Decadal Survey , ://doi.org/10.17226/13405. The growth in Landsat use is Washington, D.C.: The National Academies Press, https discussed in M.A. Wulder et al., 2016, The global Land sat archive: Status, conso Remote lidation, and direction, Sensing of Environment 185:271-283. To manage its growing data archives, NOAA has initiated a “Big Data Project;” see http://www.noaa.gov/big-data-project. 3 As inferred by the size of data archives and the number of data users and data retrievals. For example, see, in Presentation by Program Executive for Earth Science Data Systems Earth Science Divisi on (DK), Science Mission Directorate, NASA Headquarters, “NASA’s Earth Science Data Systems Program,” on February 16, 2016, https://smd-prod.s3.amazonaws.com/science-blue/s3fs- public/atoms/files/5-Big_Data-Earth_Science-tagged.pdf, slide 18. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION xi Copyright National Academy of Sciences. All rights reserved.

13 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space tion of the staff and volunteers at the Board on Studies Board with the close collaboration and coopera Atmospheric Sciences and Climate, the Board on Earth Sciences and Resources, the Ocean Studies Board, the Polar Research Board, and th e Water Sciences and Technology Board. The survey was carried out by an appointed st eering committee, which was solely responsible for tions, and five appointed interdisciplinary study this final report, including all findings and recommenda panels. In addition, the steering committee was info rmed by several informal working groups, some focusing on specific elements of the task statement and others focusing on cross-disciplinary topics (e.g., technology and innovation) and “integrating themes” (e.g., the carbon, water and energy cycles). This r a rich and comprehensive study process by structure—one of several considered—allowed fo approaching the topics in the statement of task from multiple vantage points. Designated “liaisons”—from the steering committee to each of the panels and from each panel to ove piping of information. In addition, steering each of the other four panels—helped to avoid the st committee liaisons attended panel meet ings, and panel liaisons had the opp ortunity to attend other panel meetings. Panels met three times during the course of the study; at two of these meetings, joint sessions with a concurrently held steering committee mee ting took place. The steering committee held seven in- between meetings, both the steering committee and the person meetings during the course of the study. In panels held numerous virtual meetings via WebE x. Further information on the decadal survey’s organization is available at http://www.nas.edu/esas2017. e decadal survey took place within the study panels. Their focus Much of the initial work of th areas/themes were chosen so that together they sp anned the major components of the Earth system. The panel organization, which was devised and confirme d by the steering committee early in the survey 4 process, was also informed by co mmunity input received in the first request for information (RFI; see Appendix D). Other considerations included the desire for a structure that was responsive to the agency missions and goals of the sponsors and consistent with the decadal survey statement of task. The panels were responsible for receiving a nd analyzing community input; in particular, community responses to the survey-issued second RFI. Each panel included members whose collective expertise spanned the panel’s topical focus areas from science to applications. With input from the panels, ng system priorities that integrated goals for the steering committee then developed proposed observi understanding and monitoring the Earth system with those that emphasize the use of observations in a 5 and their focus areas were as follows: range of applied settings. The panels I. Global Hydrological Cycles and Water Resources The movement, distribution, and availability of water and how these are changing over time. II. Weather and Air Quality: Minutes to Subseasonal Atmospheric Dynamics, Thermodynamics, Chemis try, and their interactions at land and ocean interfaces. III. Marine and Terrestrial Ecosystems and Natural Resource Management Biogeochemical Cycles, Ecosystem Functioning, Biodiversity, and factors that influence health and ecosystem services. IV. Climate Variability and Change : Seasonal to Centennial 4 The first request for information (RFI) was issued in advance of the initiation of the survey and requested community input to help understand the role of space-based observations in addressing the key challenges and questions for Earth System Science in the coming decade. By design, it did not ask the community for ideas on how to address an identified challenge or qu estion. Building on the first RFI, the second RFI requested ideas for specific science and applications targets (i.e., objectives) that promised to substantially advance understanding in one or more of the Earth System Science themes as sociated with the su rvey’s study panels. 5 abbreviated as follows: Global Hydrological Cycles and Throughout this report, references to panels are also Water Resources = “H” or “Hydrology;” Weather and Air Quality: Minutes to Subseasonal = “W” or “Weather,” Marine and Terrestrial Ecosystems and Natural Resource Management = “E” or “Ecosystems,” Climate Variability and Change: Seasonal to Centennial = “C” or “Climate,” and Earth Surface and Interior: Dynamics and Hazards = “S” or “Solid Earth.” UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION xii Copyright National Academy of Sciences. All rights reserved.

14 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space sphere, Land, and Cryo sphere within the Forcings and Feedbacks of the Ocean, Atmo Coupled Climate System. Earth Surface and Interior: Dynamics and Hazards V. Core, mantle, lithosphere, and surface processes, system interactions, and the hazards they generate. ESAS 2017 STATEMENT OF TASK To address the elements of the ESAS 2017 statement of task, the steering committee (the the following four broad areas: “committee”) focused its work in 1. Assessment of the past decade’s progress, 2. Establishment of a Vision and Strategy for the future decade, 3. Prioritization of science and applications targets and mapping these to an observing plan, 4. the plan specific to the requests made by Development of guidance on implementation of a. NASA b. NOAA and USGS. Within areas 2 and 3 of this list, the statement of task requests that priorities focus on science, applications, and observations, rather than the in struments and missions required to carry out those observations. In particular, the statement of task requests that the committee “recommend NASA research activities to advance Earth system science and applications by means of a set of prioritized strategic “ science targets ” [expanded by the steering committee to be sci ence and applications targets] for the space-based observation opportunities in the decade 2018- 2027.” As described in more detail in Chapter 3, a by a common space-based observable.” The is “a set of science objectives related science target steering committee defined the observable associated with each science target targeted observable . as a ESAS 2017: STRUCTURE AND KEY FEATURES OF THE REPORT The structure and key features of this report reflects its rather detailed statement of task. In particular,  As requested, the committee’s recommended strategy is one that will advance fundamental understanding of the Earth system and provide knowledge that can be applied in service to society.  The report, per the statement of task , provides recommended approaches to facilitate the development of a robust, resilient , and appropriately balanced U.S. program of Earth observations from space. Responding to task elements specific to NASA, the report provides a prioritized list of — top-level science and application objectives , with attention to gaps and opportunities in the program of record and the feasibility of measurement approaches. Task elements pertaining to NASA also include specific request s for an analysis of the balance between major program elements in the Earth Science Di vision and, within its flight element, the balance among investments into the various program elements. — Task elements pertaining specifically to NOAA and the USGS focus on how to make existing and planned programs more effective with respect to their utility to users and their cost-effectiveness, including through technology innovation. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION xiii Copyright National Academy of Sciences. All rights reserved.

15 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space 6 Per the 2008 NASA Authorization Act,  ESAS 2017 arranged for an independent cost and 7 technical evaluation (CATE) of the major candidate investments being considered for prioritization. This analysis was performed by the Aerospace Corporation, which also performed CATEs as part of recent National Academies decadal surveys in solar and space physics, planetary science, and astronomy and astrophysics (see Chapter 3 for details).  To facilitate the development and implementa tion of its recommended program for NASA’s Earth Science Division, the ESAS 2017 committee assumed the availability of resources at the 8 levels anticipated at the time the survey was initiated. It also provides “decision rules” to guide responses in the event of unexpected technical or budgetary problems.  The ESAS 2017 steering committee and its study pa nels carefully considered opportunities to lower the cost of making research-quality Earth observations by leveraging advances in technology, international partne ng in the commercial sector. rships, and the capabilities emergi Attention was also given to the exploitation of “big data” for Earth science.  NASA, like all federal agencies, is faced with difficult choices among competing priorities for these choices include whether to invest in the investment. Within the Earth Science Division, ta stream over another, or to develop a new measurement continuation of one existing da developing a recommende d program that could capability sought by the research community. In be executed within the highly constrained budgets the steering committee anticipated by NASA, and panels also faced the difficult challenge of striking an appropriate balance between these competing demands. The transfer of responsib ility from NOAA to NASA for several “continuity” measurements without budget increases commensur ate with the new responsibility added to the 9 challenge. Survey deliberations benefited from a close read of several high-level guidance documents from  10 the executive branch. Finally, this report would not have been possible without the assistance of the sponsoring agencies and colleagues in the research and applications community. The steering committee is grateful to leaders across NASA, NOAA, and US GS for their support of the survey effort; in particular, they committee and panels requi provided the detailed programmatic information the red to understand the context for their prioritization. In addition, the decad al survey could not have been completed without the substantial and substantive work colleagues put into the composition of white pape rs and participation in town hall meetings. These inputs were especially import ant to the work of the interdisciplinary panels whose outputs form the basis of the exciting science and applications that are the foundation of the survey’s recommended program. We would also lik e to acknowledge the assistance of the Aerospace 6 Section 1104 of the 2008 Act, “Directs the Administrator to enter into agreements periodically with the National Academies for decadal surveys to take stock of the status and opportunities for Earth and space science discipline fields and aeronautics research and to recomme nd priorities for research and programmatic areas over the next decade.” Further, the Act, “Requires that such agr eements include independent estimates of life cycle costs and technical readiness of missions assessed in the survey s whenever possible.” See Na tional Aeronautics and Space Administration Authorization Act of 2008, P.L. 110-422, Section 1104 (October 15, 2008). 7 The Space Sciences Decadal Surveys See Appendix B, “Implementing the CATE Process,” in , op. cit. fn. 1. 8 e survey with a budget hist As explained in Chapter 3, NASA officials provided th ory and indicated that large scale changes to recent funding levels were not anticipated. Recommendations in the present report are based on assumption that the then current budget would only grow with inflation. 9 The present survey benefited from the analysis fra mework presented in National Academies of Sciences, Engineering, and Medicine, 2015, Continuity of NASA Earth Observations from Space: A Value Framework , Washington, D.C.: The National Academie s Press, https://doi.org/10.17226/21789. 10 These are discussed in Tim Stryker, Director, U.S. Group on Earth Observations Program, “National Civil Earth Observations Planning and As sessment,” presented at the ASPRS 2015 Annual Conference, May 7, 2015, https://calval.cr.usgs.gov/wordpress/wp-conten t/uploads/ASPRS-slides_Stryker_final.pdf. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION xiv Copyright National Academy of Sciences. All rights reserved.

16 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space the cost and technical feasabilty of options to Corporation who provided an independent analysis of realize survey science priorities. OUTLINE OF THE REPORT ws; shown in bold is the major theme of each This report is organized in two parts as follo chapter: PART I  Summary of the Steering Committee’s Report  The Full Steering Committee Report: Chapter 1. A Vision for the Decade Chapter 2. A Decadal Strategy This chapter reviews progress over th e last decade, assesses emerging scientific and societal needs, and builds from that foundation to identify a strategic framework for the next decade. Program for Science, Applications, and Observations Chapter 3. A Prioritized This chapter describes the process used by the committee to identify and prioritize observational needs, and presents the recommended strategy to provide a robust and balanced U.S. program of Earth observations from ncy-provided budget expectations. space that is consistent with age Chapter 4. Agency Programmatic Context This chapter addresses some of the key agency-specific issues identified as being important programmatically in the implementation of the recommended program. Chapter 5. Conclusion PART II Chapters Contributed by the Five Study Panels  Chapter 6. Global Hydrological Cycles and Water Resources Chapter 7. Weather and Air Quality: Minutes to Subseasonal Chapter 8. Marine and Terrestrial Ecosystems and Natural Resource Management Chapter 9. Climate Variability and Change: Seasonal to Centennial Chapter 10. Earth Surface and Interior: Dynamics and Hazards UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION xv Copyright National Academy of Sciences. All rights reserved.

17 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Copyright National Academy of Sciences. All rights reserved.

18 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Acknowledgement of Reviewers This Consensus Study Report was reviewed in dr aft form by individuals chosen for their diverse is to provide candid and perspectives and technical expertise. The purpose of this independent review critical comments that will assist the National Academ ing, and Medicine in ies of Sciences, Engineer making each published report as sound as possible and to ensure that it meets the institutional standards for quality, objectivity, evidence, and responsiveness to the study charge. The review comments and draft manuscript remain confidential to protect th e integrity of the deliberative process. We thank the following individuals for their review of this report: Mark Abbott, Woods Hole Oceanographic Institution, Kevin R. Arrigo, Stanford University, Jean-Philippe Avouac, California Institute of Technology, Mike Behrenfeld, Oregon State University, Lance F. Bosart, University at Albany, SUNY, Roland Burgmann, University of California, Berkeley, Simon A. Carn, Michigan Technological University, Anny Cazenave, NAS, Centre National d’études Spatiales, Scott Denning, Colorado State University, Mark Drinkwater, European Space Agency, David P. Edwards, National Center for Atmospheric Research, Pamela Emch, Northrop Grumman, Sara J. Graves, University of Alabama, Huntsville, Tracey Holloway, University of Wisconsin, Madison, Ian Joughin, University of Washington, Chris Justice, University of Maryland, College Park, Michael D. King, NAE, University of Colorado, Boulder, Dennis P. Lettenmaier, NAE, University of California, Los Angeles, Jay Mace, University of Utah, Anne W. Nolin, Oregon State University, Theodore Scambos, National Snow and Ice Data Center, Walter Scott, DigitalGlobe, J. Marshall Shepherd, University of Georgia, Adrian Simmons, European Centre fo r Medium-Range Weather Forecasts, Richard W. Spinrad, National Oceanic and Atmospheric Administration (retired), William F. Townsend, Consultant, Annapolis, Maryland, Kevin E. Trenberth, National Center for Atmospheric Research, Eric Wolff, University of Cambridge, Robert Wood, University of Washington, and Carl Wunsch, NAS, Harvard University. Although the reviewers listed above provided ma ny constructive comments and suggestions, they were not asked to endorse the conclusions or reco mmendations of this report nor did they see the final UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION xvii Copyright National Academy of Sciences. All rights reserved.

19 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space draft before its release. The review of this repor t was overseen by Charles F. Kennel, University of California, San Diego, and Thomas H, Vonder Haar, Colorado State University. They were responsible for making certain that an independent examination of this report was carried out in accordance with the standards of the National Academies and that all review comments were carefully considered. Responsibility for the final content rests entirely with the authoring committee and the institution. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION xviii Copyright National Academy of Sciences. All rights reserved.

20 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space In Memory of Molly Macauley Molly Macauley, a member of the steering committee, passed away during the committee’s tenure in July 2016. Molly was a very special pers on, a true friend to many of us, and a tremendous ective of an economist, impacted the entire field of colleague to all. Her contributions, from the persp Earth observation. Her clarity of thought strongly influenced the early directions of this committee; that work. Molly had an unparalleled talent for voicing clarity was deeply missed during the remainder of our unanticipated perspectives that redirected discussions and brought difficult issues into instant focus. She ensured we stayed grounded in the reality of how our work directly and deeply impacts people’s lives. She drove us to quantify that value and communicate it clearly. Her loss will continue to be felt by our entire community for a long time. Waleed Abdalati and Bill Gail On Behalf of the Steering Committee, Panels, and Staff UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION xix Copyright National Academy of Sciences. All rights reserved.

21 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Copyright National Academy of Sciences. All rights reserved.

22 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Contents PART I: REPORT OF THE STEERING COMMITTEE SUMMARY 1 A VISION FOR THE DECADE A DECADAL STRATEGY 2 3 A PRIORITIZED PROGRAM FOR SCIENCE, APPLICATIONS, AND OBSERVATIONS 4 AGENCY PROGRAMMATIC CONTEXT 5 CONCLUSION PART II: PANEL INPUTS GLOBAL HYDROLOGICAL CYCLES AND WATER RESOURCES 6 7 WEATHER AND AIR QUALITY: MINUTES TO SUBSEASONAL 8 MARINE AND TERRESTRIAL ECOSYSTEMS AND NATURAL RESOURCES MANAGEMENT CLIMATE VARIABILITY AND CHANGE: SEASONAL TO CENTENNIAL 9 10 EARTH SURFACE AND INTERIOR: DYNAMICS AND HAZARDS APPENDIXES A Program of Record B Science and Applications Traceability Matrix C Targeted Observables Table D RFI Responses E Statement of Task F Committee Members and Staff Biographies G Acronyms and Abbreviations UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION xxi Copyright National Academy of Sciences. All rights reserved.

23 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Copyright National Academy of Sciences. All rights reserved.

24 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Summary ence and applications from space This decadal survey of Earth sci is the second such decadal survey produced by the National Academies of Sciences, Engineering, and Medicine for the field of Earth sciences and applications, following on the first that was released in 2007 (NRC, 2007). This summary provides a comprehensive overview of the report, but includes only selected findings and recommendations from among the 17 findings and 18 recommendations in the report. EARTH OBSERVATION FROM SPACE: A TRANSFORMATIVE CAPABILITY From the time of the earliest humans, knowledge about Earth has been fundamental to our fate and prospects. Over the past 60 years, particularly rapid progress has been achieved in acquiring such scientific and practical knowledge, due in large part to the special perspective provided by satellite- based Earth observations . The vantage-point of space enables us to see th e extent to which Earth’s ever-changing processes influence our lives. These processes operate at local spatia l-scales, such as the flows of rivers that provide fresh water and the weather and climate conditions th at determine crop yields, as well as at global spatial- scales, such as changes in the o cean currents that impact commercial fishing and contribute to global change and climate variability. The space-based vant age point also ensures we can observe processes occurring over a wide range of time scales, from the abr upt (such as earthquakes) to the decadal (such as eets), and at all time scales in between. growth and shrinkage of the world’s great ice sh ng to recognize the complex and continually Empowered by this perspective, we are comi changing ways by which Earth’s processes occur, along with the critical roles their observation and understanding play throughout our lives. Space-based Earth observations provide a global perspective of Earth that has Finding 1A: • Over the last 60 years, transformed our scientific understanding of the planet, revealing it to be an integrated system of dynamic inte ractions between the atmosphere, ocean, land, ice, and human society across a range of spatial and temporal scales, irrespective of geographic, political, or disciplinary boundaries. • societal applications that provide tremendous In the past decade in particular, enabled value to individuals, businesses, the nation, and the world. Such applications are growing in breadth and depth, becoming an essential inform ation infrastructure element for society as they are integrated into people’s daily lives. THRIVING ON OUR CHANGING PLANET This ability to observe our planet comprehensivel y matters to each of us, on a daily level. Earth information—for use in Internet maps, daily w eather forecasts, land use planning, transportation efficiency, and agricultural productivity, to name a few—is central to our liv es, providing substantial UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION S-1 Copyright National Academy of Sciences. All rights reserved.

25 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space and our personal safety. It helps ensure we are a contributions to our economies, our national security, society. thriving The Earth information we have come to rely on throughout our daily lives is the result of a sustained commitment to both exploratory and appl ied Earth science, and to what has become a sophisticated national and international infrastruct ure of observing systems, scientific research, and applications. A particular strength of the Earth scien ce and applications field is the extent to which curiosity-based science is inextricably integrated with applications-ori ented science and societal benefits. Ongoing commitment to this inspirational and practical science has returned benefits to society many times over, and will continue to do so with further support. Among the most intellectually and practically im portant revelations from the past 60 years of space-based observation is the extent to which Earth is changing , in multiple ways and for many reasons . Daily changes, such as weather, were obvious to ev en the earliest humans, even if not explainable. Longer-term changes, particularly those occurri ng on global scales, are only now becoming understood and gaining public recognition. Some of these changes are climate related, such as alteration of the El Niño Southern Oscillation, but many are not. In a ddition to climate, changes in air quality, water availability, agricultural soil nutrients, and other Earth resources are being driven largely by human actions. Successfully managing risks and identifying opportunities associated with these changes requires a clear understanding of both the human-drive n and natural processes that underlie them. A CHALLENGING VISION FOR THE DECADE AND BEYOND just from past experience. Its evolving and A changing Earth is one we can never understand emerging characteristics must be continually explored through observation. Our scientific curiosity must result from change, if we are to continue applying seek and reveal the new and altered processes that will ecisions we make this decade will be pivotal for our knowledge effectively for society’s benefit. D predicting the potential for future changes and for influencing whether and how those changes occur. Embracing this new paradigm of unde rstanding a changing Earth, and build ing a program to address it, is our major challenge for the coming decade and beyond. Earth science and applications are a key part of the nation’s information Recommendation 2.1: infrastructure, warranting a U.S. program of Earth observations from space that is robust, resilient, and appropriately balanced. NASA, NOAA, and USGS, in collaboration with other interested U.S. agencies, should ensure effi cient and effective use of U.S. resources by strategically coordinating and a dvancing this program at the national level, as also recommended in the 2007 Earth science and appli cations from space decadal survey. This context of both societal need and intellectual opportunity provided the basis for developing t. Society’s fundamental desire to thrive, the the Earth observation program proposed in this repor expanding scientific knowledge needed to support that desire, and the growing capacity to apply that committee’s recommendations. Embracing the goal of knowledge are all central motivations for this understanding Earth in pursuit of this vision— to thrive on our changing planet —motivates a new paradigm for the coming decade and beyond. Earth Science and Applications Paradigm for the Coming Decade Earth science and derived Earth information have b ecome an integral component of our daily lives, our business successes, and society’s capacity to thrive. Extending this societal progress requires that we focus on understanding and reliably pred icting the many ways our planet is changing. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION S-2 Copyright National Academy of Sciences. All rights reserved.

26 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space A STRUCTURED APPROACH TO ACHIEVING PROGRESS The next decade is one in which progress will not come easily. Financ ial and human resource constraints are likely to present challenges to progress (Chapter 1). Suceeding compels NASA, NOAA, and USGS to develop, adopt, and implement stra tegies to advance both technology and programmatic processes. The committee recommends eight elements ( numbered only for identification) of a suggested strategic framework (Chapter 2): 1. Commit to sustained science and applications; Embrace innovative methodologies for integrated science/applications; 2. 3. Amplify the cross-benefit of science and applications; 4. Leverage external res ources and partnerships; 5. Institutionalize programmatic agility and balance; 6. Exploit external trends in technology and user needs; 7. Expand use of competition; and 8. Pursue ambitious science, despite constraint. rategic thinking to overcome them, are reflected The challenges ahead, and the need for innovative and st in the following community challenge. Decadal Community Challenge ive solutions that enhance and accelerate the Pursue increasingly ambitious objectives and innovat servation and analysis to the nation and to the world science/applications value of space-based Earth ob ources are constrained, and ensures that further in a way that delivers great value, even when res investment will pay substantial dividends. The committee believes that meeting the challenge described above will motivate the scientific community to pioneer novel approaches in how it conducts its scientific research, with an emphasis on programmatic and technological innovation to accomplish more with less, with greater attention to the potential benefits of domestic a ong with the growing capability of nd international partnerships al commercial sources (Chapters 3 and 4). The committee conducted its work in close collabor ation with the decadal survey’s five study panels, each interdisciplinary and together spanning all of disciplines associated with Earth system science. The survey process is summarized in Figure S.1. It was designed to converge—from a large number of community-provided possibilities—to a final, small set of Science and Applications Priorities (shown in blue) and Observing System Priorities (shown in green) that are required to address the nation’s Earth science and applications needs. This process assumed that the existing and planned instruments in the program of record (POR) are implemented as expected . UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION S-3 Copyright National Academy of Sciences. All rights reserved.

27 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space PRIORITIZED 2017 ESAS PROGRAMMATIC ESAS 2017 4) (Chapter GUIDANCE 3) PROGRAM (Chapter OBSERVING observables Corresponding RECOMMENDATIONS ADDRESSED next in REGARDING Program of decade’s PROGRAMMATIC Record CONTEXT of Program Record (Appendix A) PROCESS Questions Science/Apps 35 : ‐ CROSS AGENCY INFORMED BY Total Objectives Supporting Science/Apps 103 : COMMUNITY 22 Total: Observables Targeted Objectives Most Important: 24 Rank ed NASA Re comme nded 5 for to Flight: Commitment • Flight RFI 7 Down for Recommended Flights: 3 to select ‐ 3 2017 ESAS • Technology Submissions: Corresponding Recommended 3 Incubation: for • Applications RFI #1: 139 Science Applications & observables NOT RFI #2: 151 Priorities Table ADDRESSED in next NOAA 2017 ESAS of Program decade’s 3.3, Appendix B ) (Table Record Observing Table Priorities System USGS 3.5) (Table applications from space decadal survey (ESAS FIGURE S.1 Roadmap for the 2017 Earth science and to identifying priorities for the coming decade, 2017) report based on the survey committee’s approach starting from community requests for information (RFIs), refining this input to determine priority science entifying new observing system priorities (assuming and applications questions and objectives, then id priorities are complemented by programmatic completion of the program of record). These recommendations. ESTABLISHING SCIENCE AND APPLICATIONS PRIORITIES ubmitted ideas, the committee, working with the Starting from an initial set of 290 community-s five interdisciplinary panels, narrowed this large set of ideas to a set of 35 key Earth science and applications questions to be addressed over the next decade. Together, these questions comprehensively in both curiosity-driven and practically focused address those areas for which advances are most needed Earth information. To identify the observational Earth science and the corresponding practical uses of mmittee then defined a set of underlying science capabilities required to answer these questions, the co and applications objectives, evaluating and assigning each to one of three prioritization categories: most important (MI), very important (VI), and important (I). mmendation 3.1 that NASA, NOAA, and USGS This process informed the committee’s Reco rized in Table S.1 (and described in more detail pursue the key science and applications questions summa in the body of the report). These questions address the central science and applications priorities for the coming decade. NASA, NOAA, and USGS, working in coordination, according to their Recommendation 3.1: mission and priorities, should implement a appropriate roles and recognizing their agency ence and applications that is based on the programmatic approach to advancing Earth sci questions listed in Table S.1, ’“Science and Applications Priorities for the Decade 2017- and objectives 2027” UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION S-4 Copyright National Academy of Sciences. All rights reserved.

28 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space TABLE S.1 Science and Applications Priorities for the Decade 2017-2027 Science and Applications Questions Science and Applications Addressed by MOST IMPORTANT Objectives Area e changes in evapotranspiration and (H-1) How is the water cycle changing? Ar Coupling of the Water precipitation accelerating, with greater rates of evapotranspiration and thereby and Energy Cycles pressed in the space-time distribution of precipitation, and how are these changes ex the frequency and magnitude of extremes rainfall, snowfall, evapotranspiration, and such as droughts and floods? (H-2) How do anthropogenic changes in climate, land use, water use, and water storage interact and modify the water an d energy cycles loca lly, regionally and globally and what are the short- and long-term consequences? What are the structure, function, and biodiversity of Earth’s ecosystems, and (E-1) Ecosystem Change how and why are they changing in time and space? What are the fluxes (of carbon, between (E-2) water, nutrients, and energy) d the solid Earth, and how and why are ecosystems and the atmosphere, the ocean an they changing? (E-3) What are the fluxes (of carbon, water, nutrients, and energy) within ecosystems, and how and why are they changing? (W-1) What planetary boundary layer (PBL) pr ocesses are integral to the air-surface Extending and (land, ocean and sea ice) exchanges of energy, momentum and mass, and how do Improving Weather and these impact weather forecasts and air quality simulations? Air Quality Forecasts (W-2) How can environmental predictions of w eather and air quality be extended to forecast Earth System conditions at l ead times of 1 week to 2 months? (W-4) Why do convective storms, heavy precipitation, and clouds occur exactly when and where they do? (W-5) What processes determine the spatio-temporal structure of important air pollutants and their concomitant adverse im pact on human health, agriculture, and ecosystems? How can we reduce the uncertainty in the amount of future warming of the (C-2) Reducing Climate Earth as a function of fossil fuel emissions, improve our ability to predict local and Uncertainty and regional climate response to natural and anthropogenic forcings, and reduce the Informing Societal uncertainty in global climate sensitivity that drives uncertainty in future economic Response impacts and mitigation/adaptation strategies? and regionally, over the next decade (C-1) How much will sea level rise, globally Sea Level Rise and beyond, and what will be the role of ice sheets and ocean heat storage? (S-3) How will local sea level change along coastlines around the world in the next decade to century? ds be accurately forecasted and How can large-scale geological hazar (S-1) Surface Dynamics, eventually predicted in a socially relevant timeframe? Geological Hazards and (S-2) How do geological disasters directly impact the Earth system and society Disasters following an event? (S-4) What processes and interactions determine the rates of landscape change? IMPORTANT (summarized) VERY IMPORTANT (summarized) (H-3) Influence of water cycle on natural Fresh water availability and impacts on ecosystems/society (H-4) (W-6) Long-term air pollution trends and impacts hazards and preparedness (W-7) h surface variations (W-3) Processes influencing tropospheric ozone and its Influence of Eart atmospheric impacts on weather and air quality cle variations on Impacts of carbon cy (C-3) ( W-8) Methane variations and impacts on tropospheric climate and ecosystems composition and chemistry Earth system response to air-sea (C-4) (W-9) Cloud microphysical property dependence on aerosols and precipitation interactions UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION S-5 Copyright National Academy of Sciences. All rights reserved.

29 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space IMPORTANT (summarized) VERY IMPORTANT (summarized) Impact of aerosols on global warming Cloud impacts on radiative forcing and weather (W-10) (C-5) Improving seasonal to decadal climate predictability (C-6) forecasts Quantifying carbon sinks and their changes (E-4) (C-7) Stability of carbon sinks (E-5) Changes in decadal scale Impacts of ozone layer change (C-9) atmospheric/ocean circulation and impacts (C-8) Improving discovery of energy, mineral, and soil resources (S-7) Consequence of amplified polar climate change on Earth system (S-5) How energy flows from the core to Earth’s surface (S-6) Impact of deep underground water on geologic processes and water supplies NOTE: The highest-priority questions (d efined as those associated with “ most important ” objectives) are listed in full; other questions associated with “ very important ” or “ important ” objectives are briefly summarized. No further priority is assumed within categories, and the topics ar e listed alphabetically. Letter and number combinations in parenthesis refer to the panel (H = “Hydrology,” W = “W eather,” E = “Ecosystems,” C = “Climate,” S = “Solid Earth”) and numbering of each panel’s questions. Complete versions of this table are provided in Table 3.3 and Appendix B. By pursuing these priorities, important advances will be made in areas that are both scientifically challenging and of direct impact to how we liv e. A major component of the committee’s observing program recommendations is a commitment to a set of observation capabilities, outlined in the next section, that will enable substantial progress in all of the following science and applications areas: , which  make-up and distribution of aerosols and clouds Providing critical information on the in turn improve predictions of future climate c onditions and help us assess the impacts of aerosols on human health;  Addressing key questions about how changing cloud cover and precipitation will affect in the future, advancing understanding of the climate, weather, and Earth’s energy balance movement of air and energy in the atmosphere and its impact on weather, precipitation, and severe storms;  Determining the extent to which the shrinking of glaciers and ice sheets , and their contributions to sea-level rise, is accelerating, d ecelerating, or remaining unchanged;  Quantifying trends in water stored on land (e.g., in aquifers) and the implications for issues such as water availability for human consumption and irrigation;  Understanding alterations to surface characteristics and landscapes (e.g., snow cover, snow melt, landslides, earthquakes, eruptions, urba nization, land-cover and land use) and the sk management and resource management; implications for applications such as ri  evolving characteristics and health of terrestrial vegetation and aquatic Assessing the ecosystems , which is important for understanding key consequences such as crop yields, carbon uptake, and biodiversity; and  Examining movement of land and ice surfaces to determine, in the case of ice, the likelihood of rapid ice loss and significantly accelerated rates of sea-level rise, and in the case of land, changes in strain rates that impact and provide criti cal insights into earthquakes, volcanic eruptions, landslides, and tectonic plate deformation. In addition, the committee is proposing competitiv e observational opportunities, also outlined in the next section, to address at least three of the following science and applications areas: UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION S-6 Copyright National Academy of Sciences. All rights reserved.

30 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space  Understanding the sources and sinks of carbon dioxide and methane and the processes that will affect their concentrations in the future;  Understanding glacier and ice sheet contributio ns to rates of sea-level rise and how they are likely to impact sea-level rise in the future; Improving understanding of ocean circulation, the exchanges between the ocean and  atmosphere, and their impacts on weather and climate;  Assessing changes in ozone and other gases and the associated implications for human health, air quality, and climate;  amount and melt rates of snow and the associated implications for water Determining the resources, weather, climate, flooding, drought, etc.;  Quantifying biomass and characterizing ecosystem structure to assess carbon uptake from the atmosphere and changes in land cover and to support resource management; and  Providing critical insights into the transport of pollutants, wind energy, cloud processes, and how energy moves between the land or ocean surfaces and the atmosphere. The recommended program will advance scientific know ledge in areas that are ripe for discovery and that have direct impact on the way we live t oday. The knowledge developed in the coming decade, through this science, holds great promise for inform ing actions and investments for a successful future. IMPLEMENTING AN INNOVATIVE OBSERVING PROGRAM Addressing the committee’s priority science and applications questions requires an ongoing commitment to existing and planned instruments and satellites in the program of record (POR). The committee’s recommended observing program builds from this, filling gaps in the POR where observations are needed to address the key science and applications objectives for the coming decade. This observing program is summarized in Table S. 2 and in the accompanying Recommendation 3.2. Most observables are allocated to two new NASA flight pr ogram elements: a committed group of observations Designated , along with a competed group termed Earth System Explorer termed . Within these two new flight program elements, eight of the priority observation needs from Table S.2 are expected to be missions. In addition, several observables are assigned implemented as instruments, instrument suites, or to a new program element called Incubation, intended to accelerate readiness of high-priority observables not yet feasible for cost-effective flight implementation. Finally, an expansion of the Venture program is proposed for competed small missions to add a focu s on continuity-driven observations. Together, these new program elements complement existing NASA fli ght program elements such as the Venture program. The foundational observations in Table S.3—the five shown in the “Designated” column that are entation, and the three to be competitively recommended specifically by the committee for implem selected from among the identified set of seven “E arth System Explorer” candidates—augment the rity science and applications questions can be existing POR and ensure that the survey’s 35 prio effectively addressed, to the extent that resources allo w. In keeping with the st udy’s statement of task, specific missions and instruments were not identifie d, ensuring that the sponsoring agencies will have discretion for identifying the most cost-effective a nd appropriate space-based approaches to implementing the recommended set of observations. Each of th e new NASA flight program elements promises innovative means for using competition and other pr ogrammatic tools to increase the cadence and quality of flight programs, while optimizing cost and risk. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION S-7 Copyright National Academy of Sciences. All rights reserved.

31 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space TABLE S.2 Observing System Priorities Candidate Measurement Targeted Science/Applications Summary Approach Observable Explorer Incubation Designated Backscatter lidar and multi- Aerosol properties, aerosol vertical profiles, and cloud properties to channel/multi-angle/polarization Aerosols X s on climate and air understand their effect imaging radiometer flown together on the same platform quality Radar(s), with multi-frequency Coupled cloud-precipi tation state and passive microwave and sub-mm for monitoring global dynamics Clouds, hydrological cycle and understanding radiometer X Convection, and contributing processes including cloud Precipitation feedback Spacecraft ranging measurement measured by Large-scale Earth dynamics the changing mass distribution within and of gravity anomaly Mass Change X between the Earth’s atmosphere, oceans, ground water, and ice sheets Hyperspectral imagery in the Earth surface geology and biology, e, snow reflectivity, visible and shortwave infrared, ground/water temperatur Surface Biology X active geologic processes, vegetation traits multi- or hyperspectral imagery in and Geology the thermal IR and algal biomass Earth surface dynamics from earthquakes Interferometric Synthetic Aperture Surface and landslides to ice sheets and permafrost Radar (InSAR) with ionospheric Deformation and X correction Change Multispectral short wave IR and and methane fluxes and trends, CO 2 global and regional with quantification of thermal IR sounders; or lidar** Greenhouse X point sources and identification of sources Gases and sinks including Global ice characterization Lidar** elevation change of land ice to assess sea level contributions and freeboard height of X Ice Elevation sea ice to assess sea ice/ocean/atmosphere interaction Doppler scatterometer currents and Coincident high-accuracy Ocean Surface vector winds to assess air-sea momentum Winds and X exchange and to infer upwelling, upper Currents ocean mixing, and sea-ice drift UV/Vis/IR microwave limb/nadir Vertical profiles of ozone and trace gases , methane, sounding and UV/Vis/IR (including water vapor, CO, NO Ozone and Trace 2 X O) globally and with high spatial and N solar/stellar occultation Gases 2 resolution Radar (Ka/Ku band) altimeter; or Snow Depth and Snow depth and snow water equivalent including high spatial resolution in lidar** X Snow Water mountain areas Equivalent Lidar** 3D structure of terrestrial ecosystem including forest canopy and above ground Terrestrial biomass and changes in above ground Ecosystem X carbon stock from processes such as Structure deforestation and forest degradation Active sensing (lidar, radar, 3D winds in troposphere/PBL for Atmospheric X X transport of pollutants/carbon/aerosol and scatterometer); or passive imagery Winds UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION S-8 Copyright National Academy of Sciences. All rights reserved.

32 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space or radiometry- b ased atmos. motion water vapor, wind energy, cloud dynamics and convection, and large-scale circulation vectors (AMVs) tracking; or lidar** Microwave, hyperspectral IR Diurnal 3D PBL thermodynamic properties and 2D PBL structure to sounder(s) (e.g., in geo or small sat constellation), GPS radio understand the impact of PBL processes on weather and AQ through high vertical and occultation for diurnal PBL Planetary X temporal profiling of PBL temperature, temperature and humidity and y er Boundar La y moisture and heights heights; water vapor profiling DIAL lidar; and lidar** for PBL height Radar; or lidar** High-resolution global topography Surface including bare surface land topography ice X Topography and topography, vegetation structure, and Vegetation shallow water bathymetry ** Could potentially be addressed by a multi-function lidar designed to address two or more of the Targeted Observables Other ESAS 2017 Targeted Observables, not Allocated to a Flight Program Element Aquatic Biogeochemistry Radiance Intercalibration Magnetic Field Changes Sea Surface Salinity Ocean Ecosystem Structure Soil Moisture NOTE: Observations (Targeted Observab les) identified by the committee as needed in the coming decade, beyond what is in the Program of Re cord, allocated as noted in the last three columns (and color-coded) to three new NASA ; as defined in the accompanying text). flight program elements ( Incubation Designated , Earth System Explorer , Within categories, the targeted observables are listed alpha betically. Targeted Observable included in the original priority consideration but not allocated to a program element are listed at the bottom of the table. Recommendation 3.2: NASA should implement a set of space-based observation capabilities based on this report’s proposed program (which was designed to be affordable, comprehensive, robust, and balanced) by implem enting its portion of the program of record and adding observations described in Tabl e S.2, “Observing System Priorities.” The implemented program should be guided by the budgetary considerations and decision rules contained in this report and accomplished through five distinct program elements: 1. Program of record. The series of existing or previously planned observations, which must be completed as planned. Execution of the ESAS 2017 recommendation requires that the total cost to NASA of the program of record flight missions from fiscal year (FY) 2018- FY27 be capped at $3.6 billion. 2. Designated. A program element for ESAS-designated cost-capped medium- and large-size missions to address observables essential to the ov erall program, directed or competed at the discretion of NASA. 3. Earth System Explorer. A new program element involving competitive opportunities for cost-capped medium-size instruments and missions serving specified ESAS-priority observations. 4. Incubation. A new program element, focused on investment for priority observation capabilities needing advancement prior to cost -effective implementation, including an innovation fund to respond to emerging needs. 5. Earth Venture . Earth Venture program element, as recommended in ESAS 2007, with the addition of a new Venture-continuity component to provide opportunity for low-cost sustained observations. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION S-9 Copyright National Academy of Sciences. All rights reserved.

33 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space The committee is confident, based on analyses of technical readiness and cost performed during the study, that the recommended observations have f easible implementations that can be accomplished on schedule and within the stated cost caps. The pr oposed program was designed to both fit within anticipated budgets (assumed for the purposes of this re port to grow only with inflation) and to ensure balance in the mission portfolio among program elem ents (Figure S.2). As appropriate, candidate instruments and missions were formally subjected to a Cost and Technical Evaluation (CATE) to assess budget needs. The committee considered management of development cost to be of critical importance to effective implementation of this program, in order to avoid impacting other programs and altering the desired programmatic balance. Should budgets be mo re or less than anticipated, the report includes decision rules for altering plans in a manner that seeks to ensure the overall program integrity. ESAS 2017 real-year dollar estimated costs (colored wedges), broken down by NASA FIGURE S.2 flight program element proposed in this report, as compared to the anticipated flight budget (black line), showing how the ESAS 2017 costs fit within the available $3.4B budget through 2027. The total NASA budget for flight elements assumes growth at the rate of inflation for years beyond the current budget projection. Only the investments re lated to ESAS 2017 recommendations are shown. The gap between the estimated costs and the av ailable budget represent funds that have been committed to other non-ESAS mission-related activities. ENABLING THE PROGRAM Finally, none of this happens without robust supporting programs at NASA, NOAA, and USGS that provide the enabling resources for developing the recommended space-based observing systems and evaluating the data they produce. In particular, these supporting programs are central to transforming scientific advances into applications and societal benefits. Th e committee has proposed a variety of programmatic actions intended to improve the ability of each agency to deliver on their space- based observation programs. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION S-10 Copyright National Academy of Sciences. All rights reserved.

34 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space associated with ensuring balanced and Key among these are findings and recommendations robust programmatic structures (Finding 4D and 4E), and for leveraging partnering opportunities (such as /Sentinel program noted in Recommendation 4.5) that enhance the European Union’s Copernicus operational efficiencies and ensure the agencies can ac complish the most possible within their available resources (Finding 4J, Recommenda tions 4.5, 4.11, and 4.12). A robust and resilient ESD program has the following attributes : Finding 4D:  A healthy cadence of small/medium missions to provide the community with regular ge advances in technologies and capabilities, and to rapidly flight opportunities, to levera respond to emerging science needs.  A small number of large cost-constrained mi ssions, whose implementation does not draw excessive resources from smaller a nd more frequent opportunities.  agencies. Strong partnerships with U.S. government and non-U.S. space  Complementary programs for airborne, in-situ, and other supporting observations.  Periodic assessment of the return on investment provided by each program element.  A robust mechanism for trading the need for continuity of existing measurement against new measurements. Finding 4E: Maximizing the success of NASA’s Earth science program requires balanced investments across its program elements, each critically important to the overall program. The flight program provides observations that the research and analysis program draws on to applied sciences program transforms the science into real- perform scientific exploration, the world benefits, and the technology program accelerates the inclusion of technology advances ross these four program elements is largely in flight programs. The current balance ac Earth science program, and can be effectively appropriate, enabling a robust and resilient maintained using decision rules such as recomme nded in this Report. Some adjustment of balance within each program element is warranted, as recommended in this report. Extension of Landsat capability thr ough synergy with other space-based Finding 4J: observations opens new opportunities for Landsat data usage, as has been proven with the libration and data sharing for Sentinel 2. European Space Agency (ESA) through cross-ca These successes serve as a model for future part nerships and further synergies with other space-based observations. Recommendation 4.5: Because expanded and extended interna tional partnerships can benefit the nation:  NASA should consider enhancing existing partnerships and seek ing new partnerships when implementing the observation priorities of this Decadal Survey.  NOAA should strengthen and expand its already strong international partnerships, by a) coordinating with partners to further ensu re complementary capabilities and operational backup while minimizing unneeded redundancy; an d b) extending partnerships to the more complete observing system life-cycle that includes scientific and technological development of future capabilities.  USGS should extend the impact of the Sustainable Land Imaging (SLI) program through further partnerships such as that . with the European Sentinel program Recommendation 4.11: NOAA should establish itself among the leading government agencies that exploit potential value of comme rcial data sources, assessing both their benefits and risks in its observational data portfolio. It should innovate new government-commercial partnerships as needed to accomplish that goal, pioneer new business models when required, UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION S-11 Copyright National Academy of Sciences. All rights reserved.

35 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space s such as international partner use rights. and seek acceptable solutions to present barrier NOAA’s commercial data partnerships should en sure access to needed information on data iate, and be robust against loss of any characteristics and quality as necessary and appropr single source/provider if the data are essential to NOAA core functions. Recommendation 4.12: NOAA should establish, with NASA, a flexible framework for joint activities that advance the capability and cost -effectiveness of NOAA’s observation capabilities. This framework should enable implementation of specific project collaborations, each of which may have its own unique requirement s, and should ensure: a) clear roles, b) mutual interests, c) life-cycle interaction, d) multi-disciplinary methodologies, e) multi-element expertise, f) appropriate budget mechanisms. ANTICIPATED PROGRESS WITHIN THE DECADE In this report, the committee identifies the science and applications, observations, and vision of understanding deeply the nature of our programmatic support needed to bring to fruition its changing planet. As described throughout this Summa ry and in the body of the report, the committee complished by the end of the decade: expects the following will have been ac Programmatic implementation within the ag encies will be made more efficient by:  Increasing Program Cost-effectiveness. Promote expanded competition with medium-size missions to take better advantage of in novation and leveraged partnerships.  Institutionalizing Sustained Science Continuity . Establish methods to prioritize and facilitate the continuation of observations deemed critical to monitoring societally-important aspects of the planet, after initial scientific exploration has been accomplished. Establish more effective means for NASA-  Enabling Untapped NASA-NOAA Synergies. NOAA partnership to jointly development the next generation of weather instruments, accelerating NOAA’s integration of advanced operational capabilities. Improved observations will enable exc iting new science and applications by: Initiating or Deploying More Than Eight Ne w Priority Observations of our Planet.  Developing or launching missions and instrument s to address new and/or extended priority observation areas that serve science and applica tions. Five are prescribed in the committee’s recommended program for NASA and three are to be chosen from among seven candidate areas prioritized by the committee to form th e basis of a new class of NASA competed medium-sized missions. These new observation priorities will be complemented by an additional two new small missions and six new instruments to be selected through NASA’s existing Earth Venture program element, and tw o opportunities for sustained observations to be selected through the new Venture-Continuity strand of this program. The existing and planned Program of Record will also be implemented as expected.  Achieving Breakthroughs on Key Scientific Questions. Advance knowledge throughout portions of the survey’s 35 key science questi ons (Table S.1) that address critical unknowns about the Earth system and promise new societal applications and benefits. Businesses and individuals will receive enhanced value from scientific advances and improved Earth information, such as:  Increased Benefits to Opera tional System End-Users. Enhanced processes will allow NOAA and USGS to have greater impact on the user communities they serve, and will provide these UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION S-12 Copyright National Academy of Sciences. All rights reserved.

36 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space agencies with improved tools to leverage low- cost commercial and international space-based observations that further their mission.  Accelerated Public Benefits of Science. Improved capacity for transitioning science to applications will make it possible to more qui ckly and effectively achieve the societal benefits of scientific exploration, and to gene rate applications more responsive to evolving societal needs. through new observations and Provide Enabling Data for Innovative Commercial Uses,  related data. Building on the success and discoveries of the last several decades, the report’s balanced program provides a pathway to realizing tremendous scientif ic and societal benefits from space-based Earth observations. It ensures the United States will continue to be a visionary leader and partner in Earth observation over the coming decade, inspiring the ne xt generation of Earth science and applications innovation and the people who make that possible. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION S-13 Copyright National Academy of Sciences. All rights reserved.

37 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space 1 A Vision for the Decade Ongoing understanding and prediction of the Eart h’s changing environment, using space-based observations, provides essential knowledge that help s make society safe, secure, and prosperous. These benefits are in turn made possible by the invest ments we choose to make in observing and exploring our planet and in transforming new discoveries into useful knowledge. From the time of the earliest humans, knowledge about Earth has been fundamental to our fate and prospects. Our ever-growing understanding of Earth’s dynamic processes and long-term changes, along with their causes, has helped en able society’s advance. Yet today Earth is changing in ways that are very different from the past, largely as a consequen ce of our own influences. From growing demand for limited resources, to air quality degr adation, to climate change, human impacts that were once local or regional are now increasingly global. Similarly, where human impacts were once largely transient, today they may last millennia. As a result, accumulated knowl edge about Earth’s past is no longer a sufficient guide to the future. Increasingly, we must observe and understand the ways th at nature’s patterns and processes are being altered—alterations that w ill likely pose significant challenges and present new prospects for both society and ecosystems, requiring environmental awareness to successfully manage. Examples are increasingly abundant. Evolving rainfall patterns may open new agricultural opportunities, but will likely bring drought to other cu rrently fertile regions. W ithout an understanding of agriculture faces economic risk. An ice-free summer where, when, and how these changes will occur, our weather, and ecosystem patterns, but will also Arctic will introduce major perturbations to climate, provide access to new resources and reduced shipping times. Managing the risks and understanding the vations and knowledge - not just of the changes opportunities inherent to a changing world requires obser that are occurring, but of the reasons fo r them and the associated implications. This tension—between society’s deep dependen ce on knowledge about our planet in order to thrive, and the challenges of acquiring and updating this knowledge as our planet changes—reflects an emerging aspect of civilization’s progress. It is the theme embodied in this report’s title Thriving on Our Changing Planet . The word thriving was chosen carefully for its breadth: it encompasses economic success, intellectual progress, societal prosperity, pe rsonal well-being, scientific exploration, and much more. The committee’s proposed program of science and applications priorities, and the observations needed to pursue them, addresses the scientific and societal challenges inherent in this tension. ALL IN A DECADE The successes of modern civilization have been achieved in no small part through advanced understanding of our planet’s behavi or and its fundamental resources. Characterizing and explaining the ways in which Earth changes over time, and identifyi ng the complex natural and human mechanisms by which that change occurs, have been critical el ements of our nation’s scientific progress. Earth continually amazes us, as we make new discoveries th at reveal its beauty, complexity, and wonder. PREPUBLICATION REPORT–SUBJECT TO FURTHER EDITORIAL CORRECTION 1-1 Copyright National Academy of Sciences. All rights reserved.

38 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space 1 owing dependence on Earth information As we move forward, society’s gr (illustrated in Figure 1.1)—for our daily lives, our businesses, and our government policies—requires ongoing investments in the observation, understanding, and prediction of Earth’s environment, including critical space-based data. Over the past few decades, the United States has b een a clear leader of the global effort to acquire sophisticated observational data from satellites. Such investments in knowledge and its applications support our efforts to continue thriving on our complex, ever-changing planet. Even a mere decade reveals the pace of advan ce and the many successes we have experienced. Over , new types of Earth information have empowered us all: the last decade  Greater access to this information has helped us as individuals by placing at our Individuals. fingertips a wide variety of vital information about th e world around us, helping each of us make important decisions. Examples range from minute- by-minute weather information to satellite in our home towns and “visit” even the most images that allow us all to explore and navigate remote places on Earth. Businesses. Scientific discoveries and the resu lting applications have helped us  advance business interests , such as making our agriculture more productive, our energy use more efficient, and our transportation more reliable. Many companies ha ve leveraged technology originally developed for Earth observations to provide valuable servi ces, ranging from consumer internet mapping to weather-based shipping optimization and much more. Society. Being able to observe the Earth in new ways has helped us prosper as a society . Our  revolutionary ability to view the world as a whol e from space allows us to watch the natural course of rivers and forests change, to observe ch anges in our climate, to discern our role within those and other changes, to understand the risks a nd benefits of our acti ons and inactions with esulting knowledge. This expanded perspective has regard to our planet, and to apply the r positioned us to benefit from th e economic opportunities it creates, increased our resilience to the tions everywhere with the wonder of Earth’s environment’s risks, and inspired citizens and na scientific challenges. nces from prior decades, confirms the special Progress during the last decade, building on adva the entire Earth in detail and to reveal new aspects ability of Earth satellites to comprehensively observe 2 of our planet’s complex behavior. Over time, we have augmented what was once a sparse surface-based observing network with a powerful space-based infr astructure for observation and prediction on global scales, making it possible to monitor aspects of the Ea rth system not previously accessible using surface- 3 based observations alone. The global view from satellite observations remains unmatched in its ability to 1 A growing body of literature characterizing how society us es Earth information and quantifying its benefits to individuals, businesses, and governments (e.g., Boulding, 1966; Daly and Townsend, 1996; Williamson et al., 2002; berth et al., 2016; Macauley, 2006; Sagoff, 2007; Sa goff, 2008; Lazo, 2011; Tren Hsiang et al., 2017; NWS, 2017). Nevertheless, there is no definitive study of the value of U.S. Earth observation to the nation. In Europe, the economic benefit-cost ratio of the toring for Environmental Security European Space Agency’s Global Moni ). Similarly, (GMES) program, now known as Copernicus, has been estimated to be 10:1 (Booz and Co., 2011 of space-based Earth observation to be 0.3% of their GDP Australia assessed the direct and indirect contributions (ACIL Tasman Pty Ltd, 2010). (See also Figure 1.1 and Box 4.1.) 2 See Earth Observations from Space: The Fi rst 50 Years of Scientific Achievements (NRC, 2008), which explores the history and value of Earth observation satellites in depth. 3 The committee fully recognizes that acc omplishing science today and achieving societal benefits from science requires treating information as an end-to-end process, involving observations, analysis, modeling, archive, automated analytics, applications, data communication, and far more. Our strategic guidance in Chapter 2 recognizes this, and the topic is addressed at a simple leve l in Chapter 4. Nevertheless, this report’s focus is the space-based observing system, so the important topic of an end-to-e nd information infrastructure is not comprehensively addressed. PREPUBLICATION REPORT–SUBJECT TO FURTHER EDITORIAL CORRECTION 1-2 Copyright National Academy of Sciences. All rights reserved.

39 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space PREPUBLICATION REPORT–SUBJECT TO FURTHER EDITORIAL CORRECTION 1-3 Copyright National Academy of Sciences. All rights reserved.

40 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space FIGURE 1.1 We all depend extensively on Earth information. Sometimes to the minute, our daily lives are guided and enhanced (often in ways we do no t readily recognize) by the many pers onal, business, and government decision that rely on knowledge about our planet. Science pr ovides the foundation that makes it all possible. J.L. Demuth, 300 Billion Served— —J.K. Lazo, R.E. Morss, and SOURCE: Data available as follows: Helping Plan Our Day Sources, perceptions, uses, and values of weather forecasts, Meteorological Society, 2009; comScore, Bulletin of the American Inc., “The U.S. Mobile App Report,” 2014. Protecting Our Health —World Health Organization, “Burden of Disease from the Joint Effects of Household and Ambient Air Pollution for 2012,” http://www.who.int/airp ollution/data/AP_jointeffect_ BoD_results_Nov2016.pdf, 2016; World Health Or ganization, “Malar ia Fact Sheet,” PREPUBLICATION REPORT–SUBJECT TO FURTHER EDITORIAL CORRECTION 1-4 Copyright National Academy of Sciences. All rights reserved.

41 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space ediacentre/factshee Keeping Us Secure — Defense One , “Cutting NASA Earth http://www.who.int/m ts/fs094/en/, 2017. Observations Would Be a Costly Mist ake,” http://www.defenseone.com/technology /2016/12/cutting-nasa-earth-observations- Mitigating Natural Disasters — GAO Highlights, “Climate Change: Information on would-be-costly-mistake/133586/, 2016. rts to Reduce Fiscal Exposur Potential Economic Effects Could Help Guide Federal Effo gao.gov/assets/ e,” https://www. Ensuring Resource Availability —McKinsey Global Institute, “How te chnology is reshaping supply and 690/687465.pdf, 2017. Nationals Water, “Coping w ith Water Scarcity: Challenge of the Twenty-First demand for natural resources,” 2017; United Century,” http://www.fao. org/3/a-aq444e.pdf, 2007. resolve the dynamics and variability of Earth process es. Using both space and in-situ observations, we intricately connected global system, increasingly understand the extent to which Earth is an within which interactions between the atmosphere, land, ice, and oceans affect us on time scales of minutes to decades. y to understanding how the Earth system functions Characterizing these Earth system interactions is ke nge in the future, and how humans influence such today, how it supports life, how conditions might cha change. The challenge is to furthe r advance this knowledge, and to pr ogressively apply it in ways that improve our lives and help us plan for the future. the next decade ? While significant By building from this knowledge base, what can we expect in progress has been made this decade and previously, it is surprising how much we still do not know about the Earth system and the human interaction with it, especially in the least accessible regions (Box 1.1). Today, Earth’s ongoing change makes the job of understa nding and predicting our planet even harder than in the past. Through both natura l variability and human influences, Earth and its environment are evolving around us—sometimes in ways we can readily predict and other times in ways we have yet to explain. To sustain prospects for adapting in the future, society needs a more comprehensive understanding of how and why our environment is cha nging and what the associated implications will be. cations, observations, a nd programmatic support This report identifies the science and appli needed to bring to fruition this vision of more d eeply understanding our changing planet. The committee expects the following will have been ac complished by the end of the decade: Programmatic implementation within the ag encies will be made more efficient by: Increasing Program Cost-effectiveness. Promote expanded competition with medium-size  novation and leveraged partnerships. missions to take better advantage of in  . Establish methods to prioritize and facilitate Institutionalizing Sustained Science Continuity to monitoring societally-important aspects of the continuation of observations deemed critical the planet, after initial scientific exploration has been accomplished.  Enabling Untapped NASA-NOAA Synergies. Establish more effective means for NASA- NOAA partnership to jointly development the next generation of weather instruments, accelerating NOAA’s integration of advanced operational capabilities. Improved observations will enable exc iting new science and applications by: w Priority Observations of our Planet.  Initiating or Deploying More Than Eight Ne s to address new and/or extended priority Developing or launching missions and instrument tions. Five are prescribed in the committee’s observation areas that serve science and applica recommended program for NASA and three are to be chosen from among seven candidate areas prioritized by the committee to form th e basis of a new class of NASA competed medium-sized missions. These new observation priorities will be complemented by an additional two new small missions and six new instruments to be selected through NASA’s existing Earth Venture program element, and tw o opportunities for sustained observations to be selected through the new Venture-Continuity strand of this program. The existing and planned Program of Record will also be implemented as expected.  Achieving Breakthroughs on Key Scientific Questions. Advance knowledge throughout portions of the survey’s 35 key science questi ons (Table S.1) that address critical unknowns about the Earth system and promise new societal applications and benefits. PREPUBLICATION REPORT–SUBJECT TO FURTHER EDITORIAL CORRECTION 1-5 Copyright National Academy of Sciences. All rights reserved.

42 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Businesses and individuals will receive enhanced value from scientific advances and improved Earth information, such as:  Increased Benefits to Opera tional System End-Users. Enhanced processes will allow NOAA and USGS to have greater impact on the user communities they serve, and will provide these agencies with improved tools to leverage low- cost commercial and international space-based observations that further their mission. Improved capacity for transitioning science to  Accelerated Public Benefits of Science. ckly and effectively achieve the societal applications will make it possible to more qui benefits of scientific exploration, and to gene rate applications more responsive to evolving societal needs.  Provide Enabling Data for Innovative Commercial Uses, through new observations and related data. PREPUBLICATION REPORT–SUBJECT TO FURTHER EDITORIAL CORRECTION 1-6 Copyright National Academy of Sciences. All rights reserved.

43 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space ********************************************************************************* BOX 1.1 A Changing Earth Creates Opportunity and Risk for Us All In our changing world, Earth observations from space are important for supporting humanity’s ability to thrive. Such observations enable scientif ic breakthroughs and have direct impacts on our economy, national security, public safety, and quality of life. sea ice provides a clear example. It presents The widely-reported multi-decadal decline in Arctic opportunity, complete with underlying international, us with a challenging confluence of risk and societal commercial, and military implications over the next d ecade. The rates of changes are extraordinarily high. Space-based observations have recorded a decline in th e summer’s ice extent at a rate of 13% per decade, along with a reduction in multi-year ice to one-quarter of its historic amount. The Northwest Passage and trans-Arctic shipping routes may open soon to regular transit. These dramatic changes are causing energy companies to examin e how access to an estimated 15% of the world’s remaining petroleum deposits coul d reduce oil and gasoline prices. The transportation industry is working to understand how a 25% reduction in ocean shipping time between Europe and Asia may improve global trade. Governments are devel oping policies aimed at pursuing these opportunities and addressing the risks. The Arctic, seemingly so far away, is changing in ways that have direct and growing impacts on our daily lives. We have seen the beginnings of all this already. On the Northern Sea Route (Figure 1.2) from Western Europe to Eastern Asia traffic increased by nearly a factor of twenty from 2010 to 2013. Russia planted a flag on the North Pole seabed in 2015 as a territorial claim. Oil e xploration is expanding. One consequence is that many nations are rapi dly building their Arctic military capacity. The US was developed with the changing Arctic in mind, to guide future Navy Arctic Roadmap 2014-2030 strategic operations. The roadmap calls for additio nal research on rising seas and improved ability to predict sea ice thickness, as well as assessments of th e surveillance and facility needs in this critical region of our world. Future observations and the asso ciated analyses will inform the federal investment decisions necessary to secure U.S. interests in the Arctic. The practical unknowns are extensive, motiva ting the need for additional monitoring and predictive capability. How will major cities or agricultu ral regions be impacted as the open Arctic Ocean alters weather patterns reaching lower latitudes? Will nations successfully develop agreements and treaties on new uses of the Arctic, and can we be confident they are complying? What are the consequences for native cultures in this region? Will delicate ecosystems adapt and thrive or struggle and decline, and what should we do to protect them? What information will be needed to address these kinds of questions and determine whether we successfully cope with the changes, manage them, or simply exploit them? These same unknowns also create new and compe lling scientific questions. How soon might the Arctic Ocean become completely ice free in su mmers? How will changes in the critical Arctic environment alter global climate? What are the mechanisms driving these changes and responses? The Arctic has never been static, but recent ch anges have been exceptionally dramatic. The needed scientific exploration has only begun, a nd the practical capabilities necessary to successfully manage and adapt to these changes require additi onal development. With the scientific, economic, political, and strategic landscape evolving so rapidly, the need for frequent ly-updated, large-scale information about the ice, ocean, land, and atmosphe re in this remote region has never been greater. Space-based Earth observations, which provide that critical information, are essential to making well- informed decisions about our nation’s ac tions and investments in the future. PREPUBLICATION REPORT–SUBJECT TO FURTHER EDITORIAL CORRECTION 1-7 Copyright National Academy of Sciences. All rights reserved.

44 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space FIGURE 1.2 2012 late summer Arctic sea ice area (in white), overlaid with current and potential Arctic shipping routes (red lines) resulting from sea-ice loss . These routes substantially decrease shipping times between Europe and Asia and other parts of the world. The orange line shows the average extent of the annual Arctic sea-ice minimum for 1979-2010, and the inset shows the monthly-average values of sea-ice extent for each September (the month at which th e ice reaches its minimum extent) during the full satellite record, 1979-2017. In 2012, th e Arctic sea-ice cover shrunk to its lowest level ever observed in the satellite record. ********************************************************************************* END OF BOX THE TRANSFORMATIVE IMPACT OF SPACE-BASED OBSERVATIONS Earth is a dynamic planet on which the interconn ected atmosphere, ocean, land, and ice interact across a range of spatial and temporal scales, irr espective of geographic, political, or disciplinary e system level, with the aim of understanding the boundaries. Today’s leading science often occurs at th linkages between these elements, th e processes that connect them, and how variability occurs among as sea level rise (Figure 1.3) illustrates the them. Even a conceptually simple phenomenon such complexity of Earth system science that must be consid ered to explain it, to predict its behavior, and to address the diverse societal impacts. Multi-decadal space-based observations are particul arly important to this understanding. They allow us to better investigate Ea rth’s variability across many scales of time and space, and to develop insights needed to understand the fundamental Earth Sy stem processes that are relevant to our lives. Since Earth is our home, our survival and quality of life depend on how well we understand its behavior. A commitment to monitoring, understanding, and pred icting complex and dynamical Earth systems is a scientific and societal imperative. PREPUBLICATION REPORT–SUBJECT TO FURTHER EDITORIAL CORRECTION 1-8 Copyright National Academy of Sciences. All rights reserved.

45 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space FIGURE 1.3 The complex interacting components of the Earth system that contribute to sea level rise and its consequences. PREPUBLICATION REPORT–SUBJECT TO FURTHER EDITORIAL CORRECTION 1-9 Copyright National Academy of Sciences. All rights reserved.

46 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space t understanding and reliably predicting the Earth The science alone is inspiring and compelling, bu vital economic, societal, and national security need as well. This need for accurate predictions system is a applies across many U.S. industries (ranging from ener gy resources to aircraft operations), for which significant functions and products de pend on effective use of Earth information. In agriculture, for example, revenue and profits de pend on efficient crop management and associated water usage that follows from an understanding of daily and seasonal weather and climate conditions. Weather variability alone—only one driver of the need for Earth informa tion—is known to influence as much as 13% of the year-to-year variability of U.S. state economies, equa l to 3.4% of U.S. GDP when aggregated over the nation (Figure 1.4). Space-based observations are a criti cal source of the needed Earth information used by companies and other providers of applications, w ith significant return on investment to the economy. FIGURE 1.4 The sensitivity (annual variation) of each state’ s Gross State Product due to routine weather 2011). The impact can be as large as 13% in some variability, such as drought and flood (Lazo, et al., states, with half of all states >5%. 4 Space-based Earth observations are also vital for national security. As an example, understanding atmospheric and oceanic processes (s uch as sea level rise and the impacts of ocean warming on ocean circulation associated with climate ch ange) and their implications is critical for naval operations. Operations of all armed services depend on environmental information, such as accurate weather forecasts, characteristics and changes of terrestrial landscapes, atmospheric conditions and processes, coastal information, an d more. Satellite observations play a crucial role in addressing these needs. On a broader level, understanding the role of climate and other environmental changes is important for anticipating future sources of geopolitical instability. 4 There is increasing academic, business, and governme nt recognition of the national security impact of Earth information, and of climate change in particular (e.g., Barnett, 2003; Smith, 2007; Nordås and Gleditsch, 2007). That recognition is less established at the public level. PREPUBLICATION REPORT–SUBJECT TO FURTHER EDITORIAL CORRECTION 1-10 Copyright National Academy of Sciences. All rights reserved.

47 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Finding 1A: Space-based Earth observations made over the last decade and longer provide a global perspective of Earth that has  Transformed our scientific understanding of the planet, revealing it to be an integrated system of dynamic interactions between th e atmosphere, ocean, land, ice and human society across a range of spatial and tempor al scales irrespective of geographic, political, or disciplinary boundaries. that provide tremendous value to individuals,  Enabled societal applications businesses, the nation, and the world. Such applications are gr owing in breadth and rmation infrastructure element for society as they depth, becoming an essential info are integrated into people’s daily lives. essential information infrastructure element for society as they are integrated into people’s daily lives. BUILDING ON PROGRESS U.S. investments in Earth observations over the last decade have led to important scientific advancement, and generated cons iderable economic value (see Chapter 2). This progress has occurred across a wide range of Earth science disciplines, a ddressing broad societal needs: assessing risks from sea level rise, understanding the genesis and evolution of severe storms and tornados, measuring the health and productivity of our lands and oceans all over th e world, managing air pollution risks, and improving e builds on these successes, weather forecasts. The decadal survey committee’s vision for the next decad 5 recognizing that society’s need for improved sci ence and Earth information is growing rapidly . This vision leads to the committee’s recognition of a new Earth science paradigm for the coming decade, building from two important prior themes . In the 1980’s and 90’s, Earth scientists and applications specialists began form ally viewing Earth as a system, moving beyond study of its individual us recognize critical system-scale processes, such land, ocean, and atmosphere components. This helped as for the El Niño/La Niña oscillation that has such enormous economic and security impacts throughout the world, and begin forecasting them. In the 2000’s, we recognized that explicitly integrating pursuit of the societal benefits of Earth research needed to be cen tral to all of our thinking. The natural extension of this thematic progress leads to the following paradigm. Earth Science and Applications Paradigm for the Coming Decade Earth science and derived Earth information have b ecome an integral component of our daily lives, our business successes, and society’s capacity to thrive. Extending this societal progress requires icting the many ways our that we focus on understanding and reliably pred planet is changing. . Decisions we make this decade regarding The coming decade is important for many reasons investments in needed capabilities will determine our capacity during the next decade and beyond to predict Earth’s future changes, including the role of human actions, and to influence the extent to which those changes will impact society. As we recognize the interdependence of a nd interconnections between 5 The proliferating use of internet mapping over the last decade is perhaps the best-known example, though integrates space-based, aeria l, and ground-based Earth merely indicative of a broad-based trend. Internet mapping observation data (obtained and used with rapidly advancing fidelity), provides the foundation for value-added services ranging from shipping logistics to commodities speculation, and is an essential information source for a growing set of applications built for financial services, energy, transportation, agricu lture, consumers, and many other sectors. PREPUBLICATION REPORT–SUBJECT TO FURTHER EDITORIAL CORRECTION 1-11 Copyright National Academy of Sciences. All rights reserved.

48 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space , there is an increasing need for reliable, science- human activities and our land, oceans, and atmosphere sions related to fisheries management, river and based guidance to support policy and investment deci water basin management, coastal construction, air quality, floods, hurricanes, droughts, changes in ecosystems, wildfires, sea level rise, navigability of the Arctic, and adaption to climate change—to name just a few. AN AMBITIOUS COMMUNITY CHALLENGE It is essential that advances in our understanding of the Earth system support the nation’s growing industrial, agricultural, and environmental needs. Sate llite observations will play a crucial role in ensuring that they do. Yet, over the last decade and more, investments in Earth observation capabilities have failed to keep pace with these needs. This is particularly evident in NASA’s Earth science program, which (as shown by the budget in Figure 1.5) has actually seen a decline in its budget from the levels that led in the 1990s to the development of NASA’s Earth Observing System (EOS) and the Mission to Planet Earth (MTPE). 2016+ ($FY2018), showing both mission and non- FIGURE 1.5 The NASA Earth Science budget 1996- mission contributions. For the period following know n budget requests, a simple inflation-adjusted increase is used. The committee recognizes that resource constraints ar e likely to remain a practical concern during 6 the next decade, and that new resources must be applied wisely when available. The importance of an 6 Various proposals for reducing agency budgets and eliminating particular Program of Record missions have been proposed over the year prior to publication of this report. While aware of the proposals and their undesirable impacts, it was not the committee’s role to speculate on potential outcomes of in-process budget proposals. Instead, the committee focused on ensuring ap propriate justification of both the Program of Record and new observing system capabilities, and on clear rules for adjusting the pr ogram when available resour ces either exceed or don’t meet the committee’s nominal budget growth expectation. To the extent that future budget issues could lead to a situation similar to that faced in the first decadal survey (NRC, 2007), which described the observing system as “at risk of collapse,” it is critical to regularly reinforce th e strategic importance of Earth observation to the nation’s governmental organizations, businesses, and individuals. PREPUBLICATION REPORT–SUBJECT TO FURTHER EDITORIAL CORRECTION 1-12 Copyright National Academy of Sciences. All rights reserved.

49 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space rise requires that our entire community of scientists effective Earth system science and applications enterp ity challenge within the next decade: and practitioners rise to the following commun Decadal Community Challenge Pursue increasingly ambitious objectives and innovat ive solutions that enhance and accelerate the science/applications value of space-based Earth ob servation and analysis to the nation and to the world in a way that delivers great value, even when res ources are constrained, and ensures that further investment will pay substantial dividends. Succeeding requires a deep commitment on the part of scientists, our government, and citizens. It will require innovation and discipline, inspiration an d dedication. But substantive progress can and must be made in the coming decade. It is a worthy and ambitious goal that will pay off many times over in civilization’s more comprehensive understanding of our changing environment, more efficient stewardship of Earth’s resources, and more effective management of risks against environmental stresses. Ultimately, a long-term goal of Earth System science research and its applications is a comprehensive capacity to understand, monitor, predic t, and steward important aspects of our Earth and its future, across all important scales of space (local to global) and time (minutes to decades), and in all relevant domains . The complexity and growing number of societal needs is in creasingly evident; the extent of the potential societal benefits presents a st rong motivation for this goal. It is a goal that should push us all to reach high, as the opportunities enab led by success—and the consequences of failure, to ourselves and to this planet—are both tremendous. THE 2017 DECADAL SURVEY , this report proposes an achievable plan of In keeping with the Decadal Community Challenge space-based observations to monitor and understand our planet over the next decade, without sacrificing pursuit of ambitious goals. Implementing this prog ram will contribute to safeguarding and improving the quality of life for all citizens. All three of the report’s sponsoring agencies pl ay essential roles. Sustained NASA, NOAA, and USGS systems are needed to ensure we have long-te rm, uninterrupted observations of the Earth system that supports many aspects of our lives. NASA missi ons already scheduled to be launched, and new science/applications proposed here, have been selected to provide a portfolio of data that will strategically build on existing capabilities, allowing us to substantia lly advance our ability to understand, explain, and manage observed changes and thus to improve Earth prediction. The recommended program will complement existing US and international programs to provide critical new and follow-on observations of the most fundamental Earth system parameters. Implem entation of this program will enable not just more accurate predictions at short time-scales (e.g., <10 day weather forecasting), but more robust projections at decadal and longer time-scales as well (e.g., sea-le vel rise, drought trends, and climate shifts) as the changing climate and other influences sh ape the world in which we will live. Building on the success and discoveries of the last several decades, the report’s balanced program provides a pathway to realizing tremendous scientif ic and societal benefits from space-based Earth observations. It ensures the United States will continue to be a visionary leader and partner in Earth observation over the coming decade, inspiring the ne xt generation of Earth science and applications innovation and the people who make it possible. PREPUBLICATION REPORT–SUBJECT TO FURTHER EDITORIAL CORRECTION 1-13 Copyright National Academy of Sciences. All rights reserved.

50 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space REFERENCES ACIL Tasman. 2010. The economic value of Earth ob servation from space: a review of the value to Australia of Earth observation from space. ACIL Tasman Pty ltd, Prepared for the Cooperative Research Centre for Spatial Informa tion (CRC-SI) and Geoscience Australia. Barnett, J. 2003. Security and climate change . Global Environmental Change XIII(1): 7-17. Behrenfeld, M.J., Y.Hu, R.T. O’Malley, E.S. Boss, C. A. Hostetler, D.A. Siegel, J.L. Sarmiento, J. Schulien, J.W. Hair, X. Lu, and S. Rodier. 2017. Annual boom-bust cycles of polar phytoplankton biomass revealed by space-b ased lidar. Nature Geoscience X(2): 118-122. Booz and Co. 2011. Final Version II, Cost-Benefit Analysis for GMES , European Commission: Directorate-General for Enterprise and Industry, London, September 19, files/library/ec_gmes_cba_final_en.pdf. https://www.copernicus.eu/sites/default/ Boulding, K.E.. 1966. The Economics of the Coming Sp aceship Earth. In H. Jarrett (ed.) Environmental Quality in a Growing Economy, pp. 3-14. Ba ltimore, MD: Resources for the Future/Johns Hopkins University Press. Daly, H.E., and K.N. Townsend. 1996. Valuing the Earth: Economics, Ecology, Ethics. MIT Press. Hsiang, S., R. Kopp, A. Jina, J. Rising, M. Delgado, S. Mohan, D. J. Rasmussen, R. Muir-Wood, P. Wilson, M. Oppenheimer, K. Larsen, and T. Houser. 2017. Estimating economic damage from climate change in the United States. Science CCCLVI(6345): 1362-1369. 016. Improving estimates of Earth’s energy imbalance. Johnson, G. C., J. M. Lyman, and N. G. Loeb. 2 Nature Climate Change VI(7) : 639-640. Lazo, Jeffrey K., et al. 2011. US economic sensitivity to weather variability. Bulletin of the American Meteorological Society XCII (6 ): 709-720. Macauley, Molly K. 2006. The value of informati on: Measuring the contribution of space-derived Earth science data to resource management. Space Policy XVII(4): 274-282. National Research Council. 2008. Earth Observations from Space: The First 50 Years of Scientific Achievements. Washington, DC: The National Academies Press. is Report: Finding on Changes in the Private NWS 2017, National Weather Service Enterprise Analys Weather Industry, 2017. (new report, just released) Nordås, R. and N. P. Gleditsch, 2007. Climate ch ange and conflict. Political Geography XXVI(6): 627- 638. Reid, W. V., D. Chen, L. Goldfarb, H. Hackman, Y. T. Lee, K. Mokhele, E. Ostron, K. Raivio, J. Roskstrom, H. J. Shellnhuber, and A. Whyte. 20 10. Earth system science for global sustainability: grand challenges. Science CCCXXX(6006): 916-917. Sagoff, Mark. 2007. The economy of the Earth: ph ilosophy, law, and the environment. Cambridge University Press. Sagoff, Mark. 2008. The economy of the Earth. Philosophy, Law, and the Environment. Smith, P. J. 2007. Climate Change, Mass Migration and the Military Response. Orbis LI(4): Pages 617- 633. Trenberth, K. E., M. Marquis, and S. Zebiak, 2016: The vital need for a climate information system. Nature Clim. Change, 6, 1057-1059, doi:10.1038/NCLIM-16101680. PREPUBLICATION REPORT–SUBJECT TO FURTHER EDITORIAL CORRECTION 1-14 Copyright National Academy of Sciences. All rights reserved.

51 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space 2 A Decadal Strategy us to first more fully understand where we are Achieving the vision set forth in Chapter 1 requires and where we are going, then define and pursue a stra tegy to accomplish the vision. This chapter reviews the strengths and weaknesses associated with our progress over the last decade, assesses the emerging scientific and societal needs we must serve, and builds from that foundation to identify a strategic framework for the next decade. PROGRESS SINCE ESAS 2007 Programmatic Overview In carrying out the 2007 Earth science and app lications from space decadal survey (“ESAS 2007”), participants endeavored to “set a new agenda for Earth observations from space in which ensuring practical benefits for humankind plays a role equa l to that of acquiring new knowledge about Earth” (NRC, 2007). The reports Earth Science and Applications from Space: Urgent Needs and Opportunities to Serve the Nation (NRC, 2005) and Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond (NRC, 2007) were the interim and final reports, respectively, that resulted from that effort. 1 ESAS 2007 called for a set of missions supporting activities that would (Tables 2.1 and 2.2) and e Earth system and provide information to enhance advance scientific understanding of key processes in th management of natural resources. Recommendations were directed to NASA, NOAA, and the USGS. discussed separately in the following text. Progress since the report for each agency is NASA Progress from ESAS 2007 For NASA, ESAS 2007 recommended 15 missions (including one joint with NOAA) for implementation. As stated in the National Academie s’ “Midterm Assessment,” issued five-years after publication of the survey (NRC, 2012), “NASA res ponded positively to the decadal survey and its recommendations and began implementing most of them ey’s release. Although immediately after the surv its budgets have never risen to the levels assumed in the survey, NASA’s Earth Science Division (ESD) has made major investments toward the missions reco mmended by the survey and has realized important technological and scientific progress as a result. Several of the survey missions have made significant advances, and operations and applications end users are better integrated into the mission teams...At the same time, the Earth sciences have advanced signi ficantly because of existing observational capabilities and the fruit of past investments, along with advances in data and information systems, computer science, 1 ESAS 2007 provided recommendations in the form of named missions. In contrast, the statement of task for ESAS 2017 requests recommended science, applications, and observations. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-1 Copyright National Academy of Sciences. All rights reserved.

52 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space and enabling technologies.” However, the Midterm Assessment authors also found that, for several 2 reasons, the survey vision was being realized at a far slower pace than was recommended. TABLE 2.1 Missions Recommended for NASA (or joint with NOAA) in ESAS 2007 Decadal Rough Cost a Orbit Mission Description Survey Instruments (FY06 Mission million) 2010-2013 CLARREO 200 Absolute, spectrally resolved Solar and Earth radiation, spectrally LEO, resolved forcing and response of the climate interferometer (NASA Precessing portion) system SMAP LEO, SSO L-band radar 300 Soil moisture and freeze/thaw for weather and water cycle processes L-band radiometer ICESat-II Ice sheet height changes for climate change 300 LEO, Non- Laser altimeter SSO diagnosis DESDynI Surface and ice sheet deformation for 700 L-band InSAR LEO, SSO understanding natural hazards and climate; Laser altimeter vegetation structure for ecosystem health 2013-2016 HyspIRI 300 Hyperspectral spectrometer LEO, SSO Land surface composition for agriculture and mineral characterization; vegetation types for ecosystem health Multifrequency laser 400 ASCENDS Day/night, all-latitude, all-season CO LEO, SSO 2 column integrals for climate emissions SWOT Ocean, lake, and river water levels for ocean 450 LEO, SSO Ku- or Ka-band radar Ku-band altimeter and inland water dynamics Microwave radiometer GEO-CAPE Atmospheric gas columns for air quality High-spatial resolution 550 GEO forecasts; ocean color for coastal ecosystem hyperspectral spectrometer Low-spatial resolution health and climate emissions imaging spectrometer IR correlation radiometer ACE Aerosol and cloud profiles for climate and LEO, SSO Backscatter lidar 800 water cycle; ocean color for open ocean Multiangle polarimeter Doppler radar biogeochemistry 2016-2020 300 LIST Land surface topogra phy for landslide Laser altimeter LEO, SSO hazards and water runoff Microwave array PATH 450 High-frequency, all-weather temperature GEO spectrometer and humidity soundings for weather b forecasting and sea-surface temperature GRACE-II High-temporal-resolution gravity fields for LEO, SSO Microwave or laser ranging 450 tracking large-scale water movement system SCLP LEO, SSO Ku and X-band radars 500 Snow accumulation for freshwater availability K and Ka-band radiometers 2 From the report: “Although NASA accepted and bega n implementing the survey ’s recommendations, the required budget assumed by the survey was not achieved, greatly slowing implementation of the recommended program. Launch failures, delays, changes in scope, and growth in cost estimates have further hampered the program.” UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-2 Copyright National Academy of Sciences. All rights reserved.

53 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Ozone and related gases for intercontinental LEO, SSO UV spectrometer 600 GACM IR spectrometer air quality and stratospheric ozone layer prediction Microwave limb sounder Tropospheric winds for weather forecasting 3D-Winds 650 LEO, SSO Doppler lidar and pollution transport (Demo) a LEO, low Earth orbit; SSO, semisynchronous orbit; GEO, geostationary Earth orbit. b Cloud-independent, high-temporal-resolution, lower-accuracy sea-surface temperature measurement to complement, not replace, global operational hi gh-accuracy sea-surface temp erature measurement. the committee. Pink, green, and blue shading indicates NOTE: Colors denote mission cost categories as estimated by large-cost ($600 million to $900 million), medium-cost ($300 million to $600 million), and small-cost (<$300 million) missions, respectively. Detailed descriptions of the missions are given in Part II [of this report], and Part III provides the foundation for their selection. SOURCE: NRC (2007). TABLE 2.2 Missions recommended for NOAA (o r joint with NASA) in ESAS 2007. Rough Cost Decadal Survey a Orbit Mission Description Instruments (FY06 Mission $million) 2010-2013 Broadband radiometers 65 CLARREO LEO, SSO Solar and Earth radiation characteristics for understanding climate forcing (instrument reflight components) High-accuracy, all-weather temperature, GPSRO LEO GPS receiver 150 water vapor, and electron density profiles for weather, climate, and space weather 2013-2016 XOVWM Sea-surface wind vectors for weather and 350 LEO, SSO Backscatter radar ocean ecosystems by the committee. Green and blue shading indicates NOTE: Colors denote mission cost categories as estimated medium-cost ($300 million to $600 million) and small-cost (<$300 million) missions, respectively. The missions are described in detail in Part II [of this report], and Part III provides the foundation for selection. a LEO, low Earth orbit; SSO, semisynchronous orbit. SOURCE: NRC 2007 Changing priorities and directions from the Presi dent and Congress also altered the expected program, notably requiring that NASA restructure its climate observing role. NASA responded to these requests/constraints by designing the (NASA, 2010; OIG 2016) , which Climate-Centric Architecture Plan also provided further guidance for implementing the ESAS 2007 recommendations and augmenting them 3 with other high-priority observations . One result of the delayed implementation and significantly higher costs of ESAS 2007 missions is that the Midterm Assessment recommended that NASA’s Earth Science Division (ESD) should implement its missions via a cost-constrained approach, requiring that cost partially or fully constrain the scope of each mission su ch that realistic science and applications objectives 3 The plan (NASA, 2010) summarized this as follows: “In addition to building the Orbiting Carbon Observatory-2 mission for laun ch in 2013, NA SA will: accelerate development of the four NRC Decadal Survey Tier 1 missions so that they are all launched by 2017; accelerate and expand the Vent ure-class line of competed, innovative small missions; initiate new space missions to ad dress continuity of high-priority climate observations; and bring two decadal survey Tier 2 missi ons forward to allo w launch by 2020. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-3 Copyright National Academy of Sciences. All rights reserved.

54 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space 4 and achievable future budget scenario. can be accomplished within a reasonable Consistent with recommendations from the Midt erm Assessment, NASA has since included cost- constraints as part of mission definition. The CLARREO mission was re-scoped as a demonstration on the International Space Station. PACE, a mission fro m the Climate-Centric Architecture initiative, is 5 being implemented as a directed cost-capped mission. As a result, the current number of missions flying noted in the Midterm Assessment (OIG, 2016). and under implementation is different from that Table 2.3 shows those missions already flying, as well as the anticipated launch dates for missions under implementation. Missions that have a legacy in the ESAS 2007 recommended missions or were selected through the ESAS-recommended Earth Venture Program are shown with an asterisk and foundational missions (those that were planned prior to ESAS 2007 and were assumed would be flown) are shown with a double asterisk. Missions in pre-formul ation are not listed. In addition, a number of joint NOAA/NASA missions as well as other “legacy” missions have either launched or are scheduled for launch, but are not listed here. Finally, as noted in ch apter 3 (Table 3.8), the science objectives of several lly or via an implementation that differs from that 2007 survey missions are being realized either partia originally envisioned. For example, the repeat-pass interferometric synthetic aperture radar (InSAR) planned for survey’s DESDynI mission will now be r ealized via NISAR (NASA-ISRO SAR), a dedicated U.S. and Indian InSAR mission scheduled for launch in 2021, and the GEDI Lidar (Global Ecosystem Dynamics Investigation Lidar) planne d for launch to the International Space Station in 2019. Together, these missions will substantially contribute to the high-resolution observations envisioned for DESDynI. Finding 2A: The NASA ESD program has made important progress during the decade, partially recovering from the underfunded state it was in a de cade ago, and extending the progress noted in the ESAS Midterm Assessment’s conclusion that “NASA responded favorably and aggressively to the 2007 decadal survey.” Since the ESAS Midterm Assessment, NASA has adeptly responded to changing  requirements and maintained a healthy cadence of Venture suborbital, instrument, and mission opportunities, managed with an improved focus on cost constraints. The Earth system science community has benef  itted from strong international partnerships and satellites exceeding their expected design lifetimes. Implementation of pre-decadal and ESAS 200  7 missions has been slowed by budgetary constraints, increases in mission costs and scope, and launch failures. NOAA Progress from ESAS 2007 of the 2007 decadal NOAA’s capability to implement the recommendations survey was hampered ements of its satellite programs, specifically the by budgetary and programmatic challenges to core el development of next generation ge ostationary and polar-orbiting oper ational environmental satellites, 6 GOES-R and NPOESS, respectively. Cost growth and delays occurred in both programs; for the polar 4 See “Establishing and Managing Mission Costs” in NAS (2012, pp. 57-59). While not recommending “missions,” the present survey follows a similar approach to constrain the costs of addressing its recommended targeted observables. 5 The PACE mission, directed by NA SA’s Goddard Spaceflight Center, is defined as a “Design to Cost” development. Details on this type of development may be found in Jeremy Werdell, PACE Project Scientist, “Project Update,” PACE Science T eam meeting, January 20-22, 2016, https://pace.oceanscie nces.org/docs/sci2 016_werdell.pdf. 6 Geostationary Operational Environmental Satellites: See, for example, Government Accontability Office, Further Actions Needed to Effectively Manage Risks , GAO-08-183T, October 23, 2007, http://www.gao.gov/products/GAO-08-183T, and Government Accontability Office, Polar-Orbiting Environmental Satellites: Agencies Must Act Quickly to Address Risks That Jeopardize the Continuity of Weather and Climate Data , GAO-10-558, 2010, http://www.g ao.gov/products/GAO-10-558. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-4 Copyright National Academy of Sciences. All rights reserved.

55 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space program, this led to utilization of NASA’s Suomi-NPP for operational data, and the initiation of the Joint 7 Polar Satellite System (JPSS) to replace NPOESS. ESAS 2007 recommended that NOAA should restore several key climate, environmental, and weather observational capabilities to its planned NPOESS, now JPSS, and GOES-R missions, following 9 8 descopes to those systems. NOAA (with NASA) was able to continue the CERES time-series and restore OMPS for JPSS; however, it was unable to do so for CMIS (microwave imager). NOAA was unable to include a temperature and humidity pr ofiling capability for GOES-R (as described in more detail in Box 4.7). 7 The first of the GOES-R satellites wa s successfully launched on November 19, 2016, and is performing well. At the time of this writing, JPSS-1 is scheduled for launch in late 2017. Notably, throughout the period of planning and development of GOES-R and NPOESS/JPSS, which ove ublication of the 2007 rlapped the decade since p decadal survey, and despite technical and budgetary challenges, the r ecent report by the NOAA NESDIS Independent Review Team (February 28 , 2017) stated that, “During this mu lti-year period, the U.S. weather forecasting and severe storm warning capability ha s functioned at a high level of performance.” 8 See “NOAA Satellite Programs” in NRC (2012, Appendix D). 9 The measurements of the Earth’s radiation budget provided by CERES instruments since 1998 will now be continued by: a) CERES on the Joint Polar Satellite System-1 (JPSS-1), and b) the Radiation Budget Instrument (RBI), a scanning radiometer capable of measuring Eart h’s reflected sunlight and emitted thermal radiation. RBI will fly on the JPSS-2mission planned for launch in No vember 2021, as well as JP SS-3 and JPSS-4. See Elena Georgieva et al., “Radiation Budget Instrument (RBI) for JPSS-2,” http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1212&context=calcon. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-5 Copyright National Academy of Sciences. All rights reserved.

56 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Planned for the 2007-2017 Decade., and Those TABLE 2.3 Status of Pre-ESAS 2007 NASA Missions Entering Implementation or Operations Since ESAS 2007 Mission Geophysical Variables Status OSTM/Jason-2** Ocean Surface Topogr aphy Launched 2008, operating OCO** CO Launch failure 2 Glory** Launch failure Aerosol and cloud particle size and optical thickness Aquarius** Sea surface salinity Mission ended Suomi NPP** Multiple variables (ATMS, VIIRS, CrIS, OMPS, Launched 2011, operating CERES) LDCM** Land use and land surface temp erature Launched 2013, operating GPM** Launched 2014, operating Precipitation (rain and snow) OCO-2 CO Launched 2014, operating 2 CYGNSS* Hurricane Winds Launched 2016, operating SMAP* Soil moisture; freeze/thaw state; su rface salinity Launch ed 2017, operating SAGE-III (on ISS) Stratospheric O , aerosols Launched 2017, operating 3 GRACE-FO Changes in Gravitational Field In Development (2017) In Development (2018) ICESat-2* Ice sheet elevation change, sea ice thickness, vegetation canopy height ECOSTRESS* In Development (2018) Plant temperature and water stress Ecosystem structure and dynamics GEDI* In Development (2018) TEMPO* Air pollution (O , NO , ...) In Development (2018) 3 2 Aerosols MAIA* In Development (2021) TROPICS* Precipitation and storm intensity In Development (2021) GeoCARB* Carbon exchanges between land and atmosphere In Development (TBD) PACE Phytoplankton communities In Development (2022) N ISAR* Surface changes from ice-sheet collapse, In Development (late 2021) earthquakes, tsunamis, volcanoes, and landslides Ocean (and freshwater) high resolution elevation, In Development (2021) SWOT* providing water storage and ocean circulation CLARREO-Pathfinder In Development (2021 High accuracy spectral re flectance with on-board calibration timeframe) on ISS* OCO-3 (on ISS) CO In Development (2018) 2 NOTE: Missions that have a legacy in the ESAS 2007 recommended missions, or were competed through the ESAS-recommended Earth Venture Program, are shown with an asterisk. Foundational missions are shown with a double asterisk. For future missions, expected launch dates are given in parentheses. Acronyms are defined in Appendix. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-6 Copyright National Academy of Sciences. All rights reserved.

57 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Eventually, as a result of the unanticipated techni cal problems that led to delays and cost growth, NOAA significantly reduced the scope of the nation’ s future polar operational environmental satellite series. This reduction included omitting observational capabilities assumed by ESAS 2007 to be part of NOAA’s future capability and failing to implem ent the three new missions recommended for NOAA 10 implementation by ESAS 2007 (the Oper ational GPS Radio Occultation Mission , the Extended Ocean Vector Winds Mission, and the NOAA portion of CLARREO). ESAS 2007 also recommended that NOAA should increase investment in identifying and facilitating the transition of demonstrably useful research to operational use. NOAA met with mixed results; it was unable to secure funding for an XOVWM for flight on the JAXA GCOM-W1 satellite, but it was successful in securing funding for the U.S. co ntribution to Jason-3 (and launching it on 17 January 11 2016) . However, while NOAA will continue to be i nvolved and play a support role, overall U.S. responsibility for continuing the series beyond Jason-3 is reverting back to NASA. The descope of CMIS buting to the potential gap in microwave coverage from JPSS and the lack of follow-on AMSR are contri (this gap is discussed in detail in Box 4.4). Finding 2B: NOAA progress during the decade was hampered by major programmatic adjustments, as summarized in the ESAS Midt erm Assessment’s conclusion that “NOAA’s capability to implement the assumed baseline and the recommended program of the 2007 Decadal Survey have been greatly diminished by budge t shortfalls and cost overruns and by sensor descopes and sensor eliminations on both JPSS and GOES-R”. NOAA’s responsibilities have since evolved to focus on those satellite programs that directly contribute to weather forecasting and warnings and, consequently, it has transferre d responsibility for many climate observations to NASA. USGS Progress from ESAS 2007 The USGS role in space-based observation during th e last decade has been significant, built around the 40+ year Landsat program and reinvigoration of this program through the new long-term Sustainable 12 Land Imaging (SLI) partnership with NASA and the decision in 2008 to make the Landsat standard data 13 products freely available through the internet. At the time of ESAS 2007 publication, continuity of the Landsat program was a serious concern. Landsat 5 was over 20 years old; Landsat 6 had failed; Landsat 7, 10 Many of the objectives of the Operational GPS Radio Occultation Mission were addressed by the FORMOSAT-3/COSMIC mission jointly implemented by NOAA and Taiwan National Space Organization and launched in 2006. A follow-on mission, FORMOSAT-7/COSMIC- 2 is scheduled to launch in two phases, with the first launching in 2017. As this report was going to press, NOAA announced it would no longer pursue the second phase of COSMIC-2. 11 The ESAS 2007 report did not explicitly recommend Jason-3; it considered Jason-2 and the NPOESS altimeter series as part of the program of record. When the NPOESS altimeter was descoped as part of the Nunn- McCurdy process, a follow-on NRC report in 2008, Ensuring the Climate Record from the NPOESS and GOES-R Spacecraft (NRC, 2008), identified Jason-3 as a e-quality continuity of the altimetry first tier priority to ensure climat record. 12 See (NRC 2013). SLI is also described in Tim Newman, Land Remote Sensing Program Coordinator, U.S. Geological Survey, “USGS Land Remote Sensing Program Update: Briefing for the National Geospatial Advisory Committee,” April 07, 2016 (available at: https://www.f gdc.gov/ngac/meetings/april-2016/landsat-program-update- ngac-apr-2016.pdf). 13 The benefit of a free archive of Landsat data is discussed in “National Geospatial Advisory Committee— Landsat Advisory Group Statement on Land sat Data Use and Charges,” available at https://www.fgdc.gov/ngac/meetings/s eptember-2012/ngac-landsat-cost-rec overy-paper-FINAL.pdf. See also, Miller, H.M., et al., 2013, “Users, uses, and value of La ndsat satellite imagery—Results from the 2012 survey of users”: U.S. Geological Survey Open-File Report 201 3–1269, 51 p., http://dx.doi.org/10.3133/ofr20131269. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-7 Copyright National Academy of Sciences. All rights reserved.

58 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space ime and had been operating since 2003 with a failed launched in 1999, was well beyond its expected lifet 14 scan line corrector; and planning for Landsat 8 was not proceeding as needed. The Landsat program had nother and unsuccessfully changing business models. a long history of moving from one agency to a Building on a NRC report (NRC 2013), an interagency study led to a commitment from the Administration for a NASA-USGS partnership creating the Sustainable Land Imaging program and extending the plan for Landsat by two decades. As a r esult of this attention, the situation has stabilized. at 9 is planned to launch in 2020+. Landsat 8 launched in 2013, and Lands Finding 2C: The USGS has transformed the Landsat program via the Sustainable Land Imaging (SLI) program by operating Landsat, connecting the scientific/user communities and the developers of new measurement technologies, and archiving/distributing data products. This has tional footing. As long as it is funded, and placed the Landsat measurements on a more opera managed as an operational program, the SLI pr ogram will support and motivate widespread usage, benefitting both the operational and scientific communities. Policy Progress from ESAS 2007 The 2007 decadal survey recommended that “the Office of Science and Technology Policy, in ultation with the science community, should develop collaboration with the relevant agencies and in cons obal Earth observations. This plan should recognize and implement a plan for achieving and sustaining gl the complexity of differing agency roles, res ponsibilities, and capabilities as well as the lessons from the 15 implementation of the Landsat, EOS, and NPOESS programs.” The 2014 National Plan for Civil Earth Observations , produced by National Science and Tec hnology Council (NSTC) and chaired by the Director of OSTP, responds to this recommendati on and provides a framework for determining when experimental Earth observations shoul d be transitioned to sustained observations for research or for the delivery of public services. However, ex ecuting the transition remains problematic. al survey, which included recommendations to As noted above, NOAA’s response to the 2007 decad sustain a number of measurements, was greatly diminish ed by budget shortfalls; cost overruns and delays, prior to its restructuring in 2010 to become the especially those associated with the NPOESS program 16 Joint Polar Satellite System (JPSS); and by sensor descopes and sensor eliminations. In 2010, NASA 17 released a Climate-Centric Architecture plan that included a set of “continuity” missions. Further illustrating a much-expanded role for NASA in sustai ning observations, the fiscal year 2014 budget of the Obama Administration directed NASA to assume responsibility for a suite of climate-relevant data record in ozone profiling, Earth radiation observations for the purpose of continuing a multi-decadal 14 The Scan Line Corrector (SLC) compensates for th e forward motion of the satellite. Without an operating g the satellite ground track and an estimated 22 percent of SLC, the sensor’s line of sight traces a zig-zag pattern alon any given scene is lost. A number of methods have been employed to fill the gaps in Landsat 7 data (See USGS, “Landsat 7,” at: https://landsat.usgs.gov/landsat-7), although for some applications this approach is still not adequate (e.g., see Xiaolin Zhu, Desheng Liu, and Jin Chen, “A ne w geostatistical approach for filling gaps in Landsat ETM+ SLC-off images,” Remote Sensing of Environment 124 (2012) 49–60. Available at: http://www.leg.ufpr.br/lib/exe/fetch.php/disciplin as:geoesalq:pira2012:rafael-artigo2.pdf). 15 Earth Science and Applications from Space: National Imperatives for the Next National Research Council, , 2007, p. 14. This same recommendation was echoed in a 2008 follow-on report, Ensuring the Decade and Beyond Climate Record from the NPOESS and GOES-R Spacecraft , which further explored in its Chapter 4 the elements needed for a long-term climate strategy. 16 of Decadal Survey Related NOAA Developments,” in Earth Science and See Table 3.1, “Summary Applications from Space: A Midterm Assessment of NASA’s Implementation of the Decadal Survey (2012). 17 NASA, Responding to the Challenge of Climate and Environmental Change: NASA’s Plan for a Climate- Centric Architecture for Earth Observations and Applications from Space , June 2010; available at http://science.nasa.gov/media/medialibrary/2010/07/01/Climate_Architecture_Final.pdf. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-8 Copyright National Academy of Sciences. All rights reserved.

59 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space ponsibility, however, has not been accompanied with budget, and total solar irradiance. This added res result, other activities at NASA are creased associated expenses. As a resources necessary to offset the in 18 impacted. The U.S. has become increasingly reliant upon th e international Earth observing community for nderstanding the Earth system and how it changes over maintaining long-term data records essential to u 19 time. In the 1970s and 1980s, NASA was essentially the sole agency with Earth observing satellites . The Europeans and Japanese soon followed. In the 1990s NAS A led the way in Earth system science with the Earth Observing System. Since that time, additi onal space agencies have developed Earth observing capabilities, and the Committee on Earth Observation Satellites—the primary forum for international space-based Earth observations—has grown to include 32 member organizations. Within the past decade, China and India have both developed ambitious programs. And today, the Europeans—ESA, EUMETSAT and the European Union (with its commitm ent to its Copernicus program)—have become have established international leadership in implementing sustained strong and capable organizations, and global Earth observations. Finding 2D: Earth Observations and the 2014 National The 2013 National Strategy for Civil Plan represent progress toward a strategy for achieving and sustaining Earth observations, as recommended by ESAS 2007. However, the U.S. has not committed the resources to collect the broad range of sustained observations need ed to monitor and understand the Earth as a system, leaving critical gaps in the implementa tion of this National Plan and a dependency on non-U.S. sources. Science and Applications Progress Scientific and applications progress as a result of the ESAS 2007 report has been substantial. The context of both a general vision for improving science and applications report articulated challenges in the knowledge and specific goals associated with particul ar science, applications, or societal benefits. Improving Weather Forecasts, b) Protecting Against Specific progress was anticipated in the areas of: a) Solid-Earth Hazards, c) Ensuring Water Resources, d) Maintaining Healthy and Productive Oceans, e) f) Protecting Ecosystems, and g) Improving Human Mitigating Adverse Impacts of Climate Change, Health. Looking back, progress over the last decade has been substantial in these as well as other areas, as a result of continuing access to space-based observations. Mission Science Example. Scientific progress resulting from the specific missions listed in Table 2.3 is just now being realized, as these missions have been launched and early research results published. The accomplishments of the OCO-2 mission provide an illustrative example. The OCO-2 Project science objectives include quan tifying variations in the column averaged atmospheric carbon dioxide (CO ) dry air mole fraction, X with the precision, resolution, and coverage CO2 2 needed to improve our understanding of: a) surface CO sources and sinks (fluxes) on regional scales 2 1000km), and b) the processes controlling their variability over the seasonal cycle. OCO-2 was ≥ ( launched in July 2014 and these goa ls are now being addressed. For example, the OCO-2 mission data have now been characterized and calibrated (Crisp et al., 2017; Eldering et al., 2017), and OCO-2 data nhouse Gases Observing SATellite (GOSAT, now called have been merged with data from the Gree t (Nguyen et al., 2017). Further progress using these Ibuki) to provide a more comprehensive data produc 20 data should include improved understand ing of the sources and sinks of CO . 2 18 (NRC 2012), p.7. 19 Russia has had Earth observing satellites since the 1970s, but access to their data, for all practical purposes, has not been feasible. 20 http://science.sciencemag.org/content/358/6360 UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-9 Copyright National Academy of Sciences. All rights reserved.

60 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Highlighted Progress. Scientific progress enabled by satellite observations during the last decade hort review, can assess and communicate. Rather than a extends far beyond what this committee, in this s comprehensive assessment, we have chosen to provid e examples that demonstrate both the progress of the 21 last decade and the opportunity for the next decade. To start, eight important examples of progress in the (Boxes) below. The examples and corresponding Boxes last decade are described in a series of sidebars are: Scientific improvements that advan ced weather prediction skill (Box 2.1).   Understanding of air/sea fluxes of sensible and latent heat. (Box 2.2).  Tracking extreme precipitation to reduce disaster risk (Box 2.3).  Enhanced monitoring to support improvements in U.S. air quality (Box 2.4).  Tracking sea level rise and its sources. (Box 2.5). stratospheric ozone (Box 2.6).  Monitoring and understanding of  Increasing global availability of satellite-based emergency mapping (Box 2.7).  Satellite ocean color and marine ecosystems - revol utionizing our understanding of life in the sea (Box 2.8). bars, seven other noteworthy examples of progress In addition to these examples highlighted in side during the last decade include:  Quantifying worldwide emissions and concentratio ns of air pollutants, and their trends. Satellite retrievals from the MISR and MODIS instruments have been used by Zhao et al. (2017) to document regional trends in aerosol AOD be tween 2001 and 2015 showing decreases over the Eastern U. S. and Western Europe. In Eastern and Central China, aerosol AOD increases prior to eases. These trends appear to be consistent 2006, fluctuates between 2006 and 2011, and then decr with emissions estimates of aerosol, precursors, and other industrial pollutants.  Use of satellite data in health impact assessment. The application of satellite retrievals in health growing rapidly since 2007, relying largely on impact assessment has been revolutionary and MODIS, MISR and related retrie vals (Chudnovsky et al., 2013; Kloog, Chudnovsky, Koutrakis, and Schwartz, 2012; Kloog et al., 2014; Liu, Franklin, Kahn, and Koutrakis, 2007; Liu, Koutrakis, and Kahn, 2007; Snider et al., 2015). This has been facilitated by both the global coverage from satellites and the improved resolutio n of estimates of particulate matter (PM)- related properties. Satellite-based estimates of PM exposures are now finer than 1 km, allowing for improved estimates of pollutant health inter actions, and the (albeit limited, at present) information from satellites on PM properties is pr oviding information on how specific sources are impacting health.  Monitoring land-use change due to both human and natural causes . The primary measurements MODIS, and VIIRS) have in the Sustainable Land Imaging suite (Landsat, sparked substantial research productivity on understanding both pro cesses and features of the land surface, due to ., 2014). For example, there is newly derived both human and natural influences (e.g., Cai et al quantification of the global distribution of irriga ted and non-irrigated cropland, and of the fact the past 50 years) explains up to 25% of the that increased agricultural productivity (ca. 50% over observed changes in seasonality of atmospheric CO (Salmon et al., 2015; Gray et al., 2014). 2  Tracking variations in ocean plankton and land vegetation as well as primary production. SeaWiFS and MODIS-Aqua data were used to provide significant advances in understanding the interannual variability and long-term trends in ma rine plankton biomass and primary productivity on a global scale as well as the relationship of regional biological variations in plankton biomass 21 It is also important to recognize that progress in th e last decade—and certainly in the earliest part of that decade—is the result of investments made prio r to the completion of the decadal survey. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-10 Copyright National Academy of Sciences. All rights reserved.

61 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space as warming (e.g., Siegel et al., 2013). Similar and physiology to ocean physical factors such ons in land vegetation greenness and primary progress also occurred for quantifying variati production combining satellite multi-spectral imager y and ecosystem models (Zhu et al., 2011; Anav et al., 2015) as well as the development of new primary production estimation approaches using measurements of solar induced fluorescence (Frankenberg et al., 2014).  r the first time on a global scale. Combinations of A-Train Seeing the rain formation process fo observations (the A-Train series of satellites is described in Box 2.9) have provided a unique glimpse into one of the important processes of the Earth system—how rain forms (e.g., Suzuki et al. 2010; Takahashi et al., 2017). This revealed many surprises both with respect to how frequently it rains on Earth (Stephens et al., 2010) and exposed significant deficiencies in the way this rain formation process is represented in Earth system models (Golaz et al., 2013; Suzuki et al. 2015) which further underscored the way this process fundamentally shapes the cloud aerosol interactions.  Cloud feedbacks contributing to the decadal cooling in the eastern tropical Pacific . Zhou et al. clouds and climate mode l simulations to show (2016) used geostationary satellite observations of how the slowdown in global mean warming th at occurred from 1998-2013 (Yan et al. 2016) was contributed to by a positive cloud feedback on localized tropical cooling.  Observation of a slowdown in sea level rise associated with flooding in Australia. Fasullo et al. (2013) used altimetry, gravity, and color imaging observations from three satellite instruments. It demonstrated that monitoring of sea level and grav ity from space is necessary in order to detect a rapid increase in sea level rise and attribute its causes, consistent with what is expected to happen eventually as the ice sheets begin to decline faster. ********************************************************************************* BOX 2.1 Progress in the Last Decade: Scientific Improvements that Advanced Weather Forecast Skill While numerical weather prediction (NWP) has improved over the last four decades, the more more accurate initial conditions due to better data rapid increases in forecast skill can be attributed to assimilation methods, more observational data, adva nces in understanding and modeling of physical processes, and increased computational resources (Bau er et al. 2015; Buizza and Leutbecher 2015). The forecast skill increase for the ECMWF model from 1981 to present is shown in Figure 2.1 (see http://www.emc.ncep.noaa.gov/gmb/STATS_vsdb/lo ngterm/ for a muli-model comparison that includes NOAA’s GFS). Forecast skill is measured he re by the correlation between the forecasts and the verifying analysis of the 500- hP a height, expressed as the anomaly with respect to the climatological height. Values greater than 60% indicate useful for ecasts, while those greater than 80% represent a high degree of accuracy. The predictive skill in the Northern and Southern Hemispheres is nearly equal today, due to the effective assimilation of satellite data th at provides global coverage. The convergence of the Hemisphere after 1999 represents the breakthrough curves for Northern Hemisphere and Southern associated with the more effective assimilation of satellite data through the use of variational data rth 2002). The improvement of weather forecast in lead time assimilation (Simmons and Hollingswo between 3-10 days has been about one day per decade. However, the 10-day and longer lead time have not yet reached the 50% level. Over the past decade, there have been signi ficant advances in the co mmunity’s capabilities for subseasonal forecasting, and several operational cente rs have implemented model-based subseasonal forecast systems that provide a bridge between th e medium-long range weather and seasonal forecasts and outlooks (e.g. s2sprediction.net;Vitart et al. 2017). Extending the useful range of forecasts beyond two weeks through the use of probabilistic forecasting is a priority for many operational NWP centers over the coming decade. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-11 Copyright National Academy of Sciences. All rights reserved.

62 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space A significant advance in the past decade has been realized with the improved agreement between infrared radiance measurements and radiances simulate d from NWP model input. This can be attributed to the advent of spectrally resolved well calibra ted IR measurements as well as better physics (line strengths, widths, mixing) in radiative transfer model calculations. These advances have, in turn, increased the impact of low Earth orbiting IR high spectral resolution sounders (AIRS, IASI, CrIS) on reducing NWP model errors. For most NWP centers, th e combined contribution to the reduction of 24- hour global forecast error for these IR sounders is no w similar to combined contribution from microwave 16). In addition, the high spectral resolution IR sounders (AMSUA, ATMS, MHS) (Auligné, et al., 20 ng, and higher vertical tion, boundary layer probi measurements have enabled surface emissivity estima resolution temperature and moisture profile determinations. Permission Pending FIGURE 2.1 Northern and Southern Hemisphere an omaly correlations for 500 hPa geopotential height r prediction (NWP) skill increase from 1981 to present forecasts, reflecting the ECMWF numerical weathe (https://www.ecmwf.int/en/for ecasts/charts/catalogue/plwww_m_hr_ccaf_adrian_ts?time=2017101100). ********************************************************************************* END OF BOX UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-12 Copyright National Academy of Sciences. All rights reserved.

63 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space ********************************************************************************* BOX 2.2 Progress in the Last Decade: Breakthrough in Understanding Air/Sea Fluxes of Sensible and Latent Heat The rate of heat exchange between the atmos phere and ocean is represented by air-sea heat -2 fluxes. Most applications necessitate fluxes with accuracies of at least 5 to 10 W m . (e.g. Bourassa et al, 2013). This is a challenging target, and in the past fl ux products disagreed substantially, often by more -2 than 20-40 W m (Bourassa et al, 2013). An important breakthrough of the last decade is the development of improved satellite-based estimates of ai r/sea fluxes of sensible and latent heat, including nding of remaining issues with the existing satellite better uncertainty information and physical understa tic and especially random errors are still higher than system (Figure 2.2; Clayson et al. 2017). Systema fluxes measured from in situ platforms like buoys (e.g . Smith et al., 2012), but with the greater spatial coverage of the satellite fields a nd improved retrieval methodologies, uncertainties vary spatially but in -2 the global mean, they are beginning to approach the 10 W m target. Determining the structural character of the uncertainties and the relation of these to other aspects of the Earth system is also important for understanding how these error sources might be a ddressed and overcome. One important source of uncertainty in estimating latent heat flux is the diffe rence between near surface humidity and its saturation value (Q closely correlate to the cloudy sky weather states that is -Q ). The error characteristics of Q -Q a a s s defined by a combination mostly clear and sha llow boundary layered clouds These analyses, and our fluxes have led to improvements that now allow for improved ability to estimate latent and sensible heat the use of the satellite flux fields for studies of ex treme weather (e.g. Liu et al. 2011); regional process studies and water budgets (Brown and Kummerow, 2014) ; and global water and energy budget studies dvances in this area could come from sensors with (L’Ecuyer et al. 2015; Rodell et al. 2015). Future a greater boundary layer sensitivity; satellite retrieval algorithm improvement; increased spatial sampling; and optimized ensembles of satellite products with increased synergy between the modeling and satellite communities. Permission Pending eat flux and the estimated total uncertainty based on FIGURE 2.2 The annual mean distribution of latent h the SeaFlux (Curry et al., 2004). ********************************************************************************* END OF BOX UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-13 Copyright National Academy of Sciences. All rights reserved.

64 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space ********************************************************************************* BOX 2.3 Progress in the Last Decade: Tracking Extreme Precipitation to Reduce Disaster Risk In the past decades, many advances in sate llite remote sensing algorithms for estimating PA (TRMM Multi-satellite Precipitation Analysis, precipitation have been made. Among these are TM ield and Kuligowski 2003) , GSMaP (Global Satellite Huffman et al. 2007), H-E (Hydro-Estimator, Scof Mapping of Precipitation, Okamoto et al. 2005), CMORPH (CPC Morphing, Joyce et al. 2004), IMERGE (Integrated Multi-Satellite Retrievals for GPM, Hu ffman et al. 2014), and PERSIANN family of systems (Hsu et al. 1997; Sorooshian et al. 2000; Hong et al. 2004; Ashouri et al. 2015). These algorithms utilize a variety of GEO IR and LEO PMW sensors for the me asurement of precipitation. Furthermore, various outputs generated by these algorithms are produced with different spatial and temporal resolutions, and time latency. From the perspective of emergency disaster mana gement including flash flooding, the availability of observations with the shortest possible time late ncy is critical. The following is an example of the Precipitation Estimation from Remotely Sensed Info rmation using Artificial Neural Networks—Cloud Classification System (PERSIANN-CCS), with approxima tely 30 to 90 minutes with high resolution of o o 0.04 degree, from 60 N to 60 S, in capturing Typhoon Haiyan. Typhoon Haiyan, with a 10-minute sustained wind speed of 230 kilometers per hour, struck Southeast Asia in November 2013. It is one of the strongest storms on record, resulting in significant damage and many casualties. Real-time monitoring of su ch large storms with the least time latency is becoming invaluable for disaster early-warning applica tions. A visualization tool that displays satellite- based precipitation estimates offers a real-time tr acking of the immense rainfall delivered by Haiyan ter and Development Inform Super Typhoon. One such tool is known as the Wa ation for Arid Lands—A Global Network (G-WADI) PERSIANN-CCS GeoS erver (http://hydis.eng.uci.edu/gwadi). The G-WADI PERSIANN-CCS GeoS erver has been under development since 2005 through collaboration between the Center for Hydrometeorology and Remote Sensing (CHRS) at the University of Scientific, and Cultural Organization’s (UNESCO) California, Irvine and the United Nations Educational, algorithm of this system, supported by NASA and International Hydrological Program (IHP). The core NOAA, extracts local and regional cloud features (col dness, geometry, and texture) from the international constellation of GEO satellites capturing Infrared (IR ) imagery and estimates rainfall at 0.04° × 0.04° spatial resolution (roughly a 4-kilometer square) every 30 minutes. Information from LEO satellites is used to then adjust the initial precipitation estimation from the ANN algorithm. In the case of Haiyan Super Typhoon, PERSIANN-CCS captured the maximum precipitation th intensity of approximately 361 milli meters per day reached on the 7 of November, while the storm was approaching the Philippines. The following days show rainfall rates steadily decreasing with the weakening of the storm as it entered the South China Sea, struck Vietnam, and then finally dissipated on th the 11 of November. This tool provides an example of how the c onfluence of machine learning algorithms and the ever-increasing capabilities of high performance co mputers can process vast amounts of observations from multiple satellites in a timely manner to allow for real-time monitoring and issuance of warning of extreme precipitation in flood prone areas and fo r use by engineers and operators managing water resources systems (Nguyen et al. 2014). UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-14 Copyright National Academy of Sciences. All rights reserved.

65 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Permission Pending FIGURE 2.3 State-of-the-art real-time global space-based precipitation estimation systems using multiple satellites and advanced machine learning techniques (A rtificial Neural Networks - ANNs) are reaching the level of maturity to monitor and capture extrem e precipitation events. The figure above provides an example of tracking precipitation of Super Typhoo n Haiyan using PERSIANN-CCS with approximately 30-minute latency at 0.04 degree resolution (Nguyen et al. 2014). ********************************************************************************* END OF BOX UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-15 Copyright National Academy of Sciences. All rights reserved.

66 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space ********************************************************************************* BOX 2.4 NASA Satellite Observations Reveal Progress in the Last Decade: Dramatic Improvements in U.S. Air Quality over the Past Decade Starting in 2003, the US EPA has acted to vigorously control fuel combustion emissions of nitrogen oxide radicals (NO ≡ NO + NO Budget Trading Program and other measures. ) through the NO x 2 x NO ulate pollution, acid deposition, and ecosystem is a major source of ozone pollution, partic x eutrophication. OMI observations of NO aboard the Aura satellite have provided a vivid demonstration of the 2 success of these emission control policies (Lu et al., 2015). Figure 2.4 shows the trends in NO 2 concentrations measured by OMI over the US from 2005 to 2015. These satellite images have been critical to communicate to the public that the air ove r the US is indeed getting cleaner in response to policy action. The quantitative NO trends observed from space and their spatial distribution are 2 consistent with the emission trends reported by the EPA, verifying compliance with the control measures but also demonstrating the value of th e satellite observations for monitoring NO emissions and their x thus documented rapid increases in NO trends worldwide. OMI observations have emissions over the x past decade in the Middle East, India, and China, a nd a leveling off in China ov er the past few years in response to new air pollution control measures. Satellite observations of air quality extend also to sulfur dioxide (SO ), formaldehyde, ammonia, ozone, and particulate matter. These observations are now 2 providing a sustained global monitoring system for air quality and are a crucial trusted resource for air quality managers. The observations have also been cruc ial in identifying air pollution as one of the top 1 environmental killers in the world. FIGURE 2.4 Decreasing US air pollution over the past decade. The figure shows annual mean tropospheric columns of nitrogen dioxide (NO ) observed by the OMI satellite instrument in 2005 and 2 2015. (Courtesy of NASA) 1 OECD (2012), OECD Environmental Outlook to 20 50: The Consequences of Inaction , OECD Publishing, Paris. http://dx. doi.org/10.1787/ 9789264122246-en ********************************************************************************* END OF BOX UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-16 Copyright National Academy of Sciences. All rights reserved.

67 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space ********************************************************************************* BOX 2.5 Global Sea Level Rise as an Interdisciplinary Problem Progress in the Last Decade: Global sea-level rise is an interdisciplinary issue with immense societal impact. Sea level rise is the combined result of thermal expansion as water warms and the addition of mass as land ice (glaciers nt of satellite altimetry in 1992 , measurements of the absolute sea and ice sheets) melts. Since the adve 1 − of sea-level rise between +2.6 ± 0.4 mm yr level from space indicate an average global rate and +2.9 ± − 1 0.4 mm yr (depending on the choice of vertical land moti on applied), more than twice the average rate 22 for the entire 20th century . This rate is consistent with measure ments of the thermal structure of the upper ocean by Argo floats, and of the mass loss of ice sheets and the associated ocean mass increase by ) satellite; they demonstrate that the contribution the Gravity Recovery and Climate Experiment (GRACE from melting ice now exceeds that from thermal expans ion. This rate is increasing due to increased warming of the oceans and melting of glaciers and ice sheets. Furthermore, the rate of global sea-level as been increasing, with the largest contribution coming from increased rise within the past two decades h melting of the Greenland ice sheet (Chen et al., 2017). Th e largest source of uncertainty in projections is the response of the Antarctic ice sheet, which cont ains 56 m of sea-level rise equivalent, and the contribution from that ice sheet is accelerating (Harig and Simons, 2015). The estimated 146 million people worldwide living along the coast within one meter or less above mean high tide—about 2% of the global population — are at direct risk this century depending on how fast global sea level continues to rise in their region. The largest impacts will be associated with storm surge and intense rainfall, which are exacerba ted by changes in local relative sea level, tidal amplitudes, local subsidence, and the nature of ex treme meteorological forces. The Intergovernmental th Panel on Climate Change in their 5 Assessment Report (2013) projects anywhere from ~25 cm to 1 m by for carbon concentration. More recent projections 2100, depending on which model scenario is used (Kopp et al., 2014) for 2100 adopted by California are larger, ranging from 0.5 to 1.2 m. Still others project higher values that could exceed 2 m. (Oppenheimer and Alley, 2016). Sea level does not rise uniformly over the whole ocean, and different climate scenarios give a range of average global sea level rise values. Moreover , coastal sea level rise depends on the relative rate to that determination is the subsidence and uplift of sea level rise, as opposed to the absolute rate. Critical of the coastal lands. Thus, accurate geodetic measurements (for example, from GPS) as well as surface displacement measurements (as can be derived from InSAR) are two critical contributions to understanding local rates of sea level rise. Addressing the impact of local sea level rise (due to the sea level itself and the corresponding coastal vertical combination of the absolute height change of for coastal facilities. The most important metric motion) requires an assessment of the implications needed for individual cites to plan to adapt to sea level rise is a prediction for when local sea level will that location under various c limate scenarios. Planners meet or exceed a particular height on the land at tions of geographically-varying sea level rise as far into the future as and engineers urgently need projec 23 rror associated with the projections possible, they need margins of e , and they need to know local rates of vertical land motion. The uncertainty in projections of sea level rise directly impacts how fast and how nd piers should be raised, where airports and other much the coast must be hardened, how high streets a infrastructure should be relocated, and/or wh ich neighborhoods should be abandoned. pacts of sea level rise result from increased In many communities, the most dramatic im vulnerability to coastal flooding. Contributing factors include not just the local sea level rise, but also storm surges and intense rainfall (e.g., Houston after Hurricane Harvey) and their dependence upon changes in local relative sea level, tidal amplitud es, local subsidence, and the nature of extreme 22 Unabated global mean sea-level rise over the satellite altimeter era, Watson, C.S., White, N.J., Church, J.A., King, M.A., Burgette, R.J. and Legresy, B. Nature Clim ate Change 5, 565–568 (2015), doi:10.1038/nclimate2635. 23 http://www.opc.ca.gov/web master/ftp/pdf/docs/rising-seas-in-ca lifornia-an-update-on-sea-level-rise- science.pdf UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-17 Copyright National Academy of Sciences. All rights reserved.

68 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space fests itself in the increasing frequency of , meteorological forces. Coastal flooding mani nuisance floods such as shown for the city of Boston in Figure 2.5. Evaluating future risks from coastal flooding and inundation—and reducing uncertainties in projections—involves an understanding of how storm frequency and intensity, offshore ocean currents, and decadal variability in the ocean is changing. This taining continuing satellite in turn depends on main observations of the variables that determine global sea level rise, (changes in ocean heat and land-ice mass) as well as observations of the variables that determine the strength of storm surges (winds, wave height, and tides), intensity of rainfall, and any local subsidence. 25 3 24 Coastal cities and regional governments across the U.S.—Seattle , San Francisco , , California 26 27 28 San Diego , and New York City among others—have developed Climate Action , Southeast Florida rise. Reducing uncertainties provides for improved Plans that evaluate concerns about sea level to stronger overall resilience to disasters. As an community adaptation and mitigation planning, leading example of the urgency of this issue, a numbe r of coastal communities are now moving beyond the Miami Beach has embark planning stage to implementation. For example, ed on a $100 million flood prevention project in the face of sea level rise. This e ffort will raise roads, install pumps and water mains, in the Mid-Beach area [Miami Herald, January 28, and redo sewer connections over the next two years 2017]. FIGURE 2.5 Predicted high tides at Boston near or exceeding the nuisance flood cm above level of 68 se (Kruel, 2016). Before 20 mean high water, and their relationship to sea level ri 10, the tides alone never exceeded flood levels; from 2011 onward, and likely into a future climate, sea level has risen and will rise ng. Catastrophic flooding can occur if a storm sufficiently that tides alone can produce nuisance floodi occurs on top of a high tide. Note: In the figure above cenario-2 refers to the , MSL=mean sea level and S “Intermediate-High” scenario of the U.S. National Climate Assessment, which is available online at: https://cpo.noaa.gov/sites/cpo/Reports/2012/NOAA_SLR_r3.pdf. 24 http://www.seattle.gov/Documents/Depa rtments/OSE/2013_ CAP_20130612.pdf 25 http://sf-planning.org/sea-level-rise-action-plan 26 https://www.sandiego.gov/sites/de fault/files/legacy/environmental- services/sustainable/pdf/action_plan_07_05.pdf 27 https://www.epa.gov/arc-x/southeast-flor ida-compact-analyzes-sea-level-rise-risk 28 http://www1.nyc.gov/office-of-the-mayor/news/122- 15/mayor-de-blasio-releases-npcc-2015-report- providing-climate-projections-2100-the-first UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-18 Copyright National Academy of Sciences. All rights reserved.

69 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space ********************************************************************************* END OF BOX ********************************************************************************* BOX 2.6 Progress in the Last Decade: Monitoring and Understanding of Stratospheric Ozone Satellite observations of atmo spheric ozone began in 1979 with the Total Ozone Mapping Spectrometer (TOMS). Observations over the past d ecade from the Ozone Monitoring Instrument (OMI), the Microwave Limb Sounder (MLS), and the Tro pospheric Emission Spectrometer (TES) aboard NASA Aura have sustained the long-term satellite record and provided further insights into the vertical distributions of ozone. The satellite record has been critical for understanding of the complex interplay between dynamic, physical, and chemical processes driving the formation of the Antarctic ozone hole. Satellite observations have enabled the monitoring of interannual variab ility and potential ozone depletion and the Antarctic. The fferences between the Arctic in the Arctic, and provided understanding of the di satellite record has also enabled tracking of ozone trends at northern mid-latitudes with sufficient information to relate these trends to their causes. Observations from satellites have provided guidance to international policies to protect the ozone layer, starting with the Montreal Protocol, and r esulting in the total ban on halocarbon production as of the late 1990s. As shown in Figure 2. 6, satellite observations of ozone over the past decade show that the depletion of the ozone layer has been halted and th ere are some early signs of recovery. The satellite observations have provided the basis for the developmen t of advanced models to simulate the chemistry of the stratosphere, and model proj ections for the future are also included in Figure 2.6. Satellites will play a central role in the coming decades for m onitoring the expected recovery of ozone and the complications associated with climate change FIGURE 2.6 Upper panels: False color images of October average total column ozone (Dobson units). The 1971 and 2015 panels are observations derived fr om the NASA Nimbus-4 BUV instrument and the UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-19 Copyright National Academy of Sciences. All rights reserved.

70 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space NASA Aura Ozone Monitoring Instrument (OMI, KNMI), respectively. The 2041 and 2065 panels are from a NASA/GSFC GEOSCCM model simulation usi ng projections of ozone depleting substances (ODSs) and greenhouse gases (GHGs). The color scale for total ozone is on the left-hand side. High Lower panel: ozone values are in red/yellow while low values are blue/purple. The lowest values from the October average over Antarctica. The black dots ar e observations from the BUV instrument, the Total Ozone Mapping Spectrometer (TOMS), and the OMI instrument. The red dots are from NASA/GSFC GEOSCCM model simulation. The red curve shows the smoothed values of the red points (10-year 29 Gaussian filter). Credit: NASA ********************************************************************************* END OF BOX ********************************************************************************* BOX 2.7 Progress in the Last Decade: Increasing global Availability of Satellite-Based Emergency Mapping (SEM) Satellite observations have been used effectively for warnings in events of high-impact natural hazards such as hurricanes, severe winter storms, and wildfires (Clark et al., 2003) over the last a few capability and information technology have led to decades. Recent advances in satellite remote sensing gional mapping systems based on rapid assessments of development of more sophisticated global and re her natural and man-made disast ers. The International Working flooding, earthquake damage, and ot ) was established after the Haiti earthquake and Group on Satellite based Emergency Mapping (IWG-SEM Pakistan flood in 2010 to improve information sh aring and cooperation of across the international community. Figure 2.7 (Voigt et al., 2016) provides a summa ry of how increased satellite availability has improved our ability to provide mapping for decisi on-makers in response to emergencies such as typhoons and earthquakes. Response capacity is impr oving rapidly. In some cases, the creation of disaster-related products has been accelerated to just a few hours or less through capabilities such as crowdsourcing, machine learning, satellite constellations , and use of day/night all weather sensors such as radar. 29 https://aura.gsfc.nasa.gov/informing_policy_makers.html UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-20 Copyright National Academy of Sciences. All rights reserved.

71 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Permission Pending FIGURE 2.7 An example of the transition of integra tive science to applied use. (Voigt et al., 2016). The references cited in in this figure include the followi ng: (41) A.S. Belward, J.O. Skøien, Who launched what, when and why; Trends in global land-cove r observation capacity from civilian Earth observation 115-128(2015), doi:10.1016/j.isprsjprs.2014.03.009; satellites. ISPRS J. Photogramm. Remote Sens. 103, Satellites and Sensors, accessed January 15, 2016, (42) University of Twente, ITC’s Database of www.itc.nl/research/products/sensordb/AllSatellit es.aspx; (43) European Space Agency, Earth ttps://directory.eoportal.org/web/eoportal/satellite- Observation Portal, accessed December 1, 2015, h missions; (44) Committee on Earth Observation Sate accessed December 20, llites, The CEOS Database, 2015, http://database.eohandbook.co m/timeline/timeline.aspx. ********************************************************************************* END OF BOX ********************************************************************************* BOX 2.8 Progress in the Last Decade: Satellite Ocean Color and Marine Ecosystems - Revolutionizing Our Understanding of Life in the Sea Satellite measurements of ocean color have revol utionized our understanding of life in the sea. Ocean color tracks variations in microscopic phytopla nkton in the upper ocean that form the base of the marine food web supporting invertebrates, fish, mari ne mammals, seabirds, and valuable commercial and color coverage has been available since mid-1997 recreational fisheries. Continuous global satellite ocean view Sensor (SeaWiFS) followed more recently by since the launch of the Sea viewing Wide-Field of MODIS and VIIRS (McClain, 2009). The amount of phytoplankton chlorophyll, the key pigment in photosynthesis, is derived by measuring light back scattered from the upper ocean in different spectral bands. Chlorophyll estimates can then be combined with information on sea surface temperature, light, nutrients, and mixed layer depth to quantify varia tions in photosynthesis or primary productivity. The large-scale geographic and seasonal variations in su rface ocean chlorophyll, well known in large part nced by ocean circulation patterns. Elevated because of satellite observations, are strongly influe chlorophyll values and highly productive marine ec osystems occur in upwelling regions along the equator, in high latitudes, and in coastal eastern boundary current regions such as off California, Oregon and Washington (Figure 2.8). ces occurred along several fronts facilitated by the Over the past decade, major scientific advan ries and development of new methods for estimating continuity of the growing global ocean color time-se . . Examples include: mapping long-term natural novel biological variables from ocean color sensors UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-21 Copyright National Academy of Sciences. All rights reserved.

72 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space biomass (Siegel et al., 2013); constraining seasonal variability in surface chlorophyll and phytoplankton d understanding of the underlying mechanisms (Blondeau-Patissier et phytoplankton blooms and improve al 2014; Behrenfeld and Boss, 2014); and improved estimates of phytoplankton primary production, functional type and size (Lee et al., 2015; Kostadinov et al., 2016; Mouw et al., 2017). A key finding is that surface chlorophyll in th e tropical ocean (bounded by the black lines in variations such as El Niño, with lower chlorophyll Figure 2.8) is linked closely with inter-annual climate found during periods with warmer sea surface temperatures as a possible indicator of responses to future climate change (Siegel et al., 2013). This climate si gnal reflects primarily physiological reductions in the amount of chlorophyll to carbon biomass in cells, likel y due to nutrient stress under more stratified upper ocean conditions. Other new satellite algorithms are opening windows on particle size and phytoplankton community composition, important ecological attribut es for connecting plankton dynamics to the carbon cycle, food-webs, and fisheries (e.g., Siegel et al ., 2014). Pilot studies using CALIOP LiDAR data are showing great promise for observing plankton deeper in the water column than passive ocean color sensors as well as resolving the vertical structure of plankton communities (Behrenfeld et al., 2017). The wealth of satellite ocean color data is being integrated with other remote sensing data, for example on ocean physics and circulation, and w ith ship and robotic ocean observations using sophisticated data science approaches and numerical models (Kavanaugh et al., 2014) for applications to marine biodiversity and fisheries. Satellite surface ocean color and temperature provi de synoptic maps for locating physical fronts and phytoplankton blooms that are often hotspot s for fish (and marine mammals and seabirds), information that is used by fisherman and recrea tional and commercial fishery forecasting services as well as ocean conservation managers. On larger scales, pr imary productivity derived from satellite data helps ntial fish catch and is central to models being explain geographic and temporal variations in pote developed for assessing climate change impacts on fish eries (e.g., Stock et al., 2017). More broadly, sensing data underpin key appli satellite ocean color and other remote cations related to evaluating water quality and ocean ecosystem health (e.g., McCa rthy et al, 2017), and NOAA produces a number of routine products for the public and natural resource managers. Examples of valuable ocean monitoring and eco logical forecasting products include data on: d runoff events to assess possible thr ocean turbidity, land pollution, an eats to coral reef health (NOAA 30 Coral Reef Watch ); the presence of harmful algal blooms (HABs) in coastal waters and the Great Lakes 31 that can endanger aquaculture, recreational and commercial fisheries, and human health ; and the 32 waters and the potential for coral bleaching magnitude of low oxygen dead zones in coastal . Synoptic information from satellite also creates a framework th at greatly enhances the value in-water physical, chemical and biological data from the U.S. Integrat ed Ocean Observing System as illustrated in the new 33 U.S. Marine Biodiversity Observation Network .. 30 ov/satellite/resear ch/oceancolor.php https://coralreefwatch.noaa.g 31 https://oceanservice.noaa.gov/hazards/hab/ 32 https://oceanservice.no aa.gov/ecoforecasting/ 33 https://ioos.noaa.gov/project/bio-data/ UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-22 Copyright National Academy of Sciences. All rights reserved.

73 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space FIGURE 2.8 Mission mean chlorophyll concentration from SeaWiFS from August 1, 1997 to December 14, 2010 from Siegel et al. (2013). The black lin es indicate the boundary between warm tropical waters and the cooler extratropics. Surface chlorophyll exhibits larger geographic variations of over a factor of 100 from low chlorophyll open ocean regions to highly productive coastal zones. To capture this wide range in values, the color bar uses a standard logar ithmic scale where each unit of variation (e.g. from -1 to 0) reflects a factor of 10 in crease in chlorophyll concentration. ********************************************************************************* END OF BOX Transitioning to the Coming Decade With Earth science and applications, scientific needs and societal needs are tightly coupled. Curiosity-driven science often leads to significant societal benefits. Scien ce driven by societal needs often reveals new intellectual challenges of a purely sci entific nature. This productive coupling between curiosity-driven science and applications-driven research is a hallmark of Earth system science. Discipline-specific advances based on observa tions from space have enabled fundamental space-based data has provi ded the foundations for discoveries across the natural sciences. In addition, integrated science of the Earth system (Jacobson et al. 2000, Reid et al. 2010, Berger et al. 2012). Many m space involve new insight into interactions of the important discoveries based on observations fro ample the atmosphere and oceans (Mechoso et al., among major components of the Earth system, for ex 2014), large ice sheets and the oceans (Pritchard et al ., 2012; Rignot et al., 2013), ocean circulation and ecosystems and the water cycle (Wrona et al., 2016). biogeochemistry (Siegel et al. 2014) or terrestrial on observations from space, many more depend on While some discoveries are grounded entirely combining information from a range of sources, in cluding field campaigns, laboratory experiments, computer modeling, and theoretical studies (Sellers et al., 1988, Sellers et al. 1995, Mechoso et al., 2014). Science based on integrating information from several approaches can lead to products where the insights from the whole are much greater than the sum of the parts. As a consequence, the value of space-based observations amplifies the returns on i nvestments across the Earth sciences. The top science priorities for the next decade all combine opportunities to drive fundamental science advances as well as contribute to important ap plications for forecasting, managing, and planning. All fit into a science and technology ecosystem that involves other kinds of measurements as well as theory, with expected returns from the integrated system that amplif y the value of each component. Some of the top priorities are important mainly from the pe rspective of one science discipline or one societal UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-23 Copyright National Academy of Sciences. All rights reserved.

74 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space plines and applications as well as for continued progress application. Others are critical for a range of disci in understanding, predicting, and managing the Earth as an integrated system. A DECADE’S OPPORTUNITY FOR RAPID PROGRESS A convergence of institutional capacity, technologi cal advance, and scientific discoveries from prior years makes possible rapid progress during the period of this Decadal Survey. Institutional Capacity to Meet Scientific and Societal Needs Since the last decadal survey, the suite of na tions with commitments to space-based observation programs has expanded, and a new generation of co mmercial satellites (especially small commercial satellites) has emerged as a viable option for some ki nds of science and applications. Close coordination can facilitate efficiency not only across nations but also between nations and the private sector. Indeed, advances in technology, coordination, and privat e sector capabilities have the potential to allow for t constraints. Also, optimal implementation of increased observation capacity within the existing budge space-based observation depends on effectively inte grating these measurements with measurements from measurements and campaigns. suborbital missions and ground-based Scientific and Technological Opportunities The coming decade will present rapidly growing and increasingly challenging needs for Earth information and the science on which it is founded. At th e same time, advances in Earth system science, and in other fields that Earth scientists draw from, promise new capabilities that can allow us to progress even more rapidly than we have in the past:  Advances in space systems technology (such as small spacecraft and active sensing) will allow us to address critical questions in ne w ways, providing the tools to observe new parameters.  New scientific methods , such as machine learning, will allow us to extend the reach of our science within limited resources.  Novel observational methodologies, such as advanced satellite c onstellations (Box 2.9), have been shown to significantly amplify the science and applications beyond that which any single satellite provides on its own  Innovative project implementation approaches, including rideshare and secondary payload opportunities, spacecraft block buys, public-p rivate partnerships and international partnerships offer the potential of lower cost missions and/or more frequent access to space.  Earth process models, data assimilation, and computational capabilities will be sufficiently robust to the point where they can make full use of all the data (tens of terabytes per day) that satellites have to offer, a capacity we cannot currently fully exploit.  The scientific community and the public have the capacity to effectively absorb new capabilities enabled by the availability of new applications, data structures, and dissemination tools .  Alternate sources of observations and analytic capabilities are rapidly emerging, particularly in the commercial sector. Th ese can augment traditional sources to enhance capability. ********************************************************************************* UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-24 Copyright National Academy of Sciences. All rights reserved.

75 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space BOX 2.9 The A-Train constellation—How Observing System Architecture Innovation Enables Breakthrough Science One of the most successful demonstrations of an integrated approach to observe the Earth system is the A-Train satellite constellation, shown in Figure 2.9. The constellation provides multiple perspectives on many Earth system processes. On April 28, 2006 two active sensors carried by the NASA CloudSat and the NASA/CNES Cloud-Aerosol Lidar and Infrared Pathfinder were added to Satellite Observations (CALIPSO) satellites the constellation. Together these sensors provided a much d eeper understanding of the combination of cloud, precipitation, and aerosol processes and the critical role of vertical profiles in understanding the effects of clouds and aerosol on the Earth’s radiation balance (L’Ecuyer et al, 2009; Han et al., 2017). The h other, offering a powerful means to interpret and radar and lidar observations mutually complement eac ssive sensors of the constellation. evaluate other information from the pa Permission Pending FIGURE 2.9 The A-Train constellation of satellites as of 2014. This constellation pioneered an integrated The A-Train r understanding of the Earth’s atmosphere. approach for observing the Earth advancing ou offered clear demonstration of both the value of cons tellation flying and the viability of tightly aligning observations from different space-borne platforms. ********************************************************************************* END OF BOX AA, and USGS hold the potential to make Supported with appropriate resources, NASA, NO tremendous advances this decade in both our scientific understanding of Earth and the use of that knowledge to benefit society. Doing so can be best accomplished with a national commitment. in space-based Earth opportunity for rapid progress This decade presents an Finding 2E: science and its application to benefit society. The recommended Earth observing system will provide previously unavailable capabilities; new modeling and analysis tools are poised to enable scientific breakthroughs ; complementary capabilities are expanding in the commercial and user community’s ability to deliver sector and other communities; and the research advances such as widespread internet access benefits is greatly enhanced by technological and mobile device use. A STRATEGIC FRAMEWORK FOR DECADAL PROGRESS The coming decade is one in which we must not only accelerate the advance of our science and applications, but do so within constrained resour ces. This perspective was summarized in the Decadal Community Challenge, as stated in Chapter 1: UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-25 Copyright National Academy of Sciences. All rights reserved.

76 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space ive solutions that enhance and accelerate the Pursue increasingly ambitious objectives and innovat observation and analysis to the nation and to the science/applications value of space-based Earth world in a way that delivers great value, even wh en resources are constrained, and ensures that further investment will pay substantial dividends. A visionary overall strategy is critical for r esponding to such a difficult challenge. Succeeding requires cost-effectively expanding the benefits of Earth science and the resulting Earth information. Doing so means addressing strategic issues: those high-level issues common to all programs and program elements within each agen cy and across agencies. Rising to this challenge requires innovation, not ju st doing things the way we have in the past but both aggressively implementing new means to be efficien t and effective in how we work. This ensures we address shortfalls in how things are done today and anticipate opportunities to improve given a changing context for the future. As a result, the committee e ndorses a strategic approach to the U.S program of Earth observation, as summarized in the recommenda tion below and detailed in the following text. Recommendation 2.1: Earth science and applications are a key part of the nation’s information infrastructure, warranting a U.S. program of Earth observations from space that is robust, resilient, and appropriately bala nced. NASA, NOAA, and USGS, in collaboration with other interested U.S. agencies, should ensure efficient and effective use of U.S. resources by strategically coordinating and adva ncing this program at the national level, as also recommended in ESAS 2007. Implementation of this recommendation is discussed in the following two sections of the report. Toward a National Strategy ESAS 2007 made two key recommendations promo ting national leadership and a national strategy for Earth observation: The U.S. government, working in concert with the private sector, ESAS 2007 Recommendation. academe, the public, and its international part ners, should renew its investment in Earth- observing systems and restore its leadership in Earth science and applications. The Office of Science and Technology Policy, in collaboration ESAS 2007 Recommendation. ith the scientific community, should develop and with the relevant agencies and in consultation w implement a plan for achieving and sustaining global Earth observations. This plan should recognize the complexity of differing agency ro les, responsibilities, and capabilities as well as the lessons from implementation of the Landsat, EOS, and NPOESS programs. Considerable progress toward this national stra tegy has occurred since, but we are far from what was originally envisioned. This progress needs to continue in order to serve critical national and societal interests. As an exampl e, the nation’s economic and security interests in the applied use of Earth information have been poorly articulated within U.S. policy. Consequently, U.S. agency responsibilities remain unclear in many areas. The resu lt is that the value of the nation’s investments in Earth observation are not fully r ealized. This is particularly the case with regard to climate, for which the most recent National Space Policy in 2010 emphasized “climate change research and 34 and “climate monitoring” within NOAA sustained monitoring” within NASA , but responsibilities and budgets for climate research a nd monitoring have since shifted substantially, as described below. 34 https://obamawhitehouse.archives.gov/sites/de fault/files/national_space_policy_6-28-10.pdf ). UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-26 Copyright National Academy of Sciences. All rights reserved.

77 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Important progress during the decade has been a r esult of the leadership of the National Science and the and Technology Council which produced the 2013 National Strategy for Civil Earth Observations ensuing 2014 National Plan for Civil Space Observations . These documents defined categories of observations, identified the important observation type s within each category, and codified the agency roles for implementing them. To be effective, the U.S. civil strategy needs to be further coordinated with strategies in the defense and intelligence agen cies, and supported with adequate funding. of the civil strategy has occurred through the Additional clarification regarding various aspects gard to responsibilities and budgets for climate budgeting and legislative processes, notably with re research and monitoring. The President’s FY 2011 B udget Request re-allocated many climate-related observing responsibilities from NOAA to NASA as part of an administration initiative called the NASA Climate-Centric Architecture, but without concomitant budget shifts. Budget-driven clarification on the 36 35 the President’s FY 2016 Budget Request roles of NASA and NOAA was included in and in the 2016 37 Senate and House Appropriations Committee Re ports. Most recently, the Weather Research and 2017 to “prioritize improving weather data, modeling, and Forecasting Innovation Act of 2017 requires NOAA computing, forecasting, and warnings for the protec tion of life and property and for the enhancement of the national economy” in the conduct of its research. Other budget-driven policy constrains possible in ternational partnerships . The 2011 Department of Defense and Full-Year Appropriations Act bega n an annual process of restricting NASA from bi- lateral collaboration with China (Hester, 2016), whereas bilateral collaboration between NOAA and China is allowed under the Atmosphere Protocol of the U.S.-China Agreement for Science and 38 Technology originally signed in 1979 . By its nature, a national strategy for Earth ob servation involves collaboration with other nations ominent coordinating bodies are the Coordination and international coordinating bodies. Among the pr Group for Meteorological Satellites (CGMS), the Committee on Earth Observation Satellites (CEOS), Systems (GEOSS). Some, such as Committee on Space and the Global Earth Observing System of ce (COSPAR), even produce their own decadal Earth- Research of the International Council for Scien system science plans (Simmons et al., 2016). The U.S. both receives data from other satellites through these collaborations and has obligations to provide its satellite data to other nations. The Program of Record (Appendix A) considered by this committee to represent the foundation of the next decade’s observing system includes a significant set of internati onal satellites formally relied on by the U.S. within its national Earth observing strategy. Despite successe s in international collaboration, access to data from non US satellites in many cases still presents challeng es for US science and applications uses for reasons of policy (as described above) or data quality. In addition, various non-governmental advisory bodi es provide strategic guidance that is valuable to a U.S. national strategy. A 2009 report addressi ng the entire breadth of U.S. civil space activities (NRC, 2009) listed one of its six strategic goals as “re establish leadership for the protection of Earth and its inhabitants through the use of space resear ch and technology”. A 2011 report addressed the boration on Earth observation sate impediments to inter-agency colla llites (NRC, 2011), and a 2012 report 35 “The FY 16 President’s Budget supports NOAA’s broad environmental mission and redefines NASA and NOAA earth observing responsibilities whereby NOAA will be responsible for satellite missions that directly contribute to NOAA’s ability to issu e weather and space weather forecasts and warnings to protect life and property.” https://obamawhitehouse.archives.gov/site s/default/files/national_space_policy_6-28-10.pdf 36 Senate Report 114-66 accompanying FY 2016 CJS Approp riations: “ NOAA is direct ed to prioritize satellite programs directly related to weather forecasting and that result in the great est reduction of risk to lives and property”. House Report 114-130 accompanying CJS Appropriations for NOAA: “The Committee recommendation t Polar Satellite System (JPSS) and Geostationary Operational Environmental focuses limited resources on the Join Satellite (GOES) program in light of th eir role in ensuring accurate and tim ely weather forecasts and warnings.” 37 Similar to the 2016 language, the 2017 House (114-605) and Senate (114-239) CJS Appropriations Reports both direct NOAA to prioritize satellite program s directly related to weather forecasting. 38 http://www.nesdisia.noaa.gov/developingpartnerships.html UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-27 Copyright National Academy of Sciences. All rights reserved.

78 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space r Service regarding actions it should take to (NRC, 2012) provided guidance to the National Weathe progress. defense industry is essential to accomplishing the Finally, engagement of the U.S. aerospace and priority measurement of this Earth science decadal survey. The use and leveraging of industry provides the U.S. economy (NRC, 2009), and was one of the extensive advantages to the agencies involved and to three major thrusts of the 2012 NRC recommendation for advancing the National Weather Service (NRC, observational satellites, U.S. industry has been a 2012). Beginning with the first Earth weather and nd implementing agent for the execution of a wide variety of Earth reliable and constructive partner a science missions. Greater industry participation, including an increasing emphasis on commercialization within the Earth science enterprise, is expected in coming decade. These benefits are expected over a wide range of scales as a result of increased compe tition, and from expanded use of public-private partnerships, data buys and other innovative ac quisition models. New entrants and the industrial capabilities in the commercial mark etplace are also expected to bring increasing opportunities for technology infusion and cost savings. As th ese opportunities become increasingly available, governmental agencies need to ensu re that commercial data meet the quality standards required for ularly in the area of climate observations for which scientific analyses and operational applications, partic to characterizing and understanding change. accuracy, precision, and stability are critical Strategic Challenges Each decade presents new opportunities and issu es. For the coming decade, optimizing the nation’s investments to achieve a successful Earth observation program, in the expected context of constrained resources, means we must do some things differently from the past. The current programs of NASA, NOAA, and USGS reflect several regarding issues that are not being strategic shortfalls adequately addressed. As long as they remain unresolved, we will not be able to achieve the full value for the nation of space-based Earth observations. Observations Continuity (Scientific and Applications). The 2013 National Strategy for Civil Earth Observations clearly define the category National Plan for Civil Space Observations and the 2014 of sustained observations, category (including those related to the important observations within that em. Despite these recent policy clarifications, a climate), and the agency roles for implementing th 39 national commitment to implementing sustained observations is lacking and funding is insufficient to match needs. Overall, it is not clear what roles NASA, NOAA, and USGS play in sustaining long-term space-based observations. Shifting responsibilities, particularly for climate observations, have exacerbated the confusion over agency roles. The co mmitment in Europe provides a strong contrast; the 40 European Union formally committed in 2014 to Copernicus, a long-term, user-driven Earth observation and monitoring program focused on the delivery of n ear-real-time products and services to meet a broad range of societal needs (Box 3.2). This is a commitment not just by a nation, but by the European Union, recognizing that the investment is returned many tim es over in the value it provides to its population and 41 business community. NASA and NOAA participate in Copernicus as pa rtners together with ESA, EUMETSAT and the the climate record of sea level rise. NASA, NOAA and EU in Sentinel-6, a series of satellites to continue USGS should continue to interact with ESA, the EU and other international space agencies to identify 39 See further discussion in Chapter 4. 40 Securing the Copernicus programme—Why EU earth observation matters, European Parliament Briefing, 12pp, April 2017, http://www.copernicus.eu/sites/de . fault/files/library/EPRS_BRI_Copernicus_matters.pdf Copernicus includes a space component and six series of Sen tinel missions are expected to be fully operational by 2023, collecting continuous, consistent observa tions of the Earth for at least a decade. 41 http://www.esa.int/Our_Activities/Observ ing_the_Earth/Copernicus/Overview4 UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-28 Copyright National Academy of Sciences. All rights reserved.

79 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space as the basis for further collaborative implementation, shared interests in the continuity of observations Copernicus. It is not just an operational building on a framework such as that established by meteorological and land imaging program, but an oper ational Earth observation program. The U.S. has no comprehensive equivalent beyond indi vidual elements such as Landsat. This report addresses three separate US agencies that manage civil Earth Fragmentation. ze Earth observation for civilian, military, and observations from space. Other US agencies utili intelligence purposes. Even within agencies, Earth ob servation can be fragmented. In NOAA for example, observation is mostly separated from the research a nd operational users of the data, and weather is organized separately from oceans. Improving resear ch-to-operations and research-to-end-use have historically faced significant challenges resu lting from institutional stove-piping (NRC, 2000). Fragmentation of roles and responsibilities with US ci vil Earth observation is a growing issue that will impede progress if not addressed. National Commitment. A U.S. commitment to Earth observation in support of economic and security progress would provide the stability and priori tization required for efficient long-term planning. ESAS 2007 recommended that Earth information be elev ated to a national strategy, a recommendation that has only been partly implemented and deser ves further attention (NRC, 2007). Today, such commitment remains inconsistent across agencies, incomplete within agencies, and lacking long-term perspective. Managing within Resources. While NASA has done an excellent job in developing a program to address the highest priority science needs and objectiv es, the US civilian space program has insufficient resources to appropriately serve the needs of the nation (for the example of NASA, see Figure 1.5). The resource limitations force a trade between sustaine d observations needed to characterize and understand changes in the Earth system, and new capabilities aime d at understanding key Earth system processes that directly impact national and societal interests. While restoring appropriate resources is the first choice, the rategies targeted to greatest success, including new realism of budget constraints implies a need for st ways of doing business, in the face of inadequate resources. Government agencies have many requ irements and constraints that limit Promoting Innovation. their ability to innovate. While some constraints may be appropriate, in today’s environment innovation has become central to progress. Strategies that break down barriers to innovation are needed, and leveraging of external innovation must be embraced. Programmatic Impediments and Vulnerabilities Strategic challenges are often related to tactical, more immediate issues having direct impact on effective program implementation. Of particular importan ce are institutional and cultural impediments or vulnerabilities that either currently exist or may arise during the next decade. Some of these are internal to the programmatic structure, with strong potential for improvement given recognition of the issue and attention to solutions. Others are ex ternal and require planning that an ticipates events or decisions outside of programmatic control. Important examples include:  Funding that is insufficient to address program priorities.  Lack of mechanisms to effectively restructure the overall program, in a manner that faithfully reflects community priorities, when changes are required.  Changes to the Program of Record due to changes in funding or direction by Congress, or due to changes in partner plans (in particular, the US is heavily reliant on the ongoing European UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-29 Copyright National Academy of Sciences. All rights reserved.

80 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space investment in the Copernicus program, which has become an increasingly important complement to the US program). Policy-mandated limitations on collaboration and/or data exchange with potential international  partners or data providers.  Lack of committed resources to collect the specifi c sustained observations needed to monitor and understand the Earth as a system (summarized in general Finding 2D).  Institutional and cultural impedime nts to interagency cooperation.  Policy, regulatory, and budgetary barriers to integration of commercial technologies and data sources.  Overreliance on unproven and unlikely technology a dvances that introduce risk and cost growth. Cost growth due to project or mission  scope creep and/or poor cost control.  to re-flight decisions and potentially additional Unanticipated launch or on-orbit failures that lead budget obligations. These issues are relevant to development of strategies in this chapter. Equally for the decade, as discussed important, however, is addressing or overcoming spec ific impediments to more effective programmatic implementation. The new programmatic approaches propo sed in Chapter 3 and Chapter 4 are specifically designed to accomplish that, consistent with the D ecadal Community Challenge presented in Chapter 1. Strategic Innovation This section describes a strategic framework, empl oying eight elements (Table 2.4), which can help the community meet our ambitious Decadal Comm unity Challenge. It is in tended to achieve three objectives: a) overcome strategic shortfalls from the p ast, b) help avoid new strategic shortfalls (cross- agency and cross-program) that threaten to emerge during the decade, and c) ensure readiness to take and improvement that arise during the decade. advantage of unplanned opportunities for advance TABLE 2.4 Elements of a decadal strategic framework. ELEMENTS OF DECADAL STRATEGY Sustained Science and Applications 1. Commit to 2. Embrace Innovative Methodologies for Integrated Science/Applications Amplify the Cross-Benefit of 3. Science and Applications 4. Leverage External Resources and Partnerships Institutionalize Programmatic Agility and Balance 5. 6. Exploit External Trends in Technology and User Needs 7. Competition Expand Use of 8. Ambitious Science , Despite Constraints Pursue Strategy Element 1 - Commit to Sustained Science and Applications . Science generally progresses initially by a first step of exploration th time-limited process: define at leads to discovery. Often, this is a and pursue an exploration (such as a space-based missi on), publish the resulting science, seek any societal benefits that emerge from the science, and move on to new scientific exploration areas. With this time- limited approach, societal benefits are often ad hoc spin-offs of this process, not explicitly planned but achieved through after-the-fact efforts on ce the societal value is recognized. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-30 Copyright National Academy of Sciences. All rights reserved.

81 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Today, we have come to recognize the important additional discoveries that are obtained when science and applications, Earth science proceeds beyond initial exploration and commits to sustained 42 enabled by continuous observing over periods requiring multiple generations of observing spacecraft . For example, continuous space-based observations enable the understanding of change in the Earth system occurring over longer time-scales than a single spacecraft lifetime. With sustained science and applications, the outcome of an initial exploration (such as a space-based mission) is reviewed for the additional science and applications. In some potential that follow-on missions could produce valuable cases, the additional science involves new discoveries. In others, new science and/or applications emerge as a consequence of extending the length of the observation over multiple years and decades. By further ensuring that sustained science involves planned implementation of applications, the latency between performing science and achieving so cietal benefits is re duced. Among NASA’s science portfolio, Earth Science is unique in the benefits that can be obtained from commitment to sustained science and applications. Sustained observations are cen tral to the progress of Earth science, and integral to the long-term achievement of societal benefits . As noted earlier, the European Union has embraced sustained science and applications, through its Copernicus program and the underlying Sentinel space- based observations (also described in more detail in Box 3.2). To achieve the needed commitment to sustaine d science and applications, and to achieve the greatest value from the nation’s investments, need be better defined and roles and responsibilities implemented for each agency, and resources need to be included in the budgeting process to allow the fulfillment of continuous observations. The importance of a sustaining a growing list of measurements for science and applications (see, for example, Box 4.5) and the lack of accompanying growth in the budget suggests that international collaboration must play a key role in any strate gy for sustained observations. ESA, EUMETSAT, the European Commission, NASA, and NOAA have recently agreed to ensure the measurement record of two future missions via the Sentinel-6 missions global and regional sea level change through at least approach to identifying measurements requiring long- within the Copernicus Programme. A systematic those are of interest to potential international term continuity (e.g., following NAS 2015) and which of determine whether similar agreements and/or frameworks are viable as partners should be undertaken to the basis for collaboration on implementation sustained measurement programs. Recommendation 2.2: NASA—with NOAA and USGS partic ipation—should engage in a formal planning effort with international partners (including, but not limited to ESA, EUMETSAT, and the European Union via its Copernicus Program) to agree on a set of measurements requiring long-te rm continuity and to develop collaborative plans for implementing the missions needed to satisfy tho se needs. This effort to institutionalize the sustained measurement record of required para meters should involve the scientific community, and build on and complement the existing dome stic and international Program of Record. Strategy Element 2 - Embrace Innovative Methodol ogies for Integrated Science and Applications. al methodological advances that are shared across One means to accelerate progress is to seek fundament disciplines and pursuits. These hold the potential to advance the field or organization as a whole. Examples include, but are not limited to: 42 The multi-decadal Landsat (now Sustained Land Imaging) program is one example. It’s 30+ year continuous data set has proven critical for understa nding the evolution of Earth’s surface, and for managing resources on that basis. Many examples of the need for sustained science and applications emerged from the Earth Observing System (EOS) program starting in the 1990s, in cluding policy-critical areas such as sea level rise, water availability, and transport of pollutants. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-31 Copyright National Academy of Sciences. All rights reserved.

82 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space : Low-cost observations and methodologies  Advanced cost-effective observation methodologies can be used to enhance and/or augment invest ments in space-based data. Examples include: a) citizen science and community-based observations, b) ad hoc and distributed observations, as from existing ground networks, automobile senso rs, and mobile phones, and c) observational sampling using compressive sensing (see Box 2.10).  : Investments in innovative an alysis capabilities accelerate the Advanced analysis methodologies ability to convert observations into scientific knowledge. Candidates include: a) data science, including big data analytics and other techniques emerging in the commercial world, b) a more integrated data analysis system that includes advances in modelling and assimilation of in situ data and data from multiple satellite sensors.  Accelerated applications : Accelerating the conversion of science into societal benefits amplifies the societal impact. Candidates include: a) applications included from the early stages of observation planning and development, b) rapid appl ications prototyping, c) rapid transition from ence of applications, to advance applications science to applications, and d) promoting the sci methodologies (Dozier and Gail, 2009). Strategy Element 3 - Amplify the Cr oss-Benefit of Science and Applications . Curiosity-inspired science will always be central to Earth observation a nd analysis. But a growing portion of our science is use-inspired or closely related to the applications it enables. The traditional paradigm for integrating science and applications can be described as pursu ing high quality and innova tive science, and then assume it will somehow find a path to app lications. Sometimes referred to as the valley of death between science and end-use (for example: NRC, 2000), or between research and applications, the issue is widely recognized even as this paradigm has been slow to evolve. cations scientists and engineers, and end-use Inspiration goes both ways: science inspires appli needs can inspire research scientists and engineers. Embedding science in the applications process often driven by those end-uses not well-recognized by reveals new and inspirational scientific questions research scientists. While we often select our pursuits by using this science-applications thinking in an implicit way, doing so more explicitly can lead to improved outcomes, particularly when resources are constrained. ********************************************************************************* BOX 2.10 Less-Costly Observing Syst ems through Optimized Sampling One example of a promising observational methodology that could reduce system cost or improve performance is compressive sensing . It is a technique for utilizing data processing to reconstruct relatively complete observational data sets from sparse measurements of those data sets. If this, or other techniques, can be used to reduce the number of space-based obser vations needed to adequately sample some aspect of the Earth system, it might be possible to build sat ellite systems with fewer satellites or less complex instruments. The technique has been applied to medical da ta, such as Magnetic Resonance Imaging (MRI) compress the very large MRI data volumes by large (Lustig et al., 2008), using wavelet transforms to amounts and translating that reduction into reduced s can times that benefit patients. MRI imagery is naturally compressible because it is not acquired in the domain of the spatially-oriented image itself but rather in a domain that is readily transformed using wavelets. Artifacts associated with data recovery need to be assessed on the basis of th e sensitivity of each end-use case. characteristics that make compressive sensing a Remote sensing imagery shares some of the candidate technique for MRI. This potential has been explored by the Department of Defense (JASON, 2012) for a variety of remote sensing data sets with di fferent characteristics (see example in Figure 2.9). Recently, it has been explored as a means for reduc ing data volume in Earth observation data sets (Ebtehaj et al., 2015), such as global observations of temperature and humidity fields from AIRS and AMSU since they admit nearly sparse representations in the wavelet domain. If fields such as this can be UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-32 Copyright National Academy of Sciences. All rights reserved.

83 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space mplified observing systems may be possible. Things reproduced with fewer space-based observations, si h remote sensing. For example, different observations are often more complicated in the real world of Eart will be more or less amenable to sampling methods . Without further work, this technology should be approaches are still possible, even if it cannot yet considered a promising example of how new observing be considered a proven solution. FIGURE 2.9 An uncompressed image (left), its wavelet coefficients (center), and a JPEG-2000 recovered image (right) made using compressive sensing with on ly 10% of the wavelet coefficients (from JASON, 2012). Although this laboratory example is not fully a pplicable, the ability to nearly reproduce original data with far fewer samples holds the pot ential for lower cost observing systems. ********************************************************************************* END OF BOX Among NASA’s diverse and inspirational scientific elements, Earth Science is special in the extent and breadth of its practical benefits to society. To its cr edit, NASA has increasingly integrated applications into flight programs and research, with results that have been embraced by both the science and applications communities. The SMAP mission has been used as a prototype for a more integrated trend will strengthen tending and expanding on this science/applications team, with positive results. Ex both science and applications. To accomplish this, pr ograms with both science and applications elements need to explicitly identify the connection, and defi ne opportunities to amplify the cross-benefit, and organization structures and processes need to be adapted when possible to integrate, rather than segregate, 43 science and operations/applications. Strategy Element 4 - Leverage External Resources and Partnerships . In a constrained resource environment, much can be done by leveraging resources. NASA, NOAA, and USGS have long- established partnerships with non-U.S. space agenci es and other organizati ons, which have already proven highly valuable in bringing additional resources to address their missions (further discussion is included in Chapter 4). In many cases, they also en able access to regional and global observations that would simply be unavailable to US agencies any other way. 43 Through its Earth Venture—Instru ments solicitation, NASA recently announced its first competitively- selected mission with societal benefit as its primary objective. The Multi-Angle Imager for Aerosols, MAIA, will investigate the connections between aerosols and human health. From the very beginning, MAIA has involved collaborations with the Environmental Protection Agency, National Institutes of Health, Centers for Disease Control and Prevention, National Oceanic and Atmospheric Administration, and World Health Organization. https://www.jpl.nasa.gov/missions/web/MAIA_Liu&Diner_PublicHealthReports.pdf UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-33 Copyright National Academy of Sciences. All rights reserved.

84 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Today, there is a strong need to build on and exte nd those partnerships, and to bring in innovative new partnerships such as commercial data sources. In particular, there is a need to: a) extend and strengthen the already strong interna tional partnerships, and b) levera ge the availability of commercial providers for resources traditionally supplied by g overnments. Specific suggestions for accomplishing that are addressed in this report. Strategy Element 5 - Institutiona . The demands we will lize Programmatic Agility and Balance lems to be solved in order to a face in the coming decade, and the prob ddress them, will arrive at an ever- increasing pace as populations grow, human impacts on the environment continue to increase, and society’s digital information use broadens. NASA, NOAA, and USGS will need to make both large and small programmatic adjustme nts over short time periods. Agility in programmatic structures, and in the authorities of staff who implement programs, is es sential to respond to new discoveries and emerging traints. At the same time, achieving and maintaining needs, particularly in the context of resource cons is critical to successful programs. programmatic balance Agility and balance do not emerge naturally in or ganizations. They must be explicitly built into the cultures and processes or they risk being ove rcome by bureaucracy. For example, the software industry moved from the traditional pre-planned “wat erfall” model to agile software management techniques to more rapidly and effectively adva nce their products (Kettunen and Laanti, 2008). t cycle for space-based observations can be as With NASA, NOAA, and USGS, the developmen long as a decade and more, impeding the ability to be responsive to changing needs and emerging science. NOAA and USGS, in their portion of the committee’s statement of t ask, have specifically sought suggestions for being more agile in terms of inte grating new science and technology. This agility is achieved in part through a balanced portfolio that incorporates both long-lead missions and activities as ive to and take advantage of emerging capabilities well as shorter-term efforts that can be more respons and opportunities. Trends in Technology and User Needs . Successful Strategy Element 6—Exploit External tively incorporate them in their organizations formally review and track key enabling trends and proac 44 activities ticipating and leveraging trends has been . Within NASA, NOAA, and USGS, success at an 45 episodic ; the agencies have been slow at times to leverage external capabilities that could have enhanced their capabilities. For NASA, NOAA, and USGS, a successful process for exploiting external trends might include, at minimum, a survey of: a) advances in scientific methodologies from outside these agencies; b) commercial methods for characterizing the diverse a pplications and information end-uses of data; c) observation technology advances in the commercial s ector; d) computing and data methodologies and tools that enable new data analysis approaches; e) community science, such as crowdsourcing and distributed observations, which has the potential to augment space-based observations; f) non-traditional partnerships such as philanthropists and non-profits; g) innovation in public-pri vate partnerships and buys, standardized spacecraft, and system block buys; and h) human acquisition alternatives such as data camps” used widely today to rapidly educate resources and education methods (such as the “boot king the workforce more effective. software engineers) targeted at ma For example, Item (b) reflects the fact that there are so many end-uses of NASA/NOAA/USGS data that the agencies no longer can simply track st raightforward metrics like grants or website data 44 moving internet field is well known. A source widely The importance of anticipating trends in the rapidly cited is industry analyst Mary Meeker, currently at the venture capital firm Kleiner Perkins, who has publicly released an annual report on internet trends for many years. The 2017 version is at http://www.kpcb.com/internet- trends. 45 There are notable exceptions. NASA has invested for many years in technology advances through the ESTO program, seeking to leverage technology progress with in the community, and has embr aced use of small satellites and funded advances in small launch vehicles. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-34 Copyright National Academy of Sciences. All rights reserved.

85 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space is the same problem faced throughout the commercial requests to know how their data are used. This et leaders such as Google, for unders tanding their user base. Since there internet world, including by intern is a clear financial stake in this information, companies are investing in developing solutions that involve more sophisticated metrics and tools, including big data techniques, federal agencies with similar needs can benefit from these investments. . Competition has already proven effective in Strategy Element 7 - Expand Use of Competition many areas of science and procurement for NASA, N OAA, and USGS as a means of inspiring innovation and creativity and delivering cost-effective appro aches to Earth observation. The committee believes these results can be extended even further for Earth sci ence, and we have embraced the use of competition within the structure of our recommendations (See Ch apter 3). Competition and collaboration (as noted in Strategy Element 4) are not necessarily in conflic t, and both should be used as appropriate. ience/Applications, Despite Constraints . Strategy Element 8 - Pursue Ambitious Sc Constraints do not imply a need to be timid. Th e committee believes that pursuing ambitious science not only leads to the greatest scientific advances, it ensures the greatest likelihood that substantial and often unanticipated societal benefits will emerge . NASA, NOAA, and USGS need to build on their history of pursuing ambitious programs to serve the nation, even when faced with resource challenges. While this requires appropriate scoping to respect constraints, it does not require losing the ability to think big. Maintaining ambition can be accomplished by: a) setting clear and far- reaching goals within all planning processes; b) exp licitly identifying mechanisms that might allow these goals to be pursued despite resource constr aints, such as creative implementation approaches; and c) pursuing ambitious observation system cap abilities, such as active sensing systems, while ed technology development as needed. ensuring acceptable risk through target REFERENCES Anav, A., P. Friedlingstein, C. Beer, P. Ciais, A. Ha rper, C. Jones, G. Murray-Tortarolo, D. Papale, N.C. Parazoo, P. Peylin, S. Piao, S. Sitch, N. Viovy, A. Wiltshire, and M. Zhao. 2015. Spatiotemporal patterns of terrestrial gross primary production: A review. Rev. Geophys. 53: 785–818. Bauer, P, Thorpe, A and Brunet, G (2015). The qui et revolution of numerical weather prediction. Nature doi.org/10.1038/nature14956. 525(7567): 47–55, DOI: https:// A. Hostetler, D.A. Siegel, J.L. Sarmiento, J. Behrenfeld, M.J., Y. Hu, R.T. O’Malley, E.S. Boss, C. Schulien, J.W. Hair, X. Lu, S. Rodier, A.J. Scarino, 2017: Annual boom–bust cycles of polar phytoplankton biomass revealed by space-b ased lidar, Nature Geoscience, 10, 118–122, doi:10.1038/ngeo2861. Berger, M., J. Moreno, J. A. Johannessen, P. F. Le velt, and R. F. Hanssen. 2012. ESA’s sentinel missions in support of Earth system science. Remote Sensing of Environment 120:84-90. Bourassa, M. A., S. Gille, D. L. Jackson, B. J. Robert s, and G. A. Wick. 2010. Ocean winds and turbulent air-sea fluxes inferred from remote sensing. Oceanography 23(4): 36-51. A. Clayson, I. Cerovecki, M. F. Cronin, W. M. Bourassa, M. A., S. T. Gille, C. Bitz, D. Carlson, C. Drennan, C. W. Fairall, R. N. Hoffman, G. Magnusdottir, R. T. Pinker, I. A. Renfrew, M. Serreze, K. Speer, L. D. Talley, and G. A. Wi ck, 2013. High-latitude ocean and sea ice surface fluxes: Challenges for climate research, Bull. Amer. Met. Soc. , 94 , 403-423. Cai, Hongyan, Xiaohuan Yang, Kejing Wang, and Linlin Xiao. “Is forest restoration in the Southwest China Karst promoted mainly by climate change or human-induced factors?.” Remote Sensing 6, no. 10 (2014): 9895-9910. Chen, X., X. Zhang, J.A. Church, C.S. Watson, M. A. King, D. Monselesan, B. Legresy and C. Harig. 2017. The increasing rate of global mean sea-le vel rise during 1993–2014. Nature Climate Change 7, 492–495. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 2-35 Copyright National Academy of Sciences. All rights reserved.

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91 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space 3 A Prioritized Program for Science, Applications, and Observations itized list of top-level science and application The committee was charged with developing a prior objectives to guide space-based Earth observations over the next ten years, and identifying gaps and opportunities in the programs of record at NASA, N OAA, and USGS in pursuit of those top-level science and application challenges. This chapter describes the process used by the co mmittee to identify and prioritize observational needs, and defines a robust a nd balanced U.S. program of Earth observations from space consistent with agency-provided budget expectations. The resulting program, built on the foundation of the US and international programs of record, addresses exciting and societally-relevant challenges in Earth system science while providing the programmatic flexibility needed to leverage innovation and opportunities that o ccur on sub-decadal timescales. THE ESAS 2017 PRIORITIZATION PROCESS Community Input Prior to the start of the decadal survey , the standing Committee on Earth Science and Applications from Space (CESAS) issued the first Request for Information (RFI-1) to the community, soliciting white paper submissions describing key cha llenges in Earth System Science. In addition to providing important input into the identification of ma jor challenges that can be substantially advanced through space-based observations, the r esponses informed the structure of the panels that were established by the steering committee, called for a submittal by the steering committee. A second RFI (RFI-2), issued objectives) that promise to substantially advance of, “specific science and applications targets (i.e. understanding in one or more Earth System Scienc e themes.” Some three-hundred white papers were anning all areas of Earth Science. submitted in response to the two calls, sp Approach and Process The 2017 decadal survey was led by a steering comm ittee and supported by five interdisciplinary panels. Steering committee members were selected to represent the broad Earth system science and applications community. Process. The steering committee, in close collabor ation with the panels, developed and implemented a process for establishing and determining the resulting Science and Application Priorities Observing System Priorities required to address them. The steps that were used to converge from a large set of possibilities to a final, small set of priorities, and the roles of the community, committee, and panels are shown using the analogy of a narrowing pyramid in Figure 3.1 and are discussed further below. From the hundreds of suggestions for science and appli cations priorities submitted through the RFI process and/or considered by the panels, onl y a much smaller number (103) were considered as formal priorities, and only a small portion of those (24) we re ranked among the highest priorities. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-1 Copyright National Academy of Sciences. All rights reserved.

92 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Recommended ‐ ESAS A Appendix Observing System 17) of Program Table ‐ Priorities 2017 2027 ‐ 8 Record 3.5 Fundamental to Targeted (2016 of many the achieving Observables prioritized science in implemented to be applications and & science priority support objectives (of applications 22 objectives candidates) final Observable Committee Objectives satisfied by Recommended ESAS ‐ 3.3 Table space existing ‐ of 103 24 Science/Applications Steering based (2016) & Science Applications Objectives observations Priorities 2027 2017 ‐ as identified Important Most Panels ‐ SATM B Appendix & Science 103 Applications Objectives supporting Questions & Science 35 Applications – D Appendix total 290 2016) 2015 Responses RFI Community describing related observations and science & desired applications May (Oct Community crementally establish priorities, shown using the FIGURE 3.1 The steps used by the committee to in with two Requests for Information (RFIs—bottom analogy of a narrowing pyramid. The process began e first RFI informed the the Earth science community. Th dark gray panel) that were distributed broadly to community to submit ideas for specific science and organization of the survey; the second invited the rstanding in one or more applications targets (i.e., objectives) that promised to substantially advance unde , panel inputs, and key reference documents, the Earth System Science themes. Drawing on the RFIs cations priorities (shown in blue). Panels then steering committee then established Science and appli developed a Science and Applications Traceability Matrix (SATM), from which observing system priorities (shown in green) were identified and compar ed with the Program of Record to determine unmet observation needs, defined as Targeted Observables. Hundreds of suggested science and applications priorities were considered, with the 24 ranked as highest priority having the strongest influence on the Observable opportunities identified as candidates for observing program selections, leading to 8 Targeted flight implementation. As summarized in this report’ s Preface, the committee’s SOT Addressing the Statement of Task. requested that priorities focus on science, applications , and observations, rather than the instruments and missions required to carry out those observations . The SOT described a multi-step requirements development process, diagramed in Figure 3.2, l eading from science to observations through a step 1 science targets referred to in the SOT as . A science target, as defined in the SOT, is “a set of science objectives” related by a common space-based obser vable. The committee defined the observable science target as a Targeted Observable. associated with each simplifying the presentation of its priorities, the In accordance with the SOT, and with the goal of committee chose to focus on two key elements of th is sequence for prioritization: a) the Science and Applications (blue, corresponding to Table 3.3), and b) the Targeted Observables (green, corresponding to Table 3.5). Example measurements and missions we re identified and evaluated only for the purpose of 1 in keeping with the nature Interpreted by the committee more broadly to be science and applications targets, of the report. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-2 Copyright National Academy of Sciences. All rights reserved.

93 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space eted Observables recommended within the ensuring cost and technical read iness feasibility for Targ NASA program, as required by the SOT. ESAS 2017 PRIORITIZED PROGRAM and (summarized Program complemented , 3.5 by of 3.3 Tables in Record ) (prioritization process performed by Committee, informed by RFI submissions) Measurements & Instruments/Missions Science & Applications Observables observables Corresponding in ADDRESSED decade’s next of Program Record Program of Record (Appendix A) A Objective 1a ‐ ‐ A Objective 1b Question A Objective ‐ 1c A ‐ 1 Measurement Measurement E) Approach A Targeted Science & Implementation ‐ Objective 1x A Submissions Observable Target 1 Applications 1 or (Instrument Measurement Objective A ‐ 2a Mission) RFI Question B Approach Science & Targeted ‐ 2 A Objective 2y A ‐ Observable Target Applications 2 2 (Appendix Objective A ‐ #a Question Targeted Science & # ‐ A Community ‐ Objective #z A 2 Observable K Target Applications ESAS 2017 ESAS 2017 Observing System Science & Applications Table Priorities Table Priorities 3.5) (Table 3.3, Appendix ) B (Table Total: Targeted 22 Observables in steps sequential Key RFI Questions : 35 Science/Apps refinement of candidate Submissions: Total for Science/Apps 5 Supporting Objectives : 103 Flight: to Commitment Recommended observing to ideas RFI 139 #1: 7 3 to select 3 ‐ Flights: Recomme nde d for Competitive Down Important: Rank ed Most 24 Objectives system priorities RFI 151 #2: Incubation: for 3 Re commended FIGURE 3.2 A notional diagram of the traceability process used by the committee to address SOT guidance and provide a prioritized program for: a) sci ence and applications (blue), b) needed observables The additional agency commi to fill gaps in the Program of Record (green). tment to the Program of Record is included (gray). Panels. Informed by the first RFI submission, the steering committee constructed a set of five gement in the decadal survey. Panel members were interdisciplinary panels to facilitate community enga sciplinary and interdisciplinary expertise. The drawn from the scientific community based on their di panels, each consisting of approximately 15 members, met 3 times, with the first and last of these meetings being conducted in a “jamboree” format in wh ich all of the panels met in parallel at the same venue to identify and discuss where their science and a pplication priorities intersected. The first panel jamboree also coincided with a meeting of the fu ll steering committee and included joint plenary sessions to identify and discuss science priorities and areas wh ere priorities might intersect. The second jamboree, and each of the stand-alone panel meetings, incl uded participation by steering committee member representatives who helped facilitate communicati ons between the steering committee and panels throughout the study. The five panels were: UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-3 Copyright National Academy of Sciences. All rights reserved.

94 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space I. Global Hydrological Cycles and Water Resources water and how these are changing over time. The movement, distribution, and availability of II. Weather and Air Quality: Minutes to Subseasonal Atmospheric Dynamics, Thermodynamics, Chemistry, and their interactions at land and ocean interfaces. III. Marine and Terrestrial Ecosystems and Natural Resource Management Biogeochemical Cycles, Ecosystem Functioning, Biod iversity, and factors that influence health and ecosystem services. IV. Climate Variability and Change: Seasonal to Centennial Forcings and Feedbacks of the Ocean, Atmosphe re, Land, and Cryosphere within the Coupled Climate System. V. Earth Surface and Interior: Dynamics and Hazards Core, mantle, lithosphere, and surface processes, system interactions, and the hazards they generate. The panel order in this list was chosen by the committee to simplify the presentation of the material and not to reflect any prioritization of the panels. This ordering is maintained throughout the discussion in this section and in various tables thr oughout the report. The panels developed science and applications priorities for their panel topic areas, b ased in large part on the input received through the RFI responses, and further informed by the expertise of the panel members and steering committee liaisons. panel RFI’s are not cited directly in this report, sin ce the intent was to use them as guidance but not to panels were directed to interpret within the report’s priorities. The suggest preference for particular RFI’s their scope broadly, considering the state of scien ce in both their encompassed traditional disciplines as well as with a broader view of Earth system science. Reports of each panel are included as chapters in this report, the important value of which is described in Box 3.1. ********************************************************************************* BOX 3.1 The Role of Panel Reports The important role of panels is identified in The Space Science Decadal Surveys: Lessons Learned and Best Practices report (NRC, 2015), which notes that “The panel reports of a decadal survey have an invaluable role in tracing how the decadal survey prioritized science and identified strategies to achieve science goals and objectives. However, panel re ports have no official standing; the survey report authored by the committee is the only source of consensus recommendations—panel reports typically provide context and history.” The ESAS 2017 steering committee chose to publish the panel reports as part of the same volume, consistent with the lesson learned and best practice identified in the 2015 report:  Lesson Learned: As the best and most detailed record of community input, a decadal survey’s panel reports are a fundamental part of the survey’s work product. It is essential that they be made public along with the final committee report. Publishing the survey committee report and the panel reports together, as has often been done , has the important advantage of providing traceability within one document of the decad al survey process of science and program prioritization. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-4 Copyright National Academy of Sciences. All rights reserved.

95 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space To make clear the utility of panel reports and to reduce ambiguity as to their use,  Best Practice: decadal committees can choose to publish the pane l reports in the same volume as the survey report, adding clear labeling that the panel reports are for reference only. ********************************************************************************* END OF BOX Integrating Themes. The steering committee identified a set of Integrating Themes to complement the panel deliberation process by ensu ring explicit considerati on of broad, thematic concepts which cut across multiple panel domains. Members of the st eering committee and representatives of each panel participated in an Integrating Themes Workshop during which priorities were considered in the context of advancing key aspects of Earth system science (e.g ., the Carbon Cycle, the Water and Energy Cycles, Extreme Events) outside of the traditional panel struct ure. While no separate re port has been prepared from this workshop, the broad thinking of the work shop is reflected in the analysis of observation priorities and the development of the committee’s recommendations. The Integrating Themes developed at this work shop were used early in the decadal survey process to ensure important Earth system priorities we re not missed by discipline-focused panels. Later, the steering committee leveraged this Integrating Themes perspective to ensure the recommended program addressed key system priorities. These them es, and their implications for the committee’s priorities, are discussed throughout this chapter. Budget Assumptions and Cost Assessment Translating the committee’s science and appli cations priorities into an observing program required that the committee assess the li kely cost of the proposed observations to ensure the program can 2 tent with agency expectations be accomplished within a budget consis . Consistent with sponsor input to the decad al survey, the committee adopted a baseline NASA budget scenario that assumes that the budget provided in the ESD Program of Record will grow only at the rate of inflation, as shown in the “sand chart” in Figure 3.3. The cost of the flight missions in the Program of Record (ICESat-2, NISAR, PACE, SWOT, Sentinel-6, GRACE-FO, RBI, TSIS-1/2 and CLARREO PF) from FY18 to FY27 results in a lien of $3.6B from the prior decade (NAS, 2015). This baseline budget then implies a total of $3.4B available to invest in the coming decade’s priorities (FY18- FY27), beyond funds already allocated and assuming ex isting program elements remain unchanged. This value corresponds to the orange portio n of Figure 3.3 It is noteworthy that in this scenario, funding for implementing this decadal survey’s flight prior ities does not emerge until ap proximately FY20, as the flight program’s resources are fully consumed with the POR until that time. 2 The statement of task says “The survey committee will work with NASA, NOAA, and USGS to understand agency expectations of futu re budget allocations and design its recommendations based on budget scenarios relative to those expectations.” NASA ESD provided a budget hi story to the committee and indicated that large scale changes to recent funding levels were not anticipated. The committee thus based its recommendations on the assumption that the current budget would grow with inflation. Decision rules are established in Chapter 4 to describe how the program can be tailored to acco mmodate modest budget shortfalls and how it can best be expanded to take advantage of any additional resources which ma y become available throughout the decade. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-5 Copyright National Academy of Sciences. All rights reserved.

96 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space FIGURE 3.3 Baseline budget scenario assumes the POR budget grows with inflation. Flight program funding for decadal survey priorities is unavailabl e until FY20. The cumulative total budget available for $3.4 billion through FY 27. Labels shown in the flight program investment in ESAS 2017 priorities is s Earth Science Division. Note: ESSP=Earth System legend refer to budgetary components within NASA’ Science Pathfinder The committee notes that its recommendations are provided with a series of decision rules (see Chapter 4) which allow NASA to readily respond with program augmentations consistent with decadal survey priorities to take advantage of any additiona available to support Earth l funds that may be made system science throughout the decade. Similarly, these decision rules provide guidance on how to implement program reductions in the face of reduced resource availability. Responsive to the study’s statement of task , the committee used an independent Cost and Technical Evaluation (CATE) process to ensure concep ts were credible and costs were of comparable fidelity when cost was a factor in prioritization. Drawing from the NRC Decadals Lessons Learned report e the relative scale of ” approach to determin (NRC, 2015), the committee first used a cost “binning investment (i.e., small, medium, large) required for each potential program augmentation prior to down- selecting which program elements required more detailed cost estimation. Full CATE studies were completed by The Aerospace Corporation for explicitl y prioritized program elements, which were binned large (>$500M). as The Program of Record The existing U.S. and international Program of Record (POR) forms the foundation upon which the committee’s recommendations are established, and is summarized in Append ix A. The POR includes NASA, NOAA, and USGS missions formally planned and budgeted per input from these agencies, and those partner missions for which either NASA or NOAA explicitly expressed a commitment to this committee. This appendix also lists anticipated additional space-based obser vation contributions from other space agencies, but the commitments to these programs were not verified by the committee (these non-verified programs have the additional challenge th at even when commitments are real, data may not be reliably available to NASA and NOAA researcher s). Some items in this list (e.g., QuikSCAT) are UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-6 Copyright National Academy of Sciences. All rights reserved.

97 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space h was taken into account by the committee in its known to currently have degraded performance, whic deliberations. To identify gaps in the POR, the steering co mmittee members and panel representatives attended a workshop during which the measurement needs to address priority science and applications objectives in the next decade as identified in the SATMs were reviewed against the POR to determine whether re adequate to meet the stated objectives. Where the POR did not existing or planned measurements we ty objective, participants identified candidate adequately meet the need to address a high priori augmentations to the POR to address the unmet need . Observations not in the POR were aggregated, as summarized in Appendix C, and became the starting point for the committee’s deliberations regarding needed unmet observations. The Program of Record, and reliable funding to ensure its implementation, are particularly important. Earth system science and applications rely on long-term sustained observations of many key components of the Earth system. The POR provides many of the coming decade’s needed continuity vestment coming from internationally coordinated measurements, with a significant portion of that in networks of operational satellites. Two such networks are the meteorological satellites coordinated by the and the more recent Sentinel satellites of the Coordination Group for Meteorological Satellites (CGMS) European Union’s Copernicus Program (see Box 3.2), which together will provide continuity for a broad range of critical Earth observations. The Sentinels will reach full operational status in 2023 and will sustain this observational capability for at least a decade. Given that the U.S. to this operational Earth has no equivalent capability observation and monitoring program in Europe, th e committee recognizes the importance of Copernicus in general and the Sentinels in particular as a l ong-term, continuing source of a variety of important observations. It is clearly in the interest of the U.S. agencies and the research community for the U.S. agencies to ensure that their inves tigators have access to Sentinel observations in a timely manner. If the and the Sentinels, U.S. agencies would benefit U.S. cannot replicate an effort like Copernicus substantially from exploring options for complementing opean effort, such as is and strengthening this Eur being done by NASA and NOAA with the Jason- CS satellite partnership for Sentinel-6. ********************************************************************************* nel Program Provides Key Element BOX 3.2 Europe’s Copernicus-Senti of Continuity in the Program of Record. 3 The Copernicus Program , led and funded by the European Union, represents a long-term EU commitment to deliver near-real-time products and ser vices to help us better understand our planet and sustainably manage the environment in which we live. Copernicus is supported by a family of sate llites—the Sentinels—that have been designed to provide continuous, consistent global data sets on an operational basis. Copernicus Services then will transform this wealth of satellite data into value-added information by processing and analyzing the data, integrating it with data from a variety of comple mentary in-situ sources and validating the results. By the end of 2017, four of the six Sentinel series should be fully deployed; all six will reach full operational status by 2023 and that capability will be maintained for at least a decade (Figure 3.4). (NASA and NOAA are partners in Jason-CS-A and -B, the first two satellites of Sentinel-6.) Discussions are currently underway in Europe concerning a possibl e expansion of the current Sentinels, as well as a long-term scenario for the next generation of Sentinels. 3 Reillon, 2017. Securing the Copernicus programme—Why EU earth observation matters, European Parliament Briefing, 12pp, April 2017, http://www.copernicus.eu/sites/default/files /library/EPRS_BRI_Copernicus_matters.pdf UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-7 Copyright National Academy of Sciences. All rights reserved.

98 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space FIGURE 3.4 The structure and schedule of the European Copernicus Sentinel program. The Copernicus full, free and open data policy (for which the U.S. Landsat program provided a pathfinder example) paves the way for innovative entrep reneurs to create new applications and services to meet societal needs. Corresponding historical datase ts will be made comparable and searchable, thus ensuring the monitoring of changes. By making the vast majority of its data, analyses, forecasts and maps freely available, Copernicus contributes towards th e development of new innova tive applications and services, tailored to the needs of specific groups of users. The Copernicus Program is coordinated and managed by the European Commission. Responsibility for the development of the Copernicus Space Component is delegated to the European Space Agency, while spacecraft operations are sp lit between ESA and EUMETSAT. The in-situ component is coordinated by the European Enviro nment Agency and the Memb er States. The services component involves the European organizations listed in Table 3.1. TABLE 3.1 Organizational responsibilities w ithin Europe’s Copernicus program. Operator Operational Service Main focus Atmosphere ECMWF Jul-15 Air quality, ozone layer, emissions and surface monitoring fluxes, solar radiation, climate forcing Marine safety, coastal environment, marine Marine environment Mercator Ocean May-15 monitoring resources, weather and climate Land monitoring Land cover, use and cover use changes, JRC EEA 2012 vegatation state, water cycle Climate change Climate variables, re-analyses and projections, ECMWF 2017 multi model seasonal forecasts Risk assessments of floods and forest fires, JRC Apr-12 Emergency management impact of naural and man-made disasters Security Border surveillan ce, maritime surveillance, 2015- Early Frontex- EMSA -- EU SatCen support for EU external action 2016 UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-8 Copyright National Academy of Sciences. All rights reserved.

99 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space opean Centre for Medium-Range Weather DATA SOURCE: Copernicus website. ECMWF: Eur ency; EMSA: European Forecasts; JRC: Joint Research Centre; EEA: Eu ropean Environmental Ag rder and Coast Guard Agency; EU SatCen: EU Maritime Safety Agenct; Frontex: European Bo private company. SOURCE: Reillon, 2017. Satellite Centre. Mercator Ocean is a French ********************************************************************************* END OF BOX The Science and Applications Traceability Matrix (SATM) Achieving traceability of both science/applications and observing system priorities was central to the committee’s work. The foundati on for this traceability was the Science and Applications Traceability Matrix (SATM) developed by the steering committee in c onjunction with the panels, with content provided primarily by the panels. It establishes th e traceability from prioritized science/applications to included in Appendix B. A shorter summary of the needed observing systems. The complete SATM is science and applications priorities within the SATM is provided as Table 3.3; this table provides the basis for the ESAS 2017 prioritized science and applications. Development of the SATM was accomplished in f our steps, as shown in Figure 3.2: 1) establish the priority science/applications question or goals , 2) identify a set of objectives (quantified when possible) needed to pursue those questions/goals, 3) determine the observables needed to fulfill those objectives, and 4) characterize the measureme nts available to make the observations. As summarized in Figure 3.1, development of the SATM began with the committee issuing a second community Request for Information (RFI-2) so liciting specific science and applications needs (i.e., specific measurements/observations, or theory and/or modeling activities) that promise to advance existing or new scientific or applications objectives, contribute to fundamental understanding of Earth System Science, and/or facilitate the connection be tween science and societal benefits. The RFI responses provided a basis for panel deliberations, with each panel considering relevant RFI responses as it pplications goals for the decade ah ead. Panels then developed their developed a set of key science and a SATM contributions to capture decadal goals and deve lop them into quantifiable objectives that might be addressed by space-based observations. The prioritization of the Earth Science/A pplications Objectives within the SATM was accomplished using three categories: cal in order to make substantive  MI — Most Important— Refers to Objectives that are criti advances in knowledge in key areas identified by the panel. These are the highest priority objectives that should be pursued even u nder the most minimal of budget scenarios.  VI — Very Important —Refers to Objectives that would cont ribute substantially to advances in knowledge in key areas identified by the panel a nd should be supported, second only to MI. Every effort should be made to accomplish these if resources are available or if they can be done opportunistically as a cost-effective add-on to an existing mission.  I — Important —Refers to Objectives of high value that should be addressed if resources allow or if cost-effective opportunities are found to address them. It is important to note that because observa tions often satisfy multiple objectives, some observations that are targeted at addressing MI prior ities will end up also addressing VI and I priorities as well. A prioritized observing program, focused on achie ving MI science and applications priorities, would be expected to achieve some (or even many ) VI and I priorities at no additional cost. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-9 Copyright National Academy of Sciences. All rights reserved.

100 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Importance Establishing SATM Ranking Methodology for and Applications Objective within the SATM The Importance values ascribed to each Science of each panel, informed by the RFI submissions and available peer- were based on the expert judgement three categories was carried out by the panels and reviewed literature. The allocation of Importance into was accomplished through an iterative process by wh ich the Importance of each Science and Application Objective was established using a score that was normalized both within and across panels. Based on these scores, the Objectives were th en binned into the categories of Most Important, Very Important, and Important. Because of the many possible considerations that can influence the assessment of scientific and applications importance, a rigid framework of sp ecific considerations was not used. However, panels were encouraged to review the cons iderations listed in Table 3.2 to guide their discussions. Each panel, ee to choose its own considerations. The steering and the individual members of that panel, was fr committee monitored the ranking process and concurred with the results. or used to evaluate Science/Applications TABLE 3.2 Considerations for the Importance fact Objectives within the SATM as presented in Appendix B (not in priority order). AREA DESCRIPTION 1. Science Science objectives that contribute to answering the most important basic and Questions stem science. These questions may span applied scientific questions in Earth Sy the entire space of scientific inquiry, from discovery to closing gaps in knowledge to monitoring change. 2. Applications and Science objectives contributing directly to addressing societal benefits Policy achievable through use of Earth System science. scientific disciplines, thematic areas, Science objectives with benefit to multiple 3. Interdisciplinary Uses or applications. 4. Long-Term Objectives that can support scientific questi ons and societal needs that may arise Science and/or in the future, even if they are not known or recognized today. Applications Science objectives that complement other objectives, either enhancing them or 5. Value to Related Objectives providing needed redundancy. 6. Readiness Are we in a position to make m eaningful progress to advance the objective, regardless of measurement? 7. Timeliness Is now the time to invest in pur suing this objective? Examples include recently occurring phenomena that require focused near-term attention and the existence of complementary observing assets that may not be available in the future. During its deliberations, the committee noted th at each SATM Question generally involved ven science, which thus led to observations that aspects of both curiosity-driven and applications-dri uity-related needs. The fact that such basic and applied science addressed both exploratory and contin categories have largely merged over the last decade is a tribute to a community success: restructuring our field in an integrated Earth context that balances sc ience with applications a nd combines exploratory and continuity-related observations. In developing this ranking approach, the committee review ed the quantitative methodology described in the report (NRC, Continuity of NASA Earth Observations from Space: A Value Framework 2015). While the merits of a fully quantitative valu ation as recommended in the Continuity Report were attractive, the practical aspects of completing such va luation over a hundred objectives, as needed for this ESAS 2017 report, precluded that approach. It would ha ve required reliable quantitative evaluation of five factors for these objectives, meaning thousands of quantitative assessments—each of which requires documented justification. Furthermore, the committee r ecognized that the factors relevant for the specific UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-10 Copyright National Academy of Sciences. All rights reserved.

101 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space task of making continuity decisions may not be sufficient for the broader task of identifying observing system priorities. As a result, the committee chose to embrace the general guidance of the Continuity Report regarding a traceable (though not fully quantitative) prio ritization process. Traceability is documented, to the extent possible, through the structure of th e SATM (Appendix B), as discussed above. A prioritized assessment of the SATM’s Science/Applications Objectives was achieved through evaluating the Importance factor in the SATM, using a rigorously normalized process guided by judgement of the committee and panels. The SATM Importance ranking (the last column in Table 3.3) thus represents the committee’s assessment of the pure science and applications priori ties (consistent with the input provided by the panels), independent of implementati on considerations such as cost. The inclusion of cost, feasibility, and readiness constraints was accomplished subsequently, when the science and applications priorities were translated into needed observations (to be ultimately implemented as instruments or missions), as discussed in the following sections. This resulted in highly ranked science and some situations where applications are not reflected in the committ ee’s observing system recommendations when the observations proved too costly or appeared not read y for implementation. In such cases, the high science and applications ranking suggests that investment in maturing the science or technology could have a substantial payoff. The steering committee interacted directly with the panels during the development of priorities, and underwent a final review to ensure concurrence with all panel input to the ranking process. While such comprehensive is a challenging process, the committee is confident that the process used was comprehensive, reliable, and largely repeatable (in other words, similar results would be expected given a different committee makeup). ESAS 2017 SCIENCE AND APPLICATIONS PRIORITIES Using the process described above, the committee developed a set of science and applications ming decade’s Earth system science and applications priorities intended to address the breadth of the co needs. Initial generation of the science/applications prio rities list was largely the responsibility of the panels. The committee reviewed and evaluated the pa nel suggestions, augmenting them with integrating theme discussions in an effort to comprehensivel y address Earth system science and applications. These integrating themes made it possible to view Earth system science in the context of thematic areas spanning multiple panels. The goal was to ensure that the depth provided by disciplinary panel experience was appropriately complemented by a broader integr ated perspective on the challenges in Earth system science. The following sections present the science and a pplications assessment itself, then provide perspectives on the assessment from both interdisciplin ary (panel) and cross-disciplinary (integrating theme) viewpoints. The Science and Applicatio ns Priorities Assessment The ESAS Integrated Science and Applications Assessment is documented in the full SATM (Appendix B) and summarized in th e abbreviated version called the Science and Applications Priorities Table (Table 3.3). Table 3.3 forms the basis for all di scussions in the remainder of this chapter. It describes the primary science and applications priorities, and it forms the basis for the observing system priorities discussed later in the chapter. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-11 Copyright National Academy of Sciences. All rights reserved.

102 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Recommendation 3.1: NASA, NOAA, and USGS, working in coordination, according to their appropriate roles and recognizing their ag ency mission and priorities, should implement ence and applications that is based on the a programmatic approach to advancing Earth sci questions and objectives in this report’s Science and Applications Priorities Table . UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-12 Copyright National Academy of Sciences. All rights reserved.

103 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Important Important Important Important Important Important Important Very Important Very Important Most Important Most Important Most Important Most Important Sci/App Importance Most Important Sci/App Importance cal ds epends on water catchments to processes controlling flash floo food production, and forest e full Science and Applications Traceability ter quality, fluxes, and storages in and nput to accurately quantify the ing, temperature, snowmelt, and ice melt, ppression, land use, and urbanization on rainfall, temperature and evaporation ective and orographic scales suitable to stainability of future water supplies. ter balance from head H4a, H4b, H4, and H4d. limation from snow and ice worldwide at scales driven by ffect evapotranspiration rates, and how these in turn affect lo mical and state variables and lated to agricultural activities, her, hydrological, and air quality forecasts at minutes to with sufficient observational i eparedness. (This is a critical socio- economic priority that d lity for human health and ecosystem services. ts to constrain estimates of evapotranspiration. 3-13 ses that cause changes in radiative forc odification of the land, including fire su arge, temperature extremes, and carbon cycling. nd anthropogenic processes that change wa oundwater, and glaciers), and response to extreme events. ugged terrain and land-margins to heavy (rain and snow/ice) worldwide at conv tly linked to H2a, H2b, Earth Science/Application Objective short-term impacts more accurately and to assess potential mitigations. Earth Science/Application Objective ally groundwater recharge, threatening su and their interactions, and to close the wa on, prediction, and pr The Science and Applications portion of th . Quantify the magnitude of anthropogenic proces Quantify rates of precipitation and its phase Understand linkages between anthropogenic m Monitor and understand hazard response in r Determine the effects of key boundary layer processes on weat Develop methods and systems for monitoring water qua Develop and evaluate an integrated Earth System analysis Quantify how changes in land use, water use, and water storage a Determine structure, productivity, and health of plan Quantify rates of snow accumulation, snowmelt, ice melt, and sub Quantify how changes in land use, land cover, and water use re Quantify key meteorological, glaciological, and solid Earth dyna Monitor and understand the coupled natural a Improve drought monitoring to forecast H-3b. topographic variability. management affect water quality and especi H-4b. H-1a. components of the water and energy cycles extremes, and strong winds at multiple temporal and spatial scales H-3a. as they alter downstream water quantity and quality continental-scale river basins. H-2b. H-4d. frequency of and response to hazards. This is tigh H-1b. H-4a. between all reservoirs (atmosphere, rivers, lakes, gr and regional precipitation systems, groundwater rech H-2a. H-3c. and rapid hazard-chains to improve detecti capture flash floods and beyond. success of addressing H1b, H1c and H4a). H-4c. H-2c. H-1c. W-1a. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION What planetary How do How does the water How is the water How do changes in Science and Applications Priorities Table rocesses to change the predictability recipitation, and how are these rovide? anthropogenic changes in climate, land p ecosystems and the services these the biotic life of streams, and affect regional freshwater availability, alter the water cycle impact local and improve preparedness and mitigation of water-related extreme events? long-term consequences? globally and what are the short- and energy cycles locally, regionally and interact and modify the water and use, water use, and water storage hazard-chains (e.g. floods, wildfires, and impacts of hazardous events and QUESTION H-2. p droughts and floods? and magnitude of extremes such as evapotranspiration, and the frequency distribution of rainfall, snowfall, changes expressed in the space-time cycle interact with other Earth System p evapotranspiration and thereby accelerating, with greater rates of WEATHER and AIR QUALITY PANEL evapotranspiration and precipitation cycle changing? Are changes in Societal or Science Question/Goal landslides, coastal loss, subsidence, QUESTION H-1. QUESTION W-1. QUESTION H-4. Societal or Science Question/Goal GLOBAL HYDROLOGICAL CYCLES AND WATER RESOURCES PANEL droughts, human health, and ecosystem health), and how do we QUESTION H-3. TABLE 3.3 Matrix (SATM) in Appendix B. Copyright National Academy of Sciences. All rights reserved.

104 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Important Important Important Very Important Most Important Most Important Most Important e, ), 3 ). 2 tes to within 1 mm/hour to and impacts on surface air 3 resolved tropospheric fields ‐ s of weather/climate variability (e.g. r forecast times of 1 week to 2 months. sport and redistribution of mass, moistur eciated particulate matter (PM), ozone (O ), and nitrogen dioxide (NO nd organization of convection and slowly nd observe total precipitation to an average 3 r the determination of controlling processes and emissions, including regional anthropogenic sources and 4 1 m/s and heavy precipitation ra ric-tropospheric exchange of O r pollution distributions and aid estimation of global air 3-14 reducing uncertainty to <10% of vertically entation of natural, low-frequency mode concentrations and in CH time of useful prediction skills by 50% fo tions in global, vertically-resolved sp 4 rface characteristics modifies regional cycles of energy, water and momentum r land and ice surfaces averaged over a 100x100 km region and two to three day time between the large-scale circulation a ecipitation and to determine convective tran enthalpy flux, and 0.1 Nm-2 in stress, a variations, including stratosphe 3 ) trends (within 20%/yr), which are necessary fo 2 Measure the vertical motion within deep convection to within Determine how spatial variability in su Improve the understanding of the processes that determine ai Reduce uncertainty in tropospheric CH Characterize tropospheric O Improve the observed and modeled repres Characterize long-term trends and varia ollution impacts on human health and ecosystems by eriod. improve model representation of extreme pr W-3a. MJO, ENSO), including upscale interactions quality and background levels. momentum, and chemical species. W-8a. W-6a. subseasonal time scales. estimation of health effects and impacts on agriculture and ecosystems. W-2a. (stress) to an accuracy of 10 Wm-2 in the W-5a. W-7a. varying boundary processes to extend the lead p accuracy of 15% over oceans and/or 25% ove and nitrogen dioxide (NO p from a process level for natural sources. (including surface concentrations) of speciated particulate matter (PM), ozone (O W-4a. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION How do spatial What processes What processes How can What processes What processes Why do convective ) variations and trends 4 ) variations and trends and 3 what are the concomitant impacts of determine the spatio-temporal QUESTION W-4. influence weather and air quality? ocean, land, cryosphere) and thereby modify transfer between domains (air, dynamics, thermal inertia, and water) (influencing ocean and atmospheric variations in surface characteristics QUESTION W-7. on human health, agriculture, and QUESTION W-6. determine observed tropospheric ozone (O structure of important air pollutants determine the long-term variations and their concomitant adverse impact these changes on atmospheric composition/chemistry and climate? and trends in air pollution and their subsequent long-term recurring and QUESTION W-3. months? conditions at lead times of 1 week to 2 seamlessly forecast Earth System and air quality be extended to environmental predictions of weather cumulative impacts on human health, QUESTION W-8. agriculture, and ecosystems? QUESTION W-5. determine observed atmospheric methane (CH they do? clouds occur exactly when and where and what are the subsequent impacts of these changes on atmospheric composition/chemistry and climate? QUESTION W-2. storms, heavy precipitation, and quality simulations? these impact weather forecasts and air momentum and mass, and how do and sea ice) exchanges of energy, integral to the air-surface (land, ocean boundary layer (PBL) processes are ecosystems? Copyright National Academy of Sciences. All rights reserved.

105 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Important Important Important Important Important Important Important Important Important Important Important Important Very Important Most Important Most Important Most Important Most Important Sci/App Importance he indicator species, etc.). distribution of marine biomass within t ition of vegetation and marine biomass, onthly temporal resolution with uncertainty < easuring the hydrometeor distribution and the boundary layer evolution. Determine the s and their rate of turnover. aining the life cycle of terrestrial and marine ecosystems and on species, invasive species, 3-15 ructure of terrestrial vegetation and 3-D nd interactions of hydrometeors by m tional traits, functional types, and compos Earth Science/Application Objective nd between ocean ecosystems and atmosphere. rrestrial and aquatic primary producers. systems related to carbon storage. gher trophic levels of food webs. tems between aquatic ecosystems. globally at spatial scales of 100-500 km and m 4 l inventory of terrestrial C pool ical properties of clouds on all scales, including small-scale cumulus clouds. of all scales on radiative fluxes, including on and CH 2 status of soils. ND NATURAL RESOURCES MANAGEMENT PANEL Quantify the effects of clouds Characterize the microphysical processes a Discover cascading perturbations in eco Quantify the global three-dimensional (3-D) st Quantify moisture Improve assessments of the globa Quantify the fluxes of CO Quantify the global distribution of the func Discover ecosystem thresholds in altering C storage. Quantify the flows of energy, carbon, water, nutrients, etc. sust Support targeted species detection and analysis (e.g., foundati Understand ecosystem response to fire events. Assess ecosystem subsidies from solid Earth. Quantify the physiological dynamics of te Understand how ecosystems support hi Constrain ocean C storage and turnover. Quantify the fluxes from land ecosys artitioning into functional types. recip rate to within 5%. E-4a. E-1b. E-5b. E-2c. E-5c. E-5a. E-2a. E-1a. E-3a. euphotic zone, spatially and over time. E-3b. E-1c. p 25% between land ecosystems and atmosphere a spatially and over time. E-4b. E-2b. E-1d. E-1e. W-9a. W-10a. structure, evolution and physical/dynam p UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION How do clouds What processes What are the fluxes How is carbon What are the fluxes Are carbon sinks What are the ecosystems and the ecosystems, and how within between roperties and their connections to Earth’s ecosystems, and how and why changing? accounted for through carbon storage, turnover, and accumulated biomass, and have we quantified all of the major carbon sinks and how are they changing in time? Earth, and how and why are they energy) energy) (of carbon, water, nutrients, and QUESTION E-2. are they changing in time and space? (of carbon, water, nutrients, and structure, function, and biodiversity of QUESTION E-1. QUESTION E-5. Societal or Science Question/Goal stable, are they changing, and why? MARINE AND TERRESTRIAL ECOSYSTEMS A subseasonal? on time scales from minutes to surface and contribute to predictability affect the radiative forcing at the QUESTION W-10. aerosols and precipitation? QUESTION E-3. p determine cloud microphysical QUESTION W-9. and why are they changing? QUESTION E-4. atmosphere, the ocean and the solid Copyright National Academy of Sciences. All rights reserved.

106 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Important Important Important Important Important Very Important Very Important Very Important Very Important Very Important Very Important Most Important Most Important Most Important Most Important Most Important Most Important Sci/App Importance ace the uncertainty and the effects e how much as been 0.3-0.5 mm/yr (e.g. Nerem 0.3mm/yr, with differing opinions se but also changes in these rates. ess). As a result, while the rate of 0.5 ng ecosystem demography.) al resolution with uncertainty better leaks from oil and gas operations. 4 uncertainty < 25% to enable regional- 1 nuously, for decades to come. , including from urban areas, from known lly in coastal regions and the southern 4 ourse of a decade and the changes in surf interannual vairability to 0.2 W/m2 (67% and CH 2 over the course of a decade. -2 forest regrowth, and changi (UTS) to climate feedbacks and change by determining how r to accommodate the inherent measurement understanding of dependence on environmental drivers such as over the course of a decade. -1 ng with a 1-sigma uncertainty of 0.05 W/m2 over the course of servoirs such as tropical forests and permafrost. over the entire ice sheets, conti and monthly temporal resolution with by net uptake of carbon by terrestrial ecosystems (i.e., determin have been detected (Nerem et al., in pr of 300 km x 300 km and monthly tempor 3-16 2 forcing uncertainty by a factor of 2. and nitrogen fertilization, 2 Earth Science/Application Objective eat storage to 0.1 W/m2 (67% conf) and ithin 1.5-2.5 mm/yr over the course of a decade (1.5 corresponds to a ~6000-km2 bon uptake by ocean to within 25% (especia rate feedback by a factor of 2. mass balance to within 15 Gton/yr over the c oud feedback by a factor of 2. ceanic heat uptake to within 0.1 Wm do feedback by a factor of 2. feedback by a factor of 2. feedback by a factor of 2. the global mean sea level rise rate over the last 25 years h for future systems is to achieve an accuracy as high as 0.1- r to properly capture not only the current rates of sea level ri ntification of emissions from large sources of CO fluxes from wetlands at spatial scales fluxes at spatial scales of 100-500 km 2 4 3, have a mission goal of 1 mm/yr, in orde m-2 day-1 in order to establish predictive process based 4 Determine regional sea level change to w Determine the change in the global o Reduce the IPCC AR5 total aerosol radiative Reliable detection and qua Reduce uncertainty in carbon cycle Reduce uncertainty in water vapor Reduce uncertainty in low and high cl Determine the global mean sea level rise to within 0.5 mm yr Quantify CO Early warning of carbon loss from large and vulnerable re Quantify CH Reduce uncertainty in snow/ice albe Reduce uncertainty in temperature lapse Determine the decadal average in global h Regional-scale process attribution for car Quantify the contribution of the upper troposphere and stratosphere Determine the changes in total ice sheet sources such as power plants, and from previously unknown or transient sources such as CH t oin scale process attribution explaining year-to-year variability C-2b. C-2g. carbon uptake results from processes such as CO C-2a. C-2f. C-3e. C-1d. ocean). C-3b. C-1a. changes in UTS composition and temperature affect radiative forci mass balance and glacier ice discharge with the same accuracy C-2c. C-3d. C-1c. p C-2e. C-3a. than 3 mg CH C-2d. C-2h. C-1b. C-3c. conf). region, 2.5 corresponds to a ~4000-km2 region) decade. et al., 2017), and acceleration rates of 0.08 mm/yr UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION How much will sea How can we reduce How large are the Current altimetry missions, such as Jason 1 heat storage? QUESTION C-2. variations in the global carbon cycle will be the role of ice sheets and ocean the uncertainty in the amount of future warming of the Earth as a function of and what are the associated climate the next decade and beyond, and what level rise, globally and regionally, over carbon emissions? fossil fuel emissions, improve our and ecosystem impacts in the context QUESTION C-1. QUESTION C-3. ability to predict local and regional climate response to natural and mitigation/adaptation strategies? of past and projected anthropogenic anthropogenic forcings, and reduce the uncertainty in global climate Societal or Science Question future economic impacts and sensitivity that drives uncertainty in CLIMATE VARIABILITY AND CHANGE PANEL and Leuliette, 2016; Ablain of seasonal and interannual variations. The current uncertainty in among experts about where it should fall in that range, in orde mm/yr reported here is in line with current capabilities, the goal Copyright National Academy of Sciences. All rights reserved.

107 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Important Important Important Important Important Important Important Important Important Important Important Very Important Very Important Very Important Very Important Very Important Very Important ulk on ed ired: e), -to- ecasts. : 67% and other GHGs. , (b) sensible and latent heat 4 2 2. Confidence levels des ectivity, lifetime, cloud phas and other gases (e.g., CO2 and nges in ENSO spatial patterns, certainty by a factor of 2) and impact ecosystems; and improve the confidence in the opriate decadal stabilities. ate the validity of Monin-Obukhov similarity gh-latitude regions reducing uncertainty in the b 0.5 deg latitude per decade (67% confidence uncertainty in IPCC 2013). Confidence level reducing the uncertainty by a factor of 2, with desir e uncertainty by a factor of , water vapor (i.e., moisture) ation change on regional scales including the occurrence of state of the ocean upon atmospheric weather patterns on decadal of natural and anthropogenic aerosols, including properties that height, and cloud properties (refl ropogenic (greenhouse gases, aerosols, land-use) forcings and nic aerosols and their precursors via observational constraints ratospheric states for initialization of seasonal-to-decadal for surface and subsurface ocean states for initialization of seasonal ecade (67% confidence desired); cha of land surface states for initialization of seasonal forecasts. itical for air quality and dominant sink for CH to within 25%, with appr 4 3-17 oceanic circulation patterns, changes on radiative fluxes (reduction in un and CH 2 NAO, ENSO, QBO) . Reduce th ta assimilation/ inverse modeling. or of 2 (relative to decadal prediction e expansion of the Hadley cell to within the mean on local or regional scales: (a) radiative fluxes to 5 W/m gas exchange, oceanic storage and impact on 2 hips at high wind speeds over the ocean. zations, particularly in extreme conditions and hi rface waves in determining wind stress; demonstr ith and modify clouds and radiation confidence desired) ilability, and inundation. , (c) winds to 0.1 m/s, and (d) CO 2 Evaluate the effect of surface CO Quantify the effect of aerosol-induced cloud Better quantify the role of su Characterize the properties and distribution in the atmosphere Decrease uncertainty, by a factor of 2, in quantification Quantify the linkage between the dynamical and thermodynamic Quantify the changes in the atmospheric and Improve the estimates of global air-sea fluxes of heat, momentum Quantify the tropospheric oxidizing capacity of OH, cr Decrease uncertainty, by a factor of 2, in quantification of Improve estimates of the emissions of natural and anthropoge Improve bulk flux parameteri Decrease uncertainty, by a factor of 2, in quantification of st Quantify the effect that aerosol has on cloud formation, cloud Quantify the linkage between global climate sensitivity and circul Quantify the linkage between natural (e.g., volcanic) and anth Improved atmospheric transport for da ) to the following global accuracy in 4 estimates and reduce uncertainties by a factor of 2. C-4a. climate (circulation, precipitation). C-5d. theory and other flux-profile relations 67%. timescales. Reduce the uncertainty by a fact C-5b. transfer coefficients by a factor of 2. C-6a. C-4b. C-7d. affect their ability to interact w decadal forecasts. C-3g. C-6b. temperature, carbon ava C-4d. C-5c. C-6c. amplitude, and phase (67% (likely). C-4c. desired); changes in the strength of AMOC to within 5% per d including semi-direct effects. oscillations in the climate system (e.g., MJO, fluxes to 5 W/m C-7c. C-7b. C-3f. confidence levels of 67% (likely in IPCC parlance). CH extremes and abrupt changes. Quantify: th C-7a. C-5a. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION How are decadal Can we significantly A. How do changes How will the Earth circulation patterns changing, and scale global atmospheric and ocean system respond to changes in air-sea QUESTION C-7. variables?* of societally-relevant climate improve seasonal to decadal forecasts events, and longer term environmental QUESTION C-6. interactions? forcings? the response of climate to its various change? aerosol signal that modifies the natural aerosols, and the anthropogenic variability of the emissions of natural better quantify the magnitude and greenhouse gases? B. How can we and offset the warming due to forcing) affect Earth’s radiation budget largest uncertainty in total climate with clouds which constitute the in aerosols (including their interactions on seasonal climate processes, extreme QUESTION C-5. QUESTION C-4. what are the effects of these changes one, so that we can better understand Copyright National Academy of Sciences. All rights reserved.

108 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Important Important Important Important Important Important Important Important Very Important Very Important Very Important Very Important Very Important Very Important Very Important Most Important Most Important Most Important Sci/App Importance a e s nd thin e coarseness of any of the beneficiary of measurements unities to increased e impact of snow/ice-albedo tive land volcano inventory with , and changes in sea level from tion) and atmospheric contamination, and deling as well as response and mitigation act on Arctic amplification. (impact on mid-latitude extreme weather rosols and dust) transported into polar mulating the observed evolution of the larg a ice cover, including sea ice fraction (wi ility of coastal comm itude of ENSO events, strength of AMOC, a fluxes, above and below ground carbon pools, e retreat and ice advance (within 5 days). stratospheric ozone and atmospheric aerosols tseismic activity over tectonically active areas on time scale climate change and associated atmosphere/ocean circulations. tion of 100 m and an accuracy of 10 mm) and damage to eat fluxes, sea ice cover, fresh water input, and ocean general nd products of Earth’s entire ac cation by quantifying the relativ ss of ice and snow cover extent Arctic, as well as their imp a volcanic eruption (hourly to daily temporal sampling). opagation, and run-up for major seafloor events. 3-18 level correspondence with the observational data)? feedbacks and linkages to global radiation. circulation, water vapor, and lapse rate feedback. ability and mid-latitude weather linkages ack carbon, soot from fires, and other ae predictability of Arctic and Antarctic se nt of surface change (<100 m spatial resolu for climate projections. Are the models si Earth Science/Application Objective nd cover changes affect turbulent heat llowing disasters for improved predictive mo se and permafrost thaw increase vulnerab the ice edge (within 1km), timing of ic rculation, such as the frequency and magn ing the surface, and relate to changes in t-eruption surface deformation a winds and storms intensify. Improve understanding of high-latitude vari Determine the changes in Southern Ocean carbon uptake due to Determine the amount of pollutants (e.g., bl Quantify the amount of UV-B reach Improve regional-scale seasonal to decadal Determine how changes in atmospheric circulation, turbulent h Observational verification of models used Measure and forecast interseismic, preseismic, coseismic, and pos Forecast, model, and measure tsunami generation, pr Measure the pre-, syn- and pos Rapidly capture the transient processes fo Determine how permafrost-thaw driven la Quantify high-latitude low cloud representation, Forecast and monitor landslides, especially those near population centers. Assess co- and post- seismic ground deformation (spatial resolu Quantify how increased fetch, sea level ri Improve our understanding of the drivers behind polar amplifi Assess surface deformation (<10 mm), exte C-9a. C-7e. 5%), ice thickness (within 20cm), location of infrastructure following an earthquake. C-8g. C-8h. S-2c. C-8c. increased melting of ice sheets and glaciers). scale patterns in the atmosphere and ocean ci resulting greenhouse gas fluxes (carbon dioxide, methane) in the the composition and temperature of volcanic products following the poleward expansion of the sub-tropical jet (to a 67% C-8b. S-2b. through optimal retasking and analysis of space data S-1a. C-8i. C-8e. and changes in storm tracks from increased polar temperatures, lo coastal inundation and erosion as regions and their impacts on snow and ice melt. time scale of days to weeks. feedback, versus changes in atmospheric and oceanic S-1b. circulation affect bottom water formation. C-8d. C-8a. ranging from hours to decades. S-2a. S-1c. C-8f. S-1d. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION How is the ozone What will be the How can large-scale How do geological forecast in a socially relevant QUESTION C-9. and projected for Antarctica on global change already observed in the Arctic consequences of amplified climate implications for Earth’s climate? system and society following an disasters directly impact the Earth timeframe? event? QUESTION C-8. geological hazards be accurately QUESTION S-2. trends of sea level rise, atmospheric QUESTION S-1. Societal or Science Question/Goal circulation, extreme weather events, EARTH SURFACE AND INTERIOR PANEL fluxes? layer changing and what are the global ocean circulation, and carbon *As noted in the text, all of the indicated measurements for Questions C-6 and C-7 would be useful, but the absence or excessiv taken to address other questions. Indicating here which measurements are already being taken is, in a way, extraneous. measurements would not be a deal-breaker. This question is best considered *not* as a motivation for a mission but rather as a Copyright National Academy of Sciences. All rights reserved.

109 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Important Important Important Important Important Important Important Important Very Important Very Important Most Important Most Important Most Important ales m mm yr- uncertainty and the effects yr, with differing opinions among as been 0.3-0.5 mm/yr (e.g. Nerem also changes in these rates. water content, and solar ss). As a result, while the rate of 0.5 mm/yr nd an order of magnitude for 2 within a factor of 2 over horizontal sc lution of 100 m and pressure of 1 kPa (0.1 nuous reshaping of Earth’s surface due to properties, soil properties/ t of near-surface materials that produce landscape of 3 for shallow aquifers a ent rates of sea level rise but accuracy as high as 0.1-0.3mm/ r to accommodate the inherent measurement at global, regional, and local scales, with uncertainty < 0.1 quivalent at resolution of 10 km. 3-19 have been detected (Nerem et al., in pre on and distribution, thermal 2 e groundwater system across the recharge area. s in situ within a factor ntle by resolving electrical conductivity to the Earth’s interior, specifically the dynamics of the Earth’s core and its changing produced by abrupt events and by conti flow in confined aquifers at spatial reso coastlines at uncertainty <1 mm yr-1. tle and lithosphere within 10 mW/m2. mantle convection and plate motions. the global mean sea level rise rate over the last 25 years h nd management of energy, mineral, agricultural, and natural resources. nt and <0.5 mm yr-1 sea-level e operly capture not only the curr future systems is to achieve an surface hydrology, and changes in ice/water conten 3, have a mission goal of 1 mm/yr, in orde and acceleration rates of 0.08 mm/yr Measure all significant fluxes in and out of th Determine the impact of water-related human activities and natural water flow on earthquakes. Determine the water content in the upper ma Determine vertical motion of land along Quantify the rates of sea-level change and its driving processes Determine the effects of convection within Map topography, surface mineralogic compositi Quantify global, decadal landscape change Determine the fluid pressures, storage, and Quantify the heat flow through the man Determine the transport and storage propertie Quantify ecosystem response to and causes of landscape change. Quantify weather events, S4c. head). S-5c. S-3b. S-6b. S4b. magnetic field and the interaction between 1 for global mean sea-level equivale S-6c. S-3a. S-5b. surface processes, tectonics, and societal activity. change. irradiance for improved development a deeper systems. S-6a. S-6d. S-4a. S-5a. of 1000 km. S-7a. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION How do we improve How will local sea How much water is What processes and How does energy Current altimetry missions, such as Jason 2 level change along coastlines around QUESTION S-6. the world in the next decade to QUESTION S-7. mineral, and soil resources? surface? QUESTION S-4. flow from the core to the Earth’s water supplies? QUESTION S-5. century? traveling deep underground and how QUESTION S-3. discovery and management of energy, landscape change? interactions determine the rates of does it affect geological processes and experts about where it should fall in that range, in order to pr and Leuliette, 2016, other references), reported here is in line with current capabilities, the goal for of seasonal and interannual variations. The current uncertainty in Copyright National Academy of Sciences. All rights reserved.

110 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Panel Perspectives and Priorities Part II of this report provides the comprehensiv e panel inputs on the science and applications . In the following sections, the steering committee underlying the SATM (Table 3.3 and Appendix B) presents a review of the panel chapters and an analys is of how the panel priorities fit into the broader context of Earth system science and applications considered by the steering committee. Global Hydrological Cycles and Water Resources h. Driven by this need, humans have established Water is the most widely used resource on Eart se, and alter our water environment, for a variety of engineering and social systems to control, manage, u uses and through a variety of organizational and individual processes. Understanding the hydrologic e therefore of critical importance to society. cycle, monitoring, and predicting its vagaries, ar Remotely sensed data have been playing a key ro le in advancing our insight into Earth’s water resources. Missions such as the Tropical Rainfall Measurement Mission (TRMM), Global Precipitation Measurement (GPM) mission, Soil Moisture Active Passive (SMAP), and the Gravity Recovery and rating sensors from the older Earth Observing Climate Experiment (GRACE)—along with still-ope System (EOS)—have provided importa nt measurements to close Earth’s energy and water cycles at various spatial and temporal scales. Among the most important contributions to hydr ologic sciences and engi neering—in addition to space-based measurements of water in its various forms—are space-based observations of shortwave and longwave radiation, as such observations provide an important ingredient for estimating fluxes of evaporation and evapotranspiration ( ET), snow and glacier extent, soil moisture, atmospheric water vapor, clouds, precipitation, terrestrial vegetation and oceanic chlorophyll, and water storage in the subsurface (Box 3.3), among many others. In their report, the Hydrology Panel rec ognized a number of high-level integrative science questions. To address these, the panel proposed remote sensing measurements that will enhance and continue developments needed to address critic al gaps in our understanding of the movement, water and its variability and cha nge over time and space. The four distribution and availability of Objectives identified by the panel as Most Important were associated with the following two Questions:  (H-1) Water Cycle Acceleration. How is the water cycle changing? Are changes in evapotranspiration and precipitation accelerating, with greater rates of evapotranspiration and thereby precipitation, and how are these chang es expressed in the space-time distribution of rainfall, snowfall, evapotranspiration, and th e frequency and magnitude of extremes such as droughts and floods?  (H-2) Impact of Land Use Changes on Water and Energy Cycles. How do anthropogenic changes in climate, land use, water use, and water storag e interact and modify the water and energy cycles locally, regionally and globa lly and what are the short- and long-term consequences? The panel recognized the importance of the coup ling between the water cycle and energetics of the Earth system as a basis for understanding how th e different water cycle facets are changing now and might change in the future. Quantifying the compone nts of the water and energy cycles at the Earth’s surface, through observations with sufficient accuracy to close the budgets at river basin scales has been an unresolved problem for many decades. Two central coupled elements of the surface water and energy balances are the precipitation that reaches the Earth’ s surface (P) and the heat fluxes associated with evaporation from the surface and from transpiration from vegetation (ET). The surface properties, including soil moisture, also strongly influence the planetary boundary layer. It, in turn, influences surface-atmosphere exchanges, further complica ting the coupling between energy and water. The panel concluded that: 1) c ouplings between water and energy are central to understanding water and energy balances on river basin scales; 2) ET is a net result of coupled processes; 3) UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-20 Copyright National Academy of Sciences. All rights reserved.

111 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space precipitation and surface water information is needed on increasingly finer spatial and temporal scales; and 4) the consequences of changes in the hydrol ogic cycle will have significant impact on the Earth population and environment. These conclusions led the panel to identify four priority societal and scientific goals associated with the hydrologic cycle: 1. Coupling the Water and Energy Cycles 2. Prediction of Changes 3. Availability of Fresh Water and C oupling with Biogeochemical Cycles 4. Hazards, Extremes, and Sea Level Rise. Related to the above four goals, the panel iden tified thirteen Science and Application Questions, and within these questions ranked the followi ng four Objectives as Most Important (MI):  Develop and evaluate an integrated Earth System (H-1a) Interaction of Water and Energy Cycles. accurately quantify the components of the water analysis with sufficient observational input to and energy cycles and their interactions, a nd to close the water balance from headwater catchments to continental-scale river basins.  (H-1b) Precipitation. Quantify rates of precipitation and its phase (rain and snow/ice) worldwide at convective and orographic scales suitabl e to capture flash floods and beyond.  (H-1c) Snow Cover. Quantify rates of snow accumulation, snowmelt, ice melt, and sublimation from snow and ice worldwide at scales driven by topographic variability.  (H-2c) Land Use and Water. Quantify how changes in land use, land cover, and water use related to agricultural activities, food production, and forest management affect water quality and especially groundwater recharge, threatening sustainability of future water supplies. Key Points Summarized by the Steering Committee:  The Hydrology Panel’s highest priorities are to develop an integrated Earth system analysis and make the measurements of rain- and snowfall, as well as accumulated snow, in order to constrain the key inputs into that analysis. In the comi ng decade, these advanced analysis systems will be the central framework upon which most of the wa ter cycle remote sensing observations will be combined to deliver high profile science and a pplications information about the hydrological cycle and changes to this cycle.  This priority evolves out of the recognition that the full character of precipitation and other critical information on surface energy and water fluxes required to address critical science and application objectives is needed on much higher spatial and temporal resolutions than can be practically addressed from space-borne observations alone. Many hydrological variables require such an an  alysis system. The multifaceted character of precipitation is one example where duration of pr ecipitation events and total water output requires dynamic analysis system. ET is another example. the integration of snap-shot observations into a This energy flux explicitly couples the water and energy cycles at the surface and is a net result of a number of complex processes that cannot be synthesized from any single remote sensing measurement alone. It is imperative, and an urgent challenge for the next decade, to accurately monitor the timing,  amount, phase (snowfall or rain), and vertical stru cture of hydrometeors of precipitating systems globally and with sufficiently high space and time re solution to detect and quantify change at the river basin scale.  In the coming decade, use of space-based observatio ns has the potential to be revolutionized by the possibility of advancing pr ocess understanding so as to properly assimilate precipitation information in advanced high-resolution models used to forecast precipitation. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-21 Copyright National Academy of Sciences. All rights reserved.

112 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space ving variables central to key processes, like  Thus, a strong case can be made that obser hydrometeor vertical velocities, will provide th e required constraints to make high quality model at 1 km and 15 min time steps a reality. based analyses and forecasts of precipitation Observations of all aspects of mountain hydrology are also a major challenge that has not been  adequately addressed. For example, estimating the spatial distribution of the extant snow water equivalent (SWE) in mountainous terrain, which is characterized by high elevation and spatially varying topography, is an important but unsolved problem. ********************************************************************************* BOX 3.3 Monitoring Groundwater Usage with Radar Interferometry Modern development and increases in populatio n have placed such great demand on water resources that in many places we have now fully e xploited easily accessible sources of surface water. Where surface supplies are limited, we often draw upon water stored in underground aquifers to meet our needs. Groundwater already provides half of U.S. drinking water and serves as a critical supply during times of drought. Moreover, it is essential for agricultu re and industry. Large-scale exploitation of groundwater resources has led to concerns about the fu ture availability of groundwater to meet growing needs. While surface waters can be monitored and t hus managed and regulated, the stocks, flows, and Recently, several US States have enacted laws to residence times of groundwater are poorly known. assess and manage groundwater reserves. term, maintain sustainability of groundwater Effective water management must, over the long ems that drain to and support river systems, that aquifers. In practice this means, for groundwater syst d doesn’t greatly reduce stream flows. In both cases, water withdrawal doesn’t exceed the recharge rate an e aquifer, known as hydraulic head, is the critical a measurement of subterranean water pressure in th metric needed to decide on and monitor actions. The sta ndard approach to monitor head in an aquifer is to record water levels in wells and surface subsid ence using leveling and precise GPS. However, these usually infrequent and sparse point measurements do not resolve seasonal variations especially over the full extent of the reservoir. Fortunately, changes in head often produce measu rable subsidence or uplift at the surface, hence repeat-pass radar interferometry (InSAR) derived defo rmation over time yields head estimates at the vastly greater coverage and finer resolution of a space borne sensor. Thus, the potential of a radar satellite mission is that it permits temporally and spatially de nser head estimates than can be obtained using wells, moreover it can yield such data worldwide. Once cal ibrated with local well-ba sed measurements, InSAR observations assimilated into a predictive model to pred ict future head levels. Thus, over the past decade InSAR has moved from a research tool to monitor a ll types of surface deformation into a mainstream applications tool for monitoring seasonal and secular va riations in vertical ground motion associated with groundwater withdrawal and recharge. The technique is now used routinely by the USGS for regional studies (Figure 3.5) as well as by many state and local water authorities to help monitor groundwater resources. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-22 Copyright National Academy of Sciences. All rights reserved.

113 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space FIGURE 3.5 Surface subsidence of regions in the Co achella Valley caused by groundwater withdrawal between 1995 and 2010. The greatest subsidence occurr ed in the most developed areas of Palm Desert, used by households and to irrigate about 125 golf Indian Wells, and La Quinta where most of the water is courses. The subsidence map is based on 93 rada r interferograms constrained by GPS point measurements. (Figure and analyses provid ed by the USGS [Sneed et al., 2014]) ********************************************************************************* END OF BOX Weather and Air Quality: Minutes to Subseasonal e coming one is that we now have a deeper A major difference between the last decade and th understanding of, and capability to model and predict, the entire coupled Earth System. In the past, we focused more on the atmosphere, ocean, sea ice, and land as separate entities. Satellite observations, combined with data assimilation and numerical pred iction models, have become essential components in the fully coupled Earth System fra mework. Working from an Earth sy stems framework is also essential for extending weather and air quality forecast skill beyond a few weeks. The societal benefits associated with achieving significant increases in weather skill, a nd extending skill to longer lead times, will be large (Box 3.4). UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-23 Copyright National Academy of Sciences. All rights reserved.

114 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space The panel identified and prioritized ten Sc ience and Application Questions. Those with Objectives ranked Most Important are: (W-1) Planetary Boundary Layer.  How is the planetary boundary layer (PBL) integral to the air- surface (land, ocean and sea ice) exchanges of energy, momentum and mass, and how do these impact weather forecasts and air quality simulations?  How can environmental predictions of weather and air (W-2) Extending Forecast Lead Times. quality be extended to seamlessly forecast Earth Syst em conditions at lead times of 1 week to 2 months?  ( W-4) Convection and Heavy Precipitation. Why do convective storms, heavy precipitation, and clouds occur exactly when and where they do? What processes determine the spatio-temporal structure of  (W-5) Mitigating Air Pollution. important air pollutants and their concomitant a dverse impact on human health, agriculture, and ecosystems? Continual increases in model resolutions enable better representation of the processes central to answering these questions and their underlying Object ives. Consequently, observations central to these Objectives require higher spatiotemporal resolution of the most basic atmospheric quantities, including profiles of temperature, humidity, wind, and atmospheric composition, along with quantitative surface characterization (e.g., snow, sea ice, surface temper ature, soil moisture) and key physical process information. The latter includes diagnostic and valida tion information associated with clouds (liquid and ice phase), convection, and precipitation. In all cases, better characterization of uncertainties in the observations is needed both for scientific inquiry a nd data assimilation purposes. Data assimilation, especially for coupled systems (e.g., atmosphere-ocean and atmosphere-land), also needs to advance in nd observations delivering information on a higher time parallel to observations in order to blend model a and space resolution. The Planetary Boundary Layer . The planetary boundary layer (PBL) has broad importance to a modynamics and wind within it address important number of Earth science priorities. Profiles of ther servations needed to advance weather and climate weather priorities. Many of the same sorts of PBL ob prediction would also enable improvements in our ab ility to track and predict the distribution of trace gases in the atmosphere. The addition of aerosol and oz one coupled to this advanced profile data would improve understanding and prediction of severe air po llution outbreaks that affect human health, as Future of Atmospheric Chemistry Research (NRC, 2016). discussed in the 2016 NRC report Advanced PBL measurements would improve our understanding of the exchanges between the biosphere and the atmosphere, and likewise the air-sea exchanges of chemical and energy fluxes. Better understanding of these exchange processes is critical for our understandi ng of biogeochemical cycles, impacts of climate change on ecological systems, and estimates of car bon storage in natural systems, among many other applications. The profiling of thermodynamics and clouds in the boundary layer and across it into the free eed for accurate, diurnally resolved, high vertical troposphere is relevant to low cloud feedbacks. The n resolution in water vapor profiling in and across the boundary has now been elevated as an Essential Climate Variable by GCOS. Accurate and high-resolution measurements and better understanding of boundary layer processes are of key importance for improving weather and climat e models and predictions. As an example, recent development of the next generation global predic tion system (NGGPS) requires better understanding and modeling of the coupling among the atmosphere, su rfaces waves, ocean, sea ice, and land in the integrated Earth system. The 2016 report Next Generation Earth System Prediction: Strategies for The Subseasonal to Seasonal Prediction (NAS, 2016) also identifies a number of boundary layer observations that would advance our prediction capabilities. The Weather and Air Quality Panel also identified important linkages between the PBL to other panels and Integrating Th emes: 1) the PBL interacts with UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-24 Copyright National Academy of Sciences. All rights reserved.

115 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space of the Hydrology Panel, the Ecosystems Panel, surface processes, which are important to the objectives and the Climate Panel (through near-surface atmos pheric quantities such as wind speed, precipitation, to-seasonal prediction will uxes); and 2) subseasonal- aerosol and trace gases, and air-sea-land surface fl ontinuum and relate to hazardous event preparedness and mitigation via bridge the weather and climate c long-lead forecast information (e.g., floods, dro ughts, wildfire potential). The strategy requires a combination of space-based observations, and expansi on of aircraft and ground-based observations, in conjunction with data assimilation and numerical mo deling representing the 3D structure of the PBL. Subseasonal-to Seasonal Prediction . The second high-priority area reflects the goal to extend environmental predictions to seamlessly predict Earth System conditions at lead times of 1 week to 2 months. The specific objective is to improve the obs erved and modeled representation of natural, low- frequency modes of weather/climate variability, in cluding upscale interactions between the large-scale circulation and organization of conv ection (e.g., MJO, ENSO) so as to reduce prediction errors by 50% at lead times of 1 week to 2 months. The panel iden tified the following steps required to advance this objective:  Developing/improving the initialization of atmospheric variables. Developing optimal strategies for initializing dete rministic and ensemble subseasonal forecasting  systems.  Constructing initial conditions that better utilize sate llite data in cloudy and precipitating regions, where significant challenges remain in data assimilation methodology.  Reducing systematic model errors in the unde rlying physical processes and subseasonal relevant phenomena that affect subseasonal forecast skill.  Developing coupled atmosphere-land-o cean data assimilation methodologies. uding measurements and metrics, for subseasonal Determining optimal verification strategies, incl  forecasts.  Translating subseasonal forecast information into actionable information for societal benefits. Convection. mospheric moist convection which exerts The third area of high importance is at profound influences on our weather and climate. Life on Earth is tightly bound to the major convective storm systems that are found throughout the tropics and mid-latitudes. Convective storms deliver the majority of the fresh water in the form of rain a nd snow and are a principal source of life-threatening severe weather. Predicting the occurrence and location of convective storms, and how they evolve into severe weather, is critical for accurate forecasting of many forms of weather and hazardous weather in particular. In addition to its role in local severe weat her, convection also impacts the large-scale atmospheric circulation. The organization of convecti on and its coupling to the larger scale flows of the atmosphere is fundamental to un derstanding the principal phenomena that influence weather on sub- seasonal to seasonal time scales, which then influence weather across the globe. Over the next decade, the spatial resolution of weather and climate models will increase to a point where cloud and convective processes will be explicitly r esolved in varying degrees, in contrast to Earth system models of today. High-resolution weather and climate modeling is necessary to make reliable projections of rainfall extremes that are important for flood forecast risk, and hence for informing decisions regarding urban planning, flood protection, and the design of resilient infrastructure. More advanced observations about convective processes will be needed in parallel to these model advances. . Exposure to elevated levels of ambient air pollution is the largest Adverse effects on Air Quality environmental health risk factor globally leading to premature death. Air pollution also has a range of detrimental effects on ecosystems. Regulatory agen cies charged with assessing and mitigating pollution levels need improved observing systems for air pollu tants, and improved unders tanding of the transport and chemical processes relating emissions to impacts. This requires the establishment and maintenance UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-25 Copyright National Academy of Sciences. All rights reserved.

116 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space spatial distribution of PM (including speciation), of a robust, comprehensive observing strategy for the O , and NO along with a modeling strategy that quantifies how pollution is transported. It is a challenge 3 2 to provide observations from space-based platforms al one, especially given this information is needed tion of space-based observations, and expansion of near ground level. The strategy requires a combina tion with chemical transport modeling to deduce aircraft and ground-based observations, in conjunc surface levels of air quality. Key Points Summarized by the Steering Committee:  Advances in weather prediction over a range of time-scales requires a comprehensive set of observations of meteorology and atmospheric composition, along with parallel advances in modeling and computation methods to assimilate data into numerical weather and air quality models. The PBL has broad importance to a number of Earth science priorities. Resolving the 3D  structure of the PBL is an unmet but important challenge, as the PBL influences not only weather prediction and air quality forecasts but is also i nherent to many other high priority Objectives connected to other panel priorities.  The specific measurements needed to advance subseasonal prediction include either sustained observations or enhanced time space resolution ob servations of: a) the 3-dimensional atmospheric state, including temperature, humidity, and winds; b) the atmospheric boundary layer; c) a number of surface characteristics and processes; and d) advanced observations of atmospheric convection, including its mesoscale organization.  Atmospheric convection exerts a profound influe nce on our weather and climate, influencing cloud, precipitation, atmospheric composition, and extreme weather processes. tion exposure globally, and developing effective Accurately characterizing the levels of air pollu  strategies to mitigate the risks, rely on a combina tion of satellite information, atmospheric models nd an understanding of the dyna mics of the boundary layer and and ground-based observations, a atmospheric transport. ********************************************************************************* BOX 3.4 Key Challenge Areas for Weather Prediction Advances in Earth science and applications will o ccur in the next decade due to the evolution of more sophisticated analysis systems. These are enable d in part by increased computer power, leading to forecast model systems of much higher fidelity in ph ysical process representation, increased temporal and spatial resolution, larger ensembles, and longer lead times. Society’s demanding applications increasingly require the outputs of these global high-resolution modeling a nd forecasting systems. Also contributing is technology innovation leading to new instrument and measurement approaches that offer the promise of providing essential new and more integrated approach es to observations for understanding fundamental Earth system processes. e will come from scientific and technological For weather forecasts, advances in the coming decad innovation in computing, the representation of physi cal processes in parameterizations, coupling of Earth- system components, the use of observations with advanced data assimilation algorithms, and the mble methods and how they interact across scales. consistent description of uncertainties through ense This progression is illustrated in Figure 3.6. The ellipses indicate key phenomena relevant for NWP as a -2 4 function of scales between 10 and 10 km resolved in numerical models and the modeled complexity of processes characterizing the small-scale flow up to the fully coupled Earth system. The boxes represent scale-complexity regions where the most significant challenges for fu ture predictive skill improvement exist. The arrow highlights the importance of error propagation across resolution range and Earth-system components. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-26 Copyright National Academy of Sciences. All rights reserved.

117 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space y is immense. Forecasts are central to NOAA’s products The value of accurate forecasts to societ and services that affect more than one-third of America’s gross domestic product. Accurate forecasts save lives, prevent economic losses from high-impact weather, and they create substantial financial revenue in many sectors of society such in ener gy, agriculture, transport and recreational sectors. Forecast systems of today are delivering life-saving and economic societal benefits by expanding into environmental services ecasts into the subseasonal to seasonal range. These (European Commission, 2016) and extending for advances depend on progress made on the connecting el ements called out in Fig. 3.9 which require mutual advances in models, data assimilation, and observations. weather forecast skill (from Bauer et al., 2015). FIGURE 3.6 The notional process for advancing ********************************************************************************* END OF BOX Marine and Terrestrial Ecosystems and Natural Resource Management Land and ocean ecosystems are essential to human we ll-being, providing food, timber, fiber, and many other natural resources. Healthy ecosystems al so help support clean air, clean water, and biodiversity among a wide range of benefits often referred to as ecosystem services. Ecosystems play a pivotal role in the planet’s cycling of carbon, nutri ents, and water as well as energy exchange with the atmosphere. One key aspect is the removal of ex cess carbon dioxide by the ocean and land biosphere, acting to slow the buildup in the atmosphere of a major greenhouse gas. Ecosystem questions are thus closely related to climate, weather, hydrology, and solid Earth questions. Information on ecosystems, and how they are changing over time, is increasingly relevant to decision-making by individuals, businesses, and governme nts. In part, this decision-making need reflects UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-27 Copyright National Academy of Sciences. All rights reserved.

118 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space often closely intertwined. Many ecosystems are the fact that human activities and ecosystems are so directly managed by people: cropl ands and rangelands for agriculture; forests harvested for timber; and protection from flooding; and coral reefs that wetlands and coasts used for fishing, aquaculture, support valuable tourism and recreation industr ies. The boundary between natural and managed ample, the threat of wildfires is changing with ecosystems is becoming more blurred with time. For ex time, because of past land management decisions, because of choices about investments in suppression, and because communities commonly begin to ab ut forests and rangeland as they grow. ce and Application Questions. Priorities related The Ecosystems panel identified fifteen Scien land and freshwater/marine ecosystems, and how broadly to the composition and dynamics of both esponse to human and natura l perturbations. Several composition and dynamics are evolving with time in r of the priority ecosystem Objectives spring from a growing body of evidence that ecosystem function depends in a variety of ways on vegetation and pla nkton composition, how the ecosystem is organized in space, and the factors governing photosynthesis or primary production. Five central interrelated Objectives, four identified as Most Important and one as Very Important, are summarized here:  (E-1a) Distribution. Quantify the global distribution of the functional traits, functional types, and composition of vegetation and marine biomass, spatially and over time.  (E-1b) Structure. Quantify the global three-dimensional (3-D) structure of terrestrial vegetation and 3-D distribution of marine biomass within the euphotic zone, spatially and over time.  (E-1c) Primary Production. Quantify the physiological dyna mics of terrestrial and aquatic primary producers.  (E-2a) Fluxes of CO Quantify the fluxes of CO globally at spatial scales of and CH and CH . 4 2 4 2 ith uncertainty < 25% between land ecosystems 100-500 km and monthly temporal resolution w and atmosphere and between ocean ecosystems and atmosphere.  (E-3a) Flows Sustaining Ecosystem Lifecycles. Quantify the flows of energy, carbon, water, nutrients, etc. sustaining the life cycle of terrestrial and marine ecosystems and partitioning into functional types. Remote sensing has allowed for bulk measu res of land vegetation cover (Box 3.5) and phytoplankton biomass (Box 2.8) as well as the rate of primary production. Only recently, however, has hyperspectral imaging technology advanced sufficientl y to distinguish different types of plants and plankton (Devred et al., 2013) (R osseaux and Gregg, 2013). This information is critical to improve estimates of primary production, nutrient, and carbon cy cling. It will also improve our understanding of how ecosystem variations propagate upward through food-webs (for example, how changes in plankton influence fisheries). Similarly, new active lida r-based sensor technologies open up opportunities to mension, yielding insights on tree-canopy height and characterize ecosystem properties in the vertical di plankton distributions in and below the mixed layer. rial and energy with the atmosphere and other Ecosystems are open systems that exchange mate rstanding of the magnitude and causes of these flows parts of the biosphere and Earth System. Better unde are critical for addressing many Earth System scientific questions and linking into integrative themes on the global carbon cycle. A specific example highlighted by the panel is characterizing the sources and sinks of key greenhouse gases, such as CO and CH with the atmosphere, as part of an effort to constrain 2 4 climate forcing and develop the tools for carbon accounting. Key Points Summarized by the Steering Committee:  Human well-being is closely tied to healthy ecosy stems, which provide a wealth of direct and indirect benefits to society.  Better information on ecosystem composition, functioning, and fluxes will support improved scientific understanding, appli cations, and decision-making. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-28 Copyright National Academy of Sciences. All rights reserved.

119 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Significant improvements in the characterization of ecosystems are now possible, in terms of both  functional traits and vertical struct ure of vegetation and plankton biomass.  between ecosystems and the atmosphere is an Characterizing the exchange of greenhouse gases essential part of understanding the globa l carbon cycle and climate forcing. ********************************************************************************* BOX 3.5 Tracking Changes in Forest Biomass Earth’s forests contain a vast amount of carbon. Recent estimates put the total carbon in trees at 450-650 billion tons (Ciais et al. 2013) . This is equivalent to more than half the quantity of carbon in carbon dioxide in the atmosphere or the amount in approximately 45-65 years of industrial emissions, at current rates. Carbon emissions from the clearing of forests represent one of the largest anthropogenic sources of greenhouse gases. In recent years, for est clearing has released about 10% as much carbon dioxide as fossil energy and industrial activity, and fires associated with climate change are increasing. On the other hand, forests and other terrestrial ecos ystems not subjected to clearing have been operating of 33% of the carbon dioxide from fossil fuels and as substantial sinks, annually taking up an average industry (Le Quéré et al. 2016). e both large, understanding their future trajectory Because the stocks and fluxes of forest carbon ar is a central challenge in climate change science. If forests become stronger sinks for carbon in coming years, then the pressure for rapid decarbonization of the industrial sector moderates. If they become weaker sinks or transition to sources, then the oppos ite is true. Incomplete knowledge about the future behavior of forests is one of the largest uncertain ties in setting a safe schedule for bringing carbon dioxide emissions to zero. But in addition, the emergence of a carbon economy means that forest biomass has an additional benefit beyond the traditional values of ha bitat and wood products. Many parts of the world have active discussions or operational programs that a llow countries and individuals to make forests key mechanisms in the portfolio of strategies they use to manage their carbon emissions. The California forest ubstantial incomes from landowners to realize s offset program, for example, provides a way for protecting or increasing forest carbon (Kelly and Schmitz 2016). tions require accurate quantification of forest Both the science questions and the management op ass estimates has been a major triumph of the last biomass. Improving the accuracy and coverage of biom role. Quantifying forest carbon stocks and fluxes is decade, with satellite remote sensing playing a central always a multi-step challenge, involving small-scal e, ground-based measurements for detailed process studies and calibration, plus satellite data for broad coverage. Usually, mathematical models are necessary raft data are important in validating concepts at for connecting observables at different scales. Often, airc ts for later deployments on satellites. intermediate scales and for testing concep The current state-of-the-science in global for est mapping was published by (Hansen et al. 2013), showing that, from 2000 to 2012, the world lost 2.3 an d gained 0.8 million square kilometers of forest (Figure 3.7). The team used Google Earth Engine to analyze over 600,000 Landsat 7 ETM+ scenes, coupled with high-resolution imagery for validation, to produce global maps of tree cover at 30m spatial resolution. Fire is one of the largest sources of for est loss and also one of the biggest unknowns for the future. A new MODIS-based analysis of global fire activity (Andela et al. 2017) finds a 24% decrease in area burned annually from 1998 to 2015, likely contributing to the forest carbon sink during that period. Other technologies have the potential to improve the accuracy and depth of analysis. Atmospheric carbon dioxide, now available from OCO-2, can be comb ined with models to constrain the locations and magnitudes of carbon flux (Hammerling et al. 2012). OC O-2 and other sensors add further information with the capability of quantifying chlorophyll fl uorescence, a proxy for instantaneous carbon dioxide uptake (Frankenberg et al. 2014). Imaging Radar, ev aluated in space on a shuttle mission in 1994, can provide detailed information on biomass (Rignot et al . 1997). One of the most powerful techniques, lidar, has been used extensively from airc raft (Gonzalez et al. 2010) and has b een successfully integrated with satellite data to provide high-resolution forest biomass maps at the scale of entire countries (Asner et al. 2010). The efficacy of radar and lidar for biomass assessment are the basi s for the upcoming NISAR UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-29 Copyright National Academy of Sciences. All rights reserved.

120 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space pean Space Agency’s BIOMASS radar mission, also (radar) and GEDI missions in the POR, and the Euro ly deployed and validated from aircraft platforms in the POR. Hyperspectral data have also been wide (Asner et al. 2017), establishing their ut ility especially in diverse forests. As the carbon economy grows, the value of accura te satellite-based assessments of forest carbon will grow in parallel. FIGURE 3.7 (A) Tree cover, (B) forest loss, and (C) forest gain. (D) shows a color composite of tree cover in green, forest loss in red, forest gain in blue , and forest loss and gain in magenta. In panel D, loss and gain are enhanced. From (Hansen et al. 2013). ********************************************************************************* END OF BOX Climate Variability and Change: Seasonal to Centennial The climate panel considered a range of pro cesses that act across time scales: short-lived processes relevant to weather, pro cesses that shape interannual variability, processes relevant to important modes of decadal variability, and longer time-scale processes associated with anthropogenic climate change. On decadal timescales, oceanic variations can imprint themselves on atmospheric weather onal shifts and changes in the occurrence of both patterns, leading to seasonal- and decadal-scale regi regularly-occurring weather patterns and extremes like ecasting these shifts, and droughts and floods. For their societal impacts, is now an active area of r esearch and one of the grand challenges of climate science. Climate variability across these time scales h as tremendous impacts on society. Understanding them requires observations for monitoring the Earth, so as to quantify what chang es are occurring, and to explore the mechanisms through which these changes occur. Advanced Earth system models provide an important tool for accomplishing these goals, through their ability to disentangle the interactions most responsible for the changes being observed. The six Objectives identified by the panel as Most Important were associated with the following two Questions: (C-1) Sea-level Rise. How much will sea level rise, globally and regionally, over the next decade  and beyond, and what will be the role of ice sheets and ocean heat storage?  (C-2) Climate Forcings and Sensitivity. How can we reduce the uncertainty in the amount of future warming of the Earth as a function of foss il fuel emissions, improve our ability to predict UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-30 Copyright National Academy of Sciences. All rights reserved.

121 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space local and regional climate response to natura l and anthropogenic forcings, and reduce the uncertainty in global climate sensitivity that dr ives uncertainty in future economic impacts and mitigation/adaptation strategies? Sea level rise: land ice contributions and ocean heat storage . Global sea level rise is one of the integrated responses of the Earth system to increased h eat stored by the planet, with potentially significant of global mean sea level impact on society’s security and prosperity. Given the expected ~25 cm to 1 m 4.6% of the global population is expected to be rise by 2100, without appropriate adaptation 0.2 to flooded annually with expected annual losses of 0.3 to 9. 3% of global gross domestic product (Hinkel et al., 2013). Accurate projection of sea level rise is essential for managing these risks. Sea level rise is tightly coupled to several aspects of the Earth syst em (Figure 1.3), and advances in predicting future change require scientific progress on a complex array of poorly understood interactions. As a result, there st is a wide spread in 21 century projections of sea level rise. The two main contributors to sea level rise are: 1) loss of land ice (mountain glaciers and the Antarctic and Greenland ice sheets), and 2) thermal expansion of the sea water as its temperature 1 increases. eat input, in conjunction with sea level rise Sustained monitoring of both ice loss and h Understanding the relative contributions to global monitoring, is required to quantify these two factors. sea level change in terms of ocean warming and ma ss changes has been made possible by simultaneous global observations of the sea surface height fro m satellite altimetry (TOPEX/Poseidon and the Jason series), ocean mass from satellite gravimetry (GRACE ), and ocean density from Argo floats (Box 3.6). Climate Forcings . There are two basic types of aerosol forcing, aerosol direct effects defined by effects where aerosol affects the energy balance aerosol influences mostly on sunlight and aerosol indirect of Earth through their effects on clouds. Of these two forcings, the aerosol indirect effect contributes by far the largest uncertainty. The coupling of cloud, pr ecipitation, and aerosol observations available from udies has enabled a deeper understanding of aerosol the A-Train (Box 2.9) and integrated into model st indirect effects and revealed the complex nature of the problem that involves various pathways primarily sses. Our understanding of the processes and complex determined by cloud physical and dynamical proce interactions relevant to all aeros ol cloud interactions is still rudimentary, and the cloud-aerosol impacts cannot be deciphered from observations alone because of the inherent ambiguity associated with assigning a cause to an observed effect. . The amount of warming of the Earth System that Climate Sensitivity and Climate Feedbacks occurs due to a given level of greenhouse gases is subs tantially determined by the climate feedbacks that act to define the eventual response to any given radi ative forcing (Box 3.6). This response is referred to as Climate Sensitivity (CS) and is defined as the amount of global average temperature ch ange per change in effective radiative forcing (IPCC, 2013). Climate sensitiv ity is an aggregate result of contributions from a wide range of feedback processes including clouds, water vapor, temperature lapse rate, surface albedo, challenges for predicted future economic impacts of and carbon cycle. Its uncertainty is one of the largest future emission scenarios (SCC, 2010). Model simula tions with high climate sensitivity and large low climate sensitivity and small (negative) aerosol (negative) aerosol forcing, as well as simulations with forcing, are able to fit past temperature changes but differ significantly in their prediction of future temperature (Penner et al., 2010). Cloud feedbacks, in particular, are the largest sour ce of uncertainty on determining this sensitivity (IPCC 2013). Cloud processes also have far reaching infl uences across the climate system. They exert a significant influence on the mass and energy balan ces over ice sheets (e.g., Von Schuckmann et al., 2016) and sea ice (Kay et al., 2009), they are a fundamental conduit of fresh water, and in the form of 1 Changes in land water storage also contribute to change in sea level. They are the dominant contributor to sea level during ENSO events and have a significant contribution to the long-term trend. Contributions to sea-level rise are discussed in chapter 3, “Contributions to Global Sea-Level Rise,” in NRC, 2012. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-31 Copyright National Academy of Sciences. All rights reserved.

122 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space her extremes and in shaping the modes of seasonal- convection they are instrumental in producing weat “reduce the uncertainty in low and high cloud interannual variability. The panel’s quantitative objective to feedback by a factor of 2” (Objective C-2a) reflects the large uncertainty in feedbacks involving high and low clouds. Two measurement approaches to advance this topic were identified: Development of observational metrics agains t which the feedback can be assessed.  This typically involves cloud observations, sustained over decades, matched to top-of atmospheric radiative flux observations.  Quantification of processes. inties are those attached to low The largest cloud feedback uncerta and high clouds. High cloud feedbacks are strongl y shaped by convective processes and, in turn, the way convection is shaped by the atmospheric circulation (Bony et al. 2015). Low cloud feedbacks are intrinsically connect ed to the main branches of the atmospheric circulation and the ith the planetary boundary layer. interaction of this circulation w Precipitation is an essential aspect to both feedb ack processes, as it both shapes the lifecycle of clouds, controls effects of aerosol on them, and c ouples to the dynamical atmosphere via the latent heating produced. Carbon cycle feedbacks, especially over land surf aces, rival those of the physical climate system (IPCC, 2013). In reality, the feedbacks that contro l water and energy exchanges within the physical system on short timescales are fundamental component s of the carbon feedbacks that operate over much longer time scales. Key Points Summarized by the Steering Committee — Sea Level and Heat Content:  Although the change of the global mean sea le vel is now well determined from space-based measurements, maintaining and improving the sea level measurement system is essential to and the rest of the Earth system. understand the linkages between the ocean  ting the rate of decadal change of sea level and A substantial amount of the uncertainty in estima of the heat storage component to the seasonal- ocean heat storage stems from the contribution interannual variability of the coupled atmosphere-ocean system. Our ability to predict the rate of sea level rise in the future is compromised by a lack of  quantitative understanding of the processes affecting sea level.  The ice sheets account for one-third of the current trend in global mean sea level (Dieng et al., 2017). Greenland and Antarctica lose about 300 Gt/y r at present. An observational system that detects changes of the total surface mass balance at the 5% level (15 GT/yr over the course of a decade) is needed to understand the interactions of ice in the Earth system at the regional scale and on a level that can test physical processes relevant to longer term change (NRC, 2015).  Careful monitoring of the radiation energy in and out of Earth continues to be essential for understanding many aspects of the changing Earth system. An important challenge is monitoring e warming of the planet, and new approaches to of the small energy imbalance associated with th monitor the changes in heat content of the plan et should be explored. The difference between the nge and ocean mass change provides a direct joint altimetric measurement of sea level cha estimate of the heat taken up by the oceans and t hus represents an indirect means for monitoring change to the planetary heat content.  Reliance on in situ Argo observations for deducin g the planetary heat imbalance will continue. Improvements in these observations are needed to better represent the oceans, particularly the implementation of deep Argo to encompass the full water column to 6000 m depth (Zilberman and Maze, 2015). Key Points Summarized by the Steering Committee — Climate Sensitivity and Feedback: UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-32 Copyright National Academy of Sciences. All rights reserved.

123 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space The largest sources of uncertainty of climate sens  itivity arise from feedbacks associated with low tanding of the connection between cloud and and high clouds. Improving our quantitative unders convection processes and clouds, water vapor, and the atmospheric circulation is essential for addressing these cloud feedback uncertainties.  Direct observation of decadal time scale cloud feedback signals from the Earth, as well as climate model predictions, requires improved accuracy and traceability to international standards for cloud property and radiative flux satellite observations. More rigorous approaches are needed to  connect quantitative objectives for cloud process observations with specific quantitative cloud fee dback objectives. A close association between observations and high-resolution cloud process m odels will be essential, and OSSEs based on these advanced model systems offer one viable approach to quantifying observational impacts. The effect of aerosols on clouds will influence th e response of clouds to climate change, so es understanding the effect of aerosols on clouds. quantification of this objective also requir  Models of the Earth system are increasing in fidelity. Advances in cloud feedback will occur based on a closer coupling between observations and models to explore cloud processes over a spectrum of time scales, from weather to seasonal, and from interannual to decadal and longer.  The coupling between carbon, water, and energy is central to understanding the carbon cycle and feedbacks that shape it. Key Points Summarized by the Steering Committee — Climate Forcings:  The largest source of uncertainty in determining c limate forcing in models is quantifying aerosol ons. Improving our understanding will require forcing, including aerosol-cloud interacti measurements capable of examining aerosol and cloud vertical profiles and sizes. Vertical profiles of aerosols are also essential for determining how and whether aerosols affect cloud microphysical properties.  Because the direct aerosol impacts on radiative fluxes and ensuing climate variables are a strong function of where in the column they occur (w hether high above the clouds and water vapor or lower in the atmosphere), measurement of the ve rtical profile of aerosol extinction is essential.  Understanding aerosol-cloud-precip itation interactions requires observations of the aerosol-cloud- precipitation cycle. Better representation of clouds themselves in climate models is essential to advance cloud-aerosol interactions. Some progress can be expected in the coming decade because of more advanced model systems that are presently in development, but joint observations of clouds, aerosols, and precipitation will be needed to support these more advanced model systems. Improved aerosol measurements from space would  also improve substantially our ability to determine the health impacts of aerosols (equivale ntly referred to as particulate matter), which are major environmental contributors to human mortality. ********************************************************************************* BOX 3.6 The Energy Imbalance of Earth Earth’s Energy Imbalance (EEI) is a fundamen tal measure of our warming planet. Earth is presently gaining energy at a rate of about 0.5-1 Wm-2 owing to increasing concentrations of greenhouse gases and the large thermal inertia of the oceans. This gain is challenging to measure directly. Our current direct measurements of radiation balance at the t op of the atmosphere are not accurate enough to quantify this small energy imbalance. Alternative methods are thus needed to address this fundamental property of the Earth system. As over 90% of the EEI is stored in the oceans, we currently rely on in situ measurements of ocean temperature change from Argo floats to deduc e this imbalance. Direct measurements, however, have some limitations, raising a numbe r of questions about how much of the heat is stored at depths not UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-33 Copyright National Academy of Sciences. All rights reserved.

124 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space ous global observations of the sea surface height from reached by Argo. As shown in Figure 3.8, simultane satellite altimetry (the Jason series) and ocean mass from satellite gravimetry (GRACE), in conjunction ssible to understand the re lative contributions to with ocean density from Argo floats, have made it po global sea level change in terms of ocean warmi ng and mass changes (and equivalently estimate the increased energy being stored in th e global oceans). Altimeter and gravimetry data, when compared to Argo, agree with each other within statistical uncertainties (Llovel et al., 2014; IPCC 2013). These data suggest that most of the heat taken up by the ocean is stored within the top 2000 m of the ocean (Llovel et al., 2014). The approach to estimate the total heat uptake of the oceans using a combination of altimeter and gravimetry measurements is currently the most pr omising way of meeting the space-based monitoring needs of this very elementary property of our warmi ng planet. This information, when combined with in situ profile data from Argo and deep Argo, offers a comprehensive way of determine both how much heat is mixed into the oceans and where this hea ting is stored within the water column. FIGURE 3.8 More than 90% of the enhanced heati ng by greenhouse gases is being taken up by the oceans. This heating contributes a large fraction of the observed sea level rise. The global mean sea-level variations are observed variations by satellite altimetry (blue). The mass contributions from land sources (mostly ice sheets) are determined from GRACE data (solid black). The steric sea level rise component (thermal expansion) is the difference (dashed black cu estimated based on in situ rve) and is independently observations (red) limited to ocean depths up to 2000 m. These data suggest most heat uptake occurs over this depth of ocean (Llovel et al, 2014). ********************************************************************************* END OF BOX UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-34 Copyright National Academy of Sciences. All rights reserved.

125 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Earth Surface and Interior: Dynamics and Hazards Continuous satellite observations of the solid Eart h enable us to document, explain, and even of spatial and temporal scales. Such dynamics anticipate Earth dynamics on an unprecedented range include volcanic eruptions, earthquakes, landslides, ground deformation due to tectonics or large-scale ers, sea level change, erosion, large scale tectonic groundwater extraction, changes in ice-sheets and glaci uplift of mountains, and even variations in Ea rth’s magnetic field. These phenomena motivate basic science questions and theories and also illuminate th e urgent needs and opportunities for developing hazard reduction programs. identified the following key goals for sustai ned, high-density space-based observation: The panel (a) quantification of the nature and pace of solid Ea rth change; (b) characterization of the precursors, impacts, and key thresholds of disruptive events (e.g ., volcanic eruptions, or wildfires); (c) delineation of incremental change in Earth’s life-sustaining surface (its “critical zone”) in response to short-lived events and to sustained trends (e.g., more frequent drought s, permafrost loss, or ecological shifts) and (d) assessment of the impact of human activity on res ources, environmental quality, sustainability, and habitability. tions Questions (Table 3.3), and within these The panel identified seven Science and Applica broader questions ranked the following six Objectives as most important: (S-1a) Volcanic eruptions. Measure the pre-, syn- and post-eruption surface deformation and  products of Earth’s entire active land volcano inventory with a time scale of days to weeks.  (S-1b) Seismic activity and earthquakes. Measure and forecast interseismic, preseismic, coseismic, and postseismic activity over tectonically active areas, on time scales ranging from hours to decades.  (S-2a) Response to disasters. Rapidly capture the transient processes following disasters for improved predictive modeling, as well as for res ponse and mitigation through optimal re-tasking and analysis of space data. and its driving processes at global,  Quantify the rates of sea-level change (S-3a) Sea-level change. -1 regional, and local scales, with uncertainty < 0.1 mm yr for global mean sea-level equivalent -1 and <0.5 mm yr sea-level equivalent at resolution of 10 km.  (S-3b) Coastline vertical motion. Determine vertical motion of land along coastlines, at -1 uncertainty <1 mm yr .  (S-4a) Landscape change. Quantify global, decadal landscape ch ange produced by abrupt events and by continuous reshaping of Earth’s surface due to surface processes, tectonics, and societal activity. Volcanic eruptions. Frequent satellite observations of volca noes can be used to document changes in their shape, their emitted air chemistry, and both the temperature and composition of crater-lake or ground surfaces. Detected changes may precede eruptions by weeks to months, and thus be used in a warning system. Vertical precision of ground change detection needs to be 1 to 10 mm. Ideally, repeat frequency of observations could be adjusted to captu re areas undergoing rapid change. Temperature and compositional estimates from hyperspectral observati ons would benefit from sampling intervals of hours to days. Seismic activity and earthquakes. Earthquake prediction remain s a grand challenge. Recent satellite-based observations have revealed transient s lip phenomenon over periods of days to years that may shed light on the physics of earthquake cycles. Measurement of four types of phenomenon will further advance the field: 1) crustal deformation betw een seismic events, 2) tem poral variation in gravity associated with large earthquakes, 3) high resolutio n bare-earth topography, and 4) high resolution seismic activity and surface deformation (from terrest rial measurements). The length and time scale of UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-35 Copyright National Academy of Sciences. All rights reserved.

126 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space on, to 1mm/week for slow slip events and repeat quantification varies from 1mm/yr, for interseismic moti measurements of less than 12 days over seismically active areas. Devastating earthquakes, tsunamis, landslides, floods, and volcanic Response to disasters. eruptions strike particular places and create a sudden local need for information to guide disaster response. Along with optical imagery, the suite of InSAR, high-resolution topography, and both hyperspectral and thermal infrared measurements fro m space provide an invaluable framework. This “rapid-response” objective will require the ability to redir ect satellites, or the creation of a constellation of satellites to provide full Earth coverage. Sea level change. The need to quantify the rates of sea- level change and its driving processes at global, regional, and local scales is of great importance as discussed by the Climate Panel. Quantifying and understanding sea level changes requires use of several satellite-based instruments including using radar altimeters over the oceans, and radar and laser altimeters over the ice sheets, along with GPS, InSAR, and GRACE gravity measurements. Gravity me asurements not only provide critical information on the contributions of ice sheets and glacier systems to sea level rise, but also changes and movement of mass throughout the Earth (Box 3.7). Coastline vertical motion. The Solid Earth panel identified the quantification of vertical land profound but poorly constrained, and hence ranked its quantification as motion on local sea-level rise as Most Important (S-3b). In many areas, land subsidence is the leading contributor to relative sea level rise. Both natural and anthropogenic pro cesses contribute to vertical land moti on. GPS can be used to quantify vertical surface deformation at spatial scales on the order of 10 km or less. High-resolution (1-m horizontal and 10 cm vertical) global topography is needed to predict the path and magnitude of inundation across subsiding areas and during large storms. Landscape change. The surface of the Earth, which includ es the ground surface and its vegetative slow, and even nearly imperceptible on seasonal to mantle, is constantly changing. Many changes are yearly level. But sustained observations from space de tect features such as the elevation change due to ice sheets, or the progressive change in vegetation tectonics, the slow shifting of rivers, the movement of much more abrupt changes in landscapes due to accompanying regional climate shifts. In addition, wildfire, earthquakes, landslides, floods, deforestation, urbanization, and agricultural practices can be uniquely quantified as a time series of change usi ng sustained and continual satellite-observations. Documentation of landscape change has wide appli cation including providing insight for theory of ologic and climate models, ecosystem analysis, landscape dynamics and evolution, information for hydr ng and land management. and data for hazard mappi Terrestrial Reference Frame. In addition to these six highest prio rity objectives, there is a critical need for protecting and extending the Terrestrial Reference Frame , an observation infrastructure system which supports all satellite missions. An accurate global terrestrial reference frame provides the framework for positioning scientific satellites and airc raft, and underpins our commerce infrastructure. 1 mm and a rate accuracy of 0.1 mm/yr. Such The reference frame must have a positional accuracy of accuracy is achieved through a combination of Ve ry Long Baseline Interferometry (VLBI) and Satellite Terrestrial Reference Frame Laser Ranging (SLR) (Davis, et al., 2015). Sustaining this invaluable requires: 1) maintaining global pa rticipation and funding support with other agencies/countries; 2) increasing capacity; 3) lowering cost; 4) upgrading ol der sites (some VLBI/SLR instruments are more than 30 years old): and 5) improving real-time capabilities for GPS/GNSS. Key Points Summarized by the Steering Committee:  Key advances in understanding an d predicting earthquakes, land scape evolution, landslides, volcanic eruptions, groundwater dynamics, ice- sheets, sea-level rise and other hazards and UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-36 Copyright National Academy of Sciences. All rights reserved.

127 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space satellite data with higher spa resources can be accomplished using tial resolution, expanded global Earth coverage, and higher temporal frequency of sampling.  Measurements from space that are most important to accomplish these goals include: InSAR, GPS, gravity, and hyperspectral observations.  Satellite-derived high resolution lidar to obtain hi gh-resolution (1 to 5 m spatial resolution) bare- earth topography globally remains a top priority, but is not yet technically feasible.  Maintaining and improving the global Terrestrial Reference Frame is critically important. ********************************************************************************* BOX 3.7 Using Satellite Gravity to Understand the Mass Change d Climate Experiment (GRACE) has provided Since launch in 2002 the Gravity Recovery an unique insights with far-reaching be nefits for understanding Earth system mass transport (Tapley et al., By measuring gravity changes over the entire Earth, the GRACE mission produces 2004) (Figure 3.9). monthly maps of how liquid water, ice and solid Ea rth components are being redistributed within and between the ocean and the continents (Fasullo et al., 2013). This info rmation has helped to understand and to quantify mass changes of ice sheets (Rignot et al., 2013), and mountain glaciers, water losses from lakes and underground aquifers (Rodell et al., 2015), and their overall contribution to sea level rise. By mapping seasonal and year-to-year changes in water storage across the landscape, GRACE contributes to our understanding of the global water cy cle. In addition, with a 15-year record of gravity measurements, it is possible to discern the comparatively small, but important, decadal trends associated with climate post glacial rebound (Ivins et al., 20 13), and the epoch-related mass change (Johnson et al., 2013), redistribution associated with large earthquakes (Chen et al., 2007; Han et al., 2016). Globally sea level is changing mainly as a resu lt of two processes, density changes due to nges due to water mass input from ice sheets, glaciers, and changes in temperature variations and mass cha ear what portion each effect had on global sea level net land water storage. Before GRACE, it was not cl change. GRACE not only gave insight into the magn itude of the mass component but also its sources and allowed an estimate of the heat absorbed by the ocean (Riva et al., 2010). The separation on the annual large underground aquifers identif ies emerging problems and allows variability from the decadal trends in future water availability for agriculture and planning for resource management with regard to consumption. Based on the significant advances in both measure ment capability and the analytical framework during the mission life span of 15 years, GRACE data is now an essential asset for a number of operational applications, such as drought forecasting within the framework of the US National Drought Monitor (Houborg et al., 2012). The ingestion of GRACE data by a Land Data Assimilation System allows significant improvement in the quality of th e total Terrestrial Water Storage estimates (higher e provision of these products supports forecast and spatial and temporal resolution). The near real-tim planning activities related to water use for agricu ltural and consumption purposes. Recent international efforts were initiated that use GRACE gravity obser vations for disaster forecasting and management response (e.g., The multination European Gravity Service for Improved Emergency Management - EGSIEM). In addition to earthquake assessment and drought forecast (http://nasagrace.unl.edu), the GRACE Total Water Column measurements provide cr ucial information for implementing a global early flood detection and prediction capability (http://egsie m.eu/project/introduction). These examples, which demonstrate the ability of the GRACE measurement as a unique tool to quantify Earth’s mass change on a global basis as well as the ability to determine the dis tinct local components of the global mass change, underscore the importance of global gravity me asurements in understanding the Earth System interactions. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-37 Copyright National Academy of Sciences. All rights reserved.

128 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space FIGURE 3.9 Over 15 years of gravimetric data from GRACE illustrates decadal mass change trends due to changes in total land surface water storage and drou ght patterns, changes in snow, ice, and ocean mass, and changes due to postglacial crustal rebound and larg e earthquakes. As shown in the local area records, the total signal involves a large annual signal with a much smaller longer-term trend. Source: Tapley, Save and Bettadpur, 2017. ********************************************************************************* END OF BOX Integrating Themes Perspective on the Assessment It is important to examine the science and applica tions priorities not just from the perspective of science perspective that establishes a more multi- the five panels, but also from an Earth System disciplinary view of the science and applications be ing recommended. In part, this system perspective ensures that important topics do not “fall through th e cracks” between panels, that we independently assess our choice of science and applications prioritie s, and that we adequately address the breadth and depth of Earth system science. Such Integrating Them e analysis allowed the co mmittee to re-examine the work of the panels, reinforce the importance of ke y topics, and uncover new science or applications not revealed by a single thematic perspective alone. As described earlier in this chapter, the inte grating themes were addressed in an Integrating Themes Workshop attended by members of the steer ing committee and representatives of each of the UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-38 Copyright National Academy of Sciences. All rights reserved.

129 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space panels. The workshop focused on four topics: 1) water and energy cycle, 2) carbon cycle, 3) extreme events, and 4) miscellaneous topics exploring other important aspects of the Earth system that do not necessarily fit under the previous three topics. The miscellaneous category included a variety of topics such as sea level rise, tipping points, and human health. The strategy for placing disciplinary science objectives into the broader framework of an integrating theme could have followed a number of dir ections. The approach adopted was to organize this discussion around the important physical cycles of the Earth system that are widely recognized as fundamental to understanding the Earth system and predicting its change. The three cycles of water, energy and carbon served as main themes for connec ting across panels. These three cycles have also served as the organizing framework of the grand Earth science challenges identified under the World Climate Research Program (Asrar et al., 2013) and as a structure for the planning of the Global Climate Observing System (Simmons et al., 2016). An add itional integrating perspective was developed around the topic of extremes given the fundamental importan ce and visibility of extreme events to society. Though not comprehensive, these formed the basis for examining the panel priorities from a more integrated Earth system perspective. Both the Integrating Themes Workshop and the ser ies of panel deliberati ons further identified modeling of the Earth system as an important integr ating theme. Models serve a fundamental basis for understanding the interactions between the subsystems that are essentia l in shaping the variability and 2 changes of Earth and its climate . Observations are now increasingly being tied to modeling of the Earth System so as to disentangle the interactions and establis h the causal relationships that determine them (see Modeling section in Chapter 4 for further discussion). Extreme Events Given the great impact and visibility of extreme events to society, the perspective of extreme important way to consider the Earth system context events was thought by the steering committee to be an the character of extreme events as the Earth system of the panel’s priorities. The potential for a change in undergoes change has many important societal and economic impacts (Box 3.8). Extremes are by definition rare and thus it takes longer time periods of monitoring and better resolution in both space and ents. The development of high-resolution data from time to characterize long-term changes in extreme ev 3 current archives is one important effort needed to address extremes . ********************************************************************************* BOX 3.8 High Impact Weather, Climate, and Geophysics Extreme Events Have a Large Societal Impact High-impact weather, climate, and geophysical extreme events occur over a wide range of temporal and spatial scales (Figure 3.10), which have significant societal impacts (e.g., human health, food and water security, etc.). These are becoming more extreme as climate changes, with wildfires being a prominent example. We can and must advance our ab ility to better observe, monitor, and predict natural hazards and extreme events to meet so ciety’s needs in a changing climate. 2 For example, the IPCC glossary formally considers th at the inclusion of the bi ogeochemical carbon cycle distinguishes an Earth-system model from the physical cl imate model where the latter provides the coupling models of the atmosphere, ocean, land and ice. 3 A prioritization of the many challenges presented by extremes is given in the World Climate Research Program’s Grand Challenges, availa ble at http://www.gewex.org/about/s cience/wcrps-grand-challenges/. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-39 Copyright National Academy of Sciences. All rights reserved.

130 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space FIGURE 3.10 High-impact weather-climate extreme events occur on time scales from minutes to centuries and beyond. Observing, monitoring, and pred icting these complex extreme events require an ary and transdisciplinary in novations to advance our integrated Earth system approach with interdisciplin capability to better understand and predict them a nd prevent natural hazards from becoming human disasters. This chart shows how Earth system observa tions, modeling, and data assimilation can be best used together for building a weat projection system to inform her-climate prediction and long-term decision-making process in response to natura l hazards and meet societal needs. There are common characteristics of extreme even ts across all time and spatial scales. They are usually results of complex interactions among va rious processes from either within or different components of the Earth system. They are relatively rare, difficult to predict, and have high impacts on society. The approach to advance our understanding of extremes and our ability to predict them is to address fundamental physical and dynamical processes th at underlie a given type of extreme event for which our understanding and prediction abilities are poor. occurrences of events from different components Extreme events are often a result of concurrent of the Earth system on different time scales. For example, landslides can be caused by extreme rainfall events over just minutes to hours. The conditions that lead to instability may develop over thousands of years as the landscapes evolve, or can result from a r ecent disturbance such as deforestation that reduces the strength of the soil. Landslide predictions use t opographic data (ideally high resolution), monitored and predicted precipitation, and estimat es of hillslope material properties, partly controlled by vegetation. Flash floods and droughts occur from hours to seasonal a nd decadal time scales (Zhang, 2013). Observing and modeling the processes leading up to these events requires knowledge of convective precipitation in the atmosphere, hydrological properties of soil mois ture and river flow, and climate dynamic processes such as ENSO on interannual time scales. There are many examples of using satellite obser vations proven to be effective in monitoring, managing and responding to hazards and extreme even ts (e.g., extreme lightning events (Lang et al. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-40 Copyright National Academy of Sciences. All rights reserved.

131 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space ellite-based global landslide model (Farahmand and 2017), emergency mapping (Voigt et al. 2016), sat nge of high-impact extreme weather events is a focus of the AghaKouchak, 2013)). A wide ra international community. The requirements for integrated Earth system observations and modeling for predicting high- impact extreme events are:  lti-scale and multi-component interactions leading Observe state variables that best represent mu to extreme events.  Monitor global and regional trends of extreme events and impacts.  Understand predictability of extreme events using advanced Earth system models.  Quantify uncertainty and improve prediction and long-term projection of extreme events in a changing climate. ********************************************************************************* END OF BOX Carbon, Energy, and Water Cycles The hydrological and carbon cycles of Earth and their interactions with the Earth energy balance are widely understood to be the foundation for unde rstanding and modeling of the Earth as a physical system. One reason is the basic importance of water to life and the central and interactive role the cycling of water plays within the Earth system. Another reason is the seminal role of energetics as a physical basis for understanding of the evolving Earth system and partly through the widespread consequences of rising levels of carbon dioxide and methane in the atmosphere. Water and Energy Cycle. The hydrological and biogeochemical cycles, and the energy cycle that couples to them, can no longer be considered to be changing solely due to natural variability. es occur across a range of space and time scales. The terrestrial Anthropogenic influences on these cycl component of the global water cycle on the regional scale, for example, is highly managed. On the larger scale, the hydrological cycle is changing due to climat e change, in ways that are not yet fully understood. One aspect of climate change is increased heat uptake by the global oceans. This heat uptake, together with an increased amount of fresh water added to the o ceans associated with melting land ice, results in rising sea levels. The water and energy cycle theme underpins a number of the most important topic areas identified across the ESAS interdisciplinary panels:  Global Hydrological Cycles and Water Resource Panel. There are a number of important water- related variables that are central to the most important hydrological science challenges and to water resource applications. These include soil mois ture, stream flow, lake and reservoir levels, snow cover, glaciers and ice mass, evaporation and transpiration, groundwater, water quality and water use. High-resolution precipitation measuremen ts however emerged as a high-priority by the panel. Numerous discussions within the precipita tion community, reflected in part by multiple white paper submissions to this decadal survey, i ndicate the need and desire to continue to (a) advance the quality of space borne instantaneous precipitation measurements not adequately covered by GPM, and (b) improve the quality as well as space/time resolution of measurements of precipitation. For the latter, in particular, th ere is growing consensus that the key to success is better process related observations coupled to fine-scale models. A second high-priority measurement that emerged is the surface flux of evapotranspiration which is a flux common to both the water and energy cycles thus linking the two. The difference between surface precipitation and evapotranspiration (P-E) is c onsidered a fundamental hydrological balance UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-41 Copyright National Academy of Sciences. All rights reserved.

132 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space and surface runoff. The latent heat flux is an quantity being a measure of ground water storage important component of the surface available energy and is a primary driver of the surface boundary layer that influences the coupling of the land with the atmosphere and a topic of high importance to weather and air quality. Weather and Air Quality Panel. This panel identifies the advancement of weather prediction skill  on sub-seasonal to seasonal (S2S) timescales as one of the most important challenges of the coming decade. The representation of physical pr ocesses in parameterizations, coupling of Earth- system components, the use of observations w ith advanced data assimilation algorithms are essential ingredients for progress. Moist processes associated with atmospheric convection and the coupling of these to the atmospheric circula tion largely determines the evolution of major modes of atmospheric variability on S2S timescal es and principally establishes the precipitation patterns associate with these modes of variability. The planetary boundary layer is also intimately Earth as it is linked to surface processes that are connected to the water and energy cycles of the important to the objectives of the Global Hydr ological Cycles and Water Resources panel, the l Resource Management panel, and the Climate Marine and Terrestrial Ecosystems and Natura Panel. These linkages are achieved through n ear-surface atmospheric quantities such as wind speed, precipitation, aerosol and trace gases, and air-sea-land surface fluxes of energy, water and carbon. Marine and Terrestrial Ecosystems Panel. Land vegetation plays a central role modulating  surface energy and water fluxes. Water availab ility, in particular, shapes the distribution, productivity, and dynamics of terrestrial ecosyst ems. Different types of vegetation and the seasonal cycle of leaf cover modify the color or albedo of the surface, especially compared to bare soil or snow, and thus the fraction of solar ra diation reflected back to space. Transpiration by plants strongly affects the partitioning of surface heat losses between sensible and latent heat flux influences the amount of precipitation reaching and surface temperatures. Vegetation cover also the surface, soil infiltration, and surface runoff.  Climate Variability and Change Panel. Processes that couple water and energy are fundamental to the most pressing climate science challenges iden tified by the climate panel. On one scale, the increased amounts of heat being absorbed by th e global oceans, together with an increased amount of fresh water added to the oceans associated with ice melt, results in the rising sea levels. Conversely, bulk measurements of the volume and mass changes of the oceans are a direct indicator of the planetary energy imbalance. Wa ter-energy coupled processes also shape the most influential climate feedback processes that determ ine the climate sensitivity through the profound and complex influences of water on energy fl ows within the Earth system. Water vapor feedbacks, carbon feedbacks, cryosphere feedback s, cloud feedbacks, aerosol-cloud forcing, and es in the availability and state of water and the precipitation are all essentially shaped by chang influence of these changes on the energy cycl e. The two most important cloud feedbacks identified by the climate panel are associated with low and high clouds and how these clouds connect both to their environment and affect th e radiation balance of Earth. Progress on these feedbacks requires process scale observations not only of cloud properties which include dynamical properties of clouds, convection and precipitation. Earth Surface and Interior Panel. Rainfall, snowmelt, and coastal storms drive erosional  processes that evolve landscapes and generate hazards, and Earth surface characteristics, such as slopes, aspect, soil permeability, and the shape, or ientation, geometry of channels and basins. determine the terrestrial pathways of water. The Earth surface processes community is actively exploring the relationships between climat e, tectonics, and topography. Higher quality UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-42 Copyright National Academy of Sciences. All rights reserved.

133 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space ace and time, will enable the advancement of precipitation observations, more resolved in sp mechanistic theories and hazard prediction. La ndslides (also Box 4.8) are mostly commonly lead to destabilizing pore water pressures on caused by exceptional precipitation events, which hillslopes, and to related issues such a gully er r-channel avulsion, bank osion, topsoil loss, rive erosion, and widespread flooding. Landslide wa rning programs have been developed that use precipitation forecasts, combined with other in formation, to anticipate periods of potential landslide activity and landslide susceptibility, and to underpin enhanced early- warning and mitigation efforts. NASA, for example, has rece ntly launched the Landslide Hazard Assessment for Situational Awareness (LHASA) for determini ng regional landslide probability in near real- time. LHASA provides web-based mapping of precipitation over various periods and the corresponding locations of potential landsliding. Carbon Cycle. The natural carbon cycle and the human-dri ven perturbations form an integrating cles, biogeochemistry and the functioning of the land theme that is closely linked to water and energy cy and ocean biosphere, and a broad range of human ac tivities that include fossil-fuel use, industry, agriculture and forestry, and other human land-uses. A central scientific focus is to document and understand the processes controlling the atmospheric levels of the greenhouse gases carbon dioxide and methane, information that is essential for improvi ng projections of future climate forcing trends. Addressing the carbon cycle is a scientific grand challe nge that requires integration across the physical, chemical, and human socio-economic aspects of th e Earth system, and the carbon cycle theme arises across a number of the most important topic areas identified by the ESAS interdisciplinary panels. Present-day atmospheric carbon dioxide levels are near ly 45% higher than preind ustrial conditions, acting as the single largest human-factor contributing to glob al climate change. Currently, a little less than half mosphere (IPCC, 2014), with the remainder removed of human carbon dioxide emissions stay in the at into ocean and land reservoirs. Ocean uptake at present is predominately caused by dissolution of elevated atmospheric carbon dioxide into the surface ocean and subsequent physical transport into the deep ocean by circulation and storage through ecosystem proce sses. The terrestrial carbon storage sink is less well understood but reflects a mixture of carbon dioxide a nd nitrogen fertilization, climate change, land management, permafrost change, a nd forest regrowth. Looking forward in time over this century and beyond, the continued build-up of carbon dioxide in th e atmosphere due to cumulative human emissions is expected to be the dominant an acks between climate change and the thropogenic climate forcing; feedb storage of carbon in land and ocean reservoirs are also important because they could amplify or damp atmospheric carbon dioxide growth and warming. For example, the release of methane from thawing permafrost in the warming Arctic constitutes a str ong positive feedback that further exacerbates warming, while increased vegetation growth at higher latit udes, can increase carbon uptake helping to reduce the rate of warming. Vegetation, however, can produ ce a darker surface, which in turn increases surface warming. The carbon cycle theme underpins a number of th e most important topic areas identified across the ESAS interdisciplinary panels.  Both the Ecosystems and Climate Panels prior itize science/applications related to the carbon cycle: the measurement of the fluxes of car bon dioxide (and methane, see below) among the atmosphere, land and ocean; the size and processes governing long-term terrestrial and ocean carbon storage; and carbon cycle-climate feedb ack mechanisms including possible thresholds such as carbon release from thawing permafrost.  Other Ecosystems Panel priorities highlight observing key underlying carbon cycle dynamics including the factors governing primary production by plants and phytoplankton and the connection of carbon fluxes to water, energy, and nutrient cycles. The direct link of carbon and UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-43 Copyright National Academy of Sciences. All rights reserved.

134 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space is also called out by Hydrology and Weather water fluxes via evapotranspiration by land plants prioritizes characterizing the interplay of human and Air Quality Panels, and the Hydrology Panel land management practices and water quality and availability. Methane, an even more potent greenhouse gas than carbon dioxide on a per molecule basis, has  accumulated in the atmosphere from the pre-industrial era at an even faster relative rate than carbon dioxide. The causes of changes in atmosphe ric methane concentration are not completely understood but likely involve a combination of emissions from natural and managed wetlands, thawing permafrost, agriculture, and the natural gas industry. Methane is also linked to air quality through its importance in producin g background tropospheric ozone. Atmospheric methane levels, fluxes with the at  natural biosphere and mosphere, and underlying human methane sources are prioritized by three panels: Ecosystems, Climate, and Weather and Air Quality. These integrating themes—(a) extreme events and (b) carbon, water, and energy cycles— provided a complementary multidisciplinary lens thro ugh which the panel priorities could be viewed. While these themes formed the core of cross- and mu lti-disciplinary examination of the panel priorities, others were considered as well, such as sea level ri se, atmospheric composition, tipping points, human health, etc. The recommended targeted observables, derived from the panel priorities and informed by these themes, address key priorities within and across disciplinary lines. In so doing, they focus the investments on making the most substantive advances in Earth system science possible. The Coupled Dynamic Earth System Framework Examination of panel priorities in the context of these sets of Integrating Themes enabled the broader interdisciplinary context, as a critical committee to consider observation priorities in a complement to the rigorous panel prioritizations that occurred. In the development and implementation of its pr ograms, it is important that NASA continue to ch in the context of their contributions to Earth approach its Earth science missions and associated resear science. After all, it was the space based persp ective that brought into much sharper relief that System Earth is a truly integrated system of complex dyna mic interactions between the atmosphere, ocean, land, and ice across a range of spatial and temporal scales. As a result, the Earth and our relationship with it can best be understood when we consider geophysical, chemical, and biological processes in the broader Earth system framework. Disciplinary focus remains crucial for understanding key processes in sufficient detail that their broader interactions and interfaces system can be examined, but it needs to be complemented by a broader Earth system view as a fundamental component of NASA’s approach to it s science activities. Such an approach will allow disciplinary phenomena to be translated to and underst ood in the context of matters of societal relevance. The Integrating Themes approach provides consideration of physical, chemical, and biological processes in an Earth system context that (a) po ses interesting scientific challenges, (b) integrates disciplinary elements of the Earth system into a broader framework to address larger and more comprehensive scientific challenges that are of relevan ce to society, and (c) provides an effective bridge between discipline-specific research, and appli cations of direct societal relevance. This approach is not new, but rather the culmination of progress over several decades. In the 1990s, the emergence of a robust Earth Observing Syst em allowed us to begin viewing the Earth as a system of interacting components. The space-based perspective enabled our examination of these components on global scales and allowed us to watch them evolve with time. The space-based perspective motivated characterization of their behavior, along with investigation of their interactions, with a view toward the ultimate goal of prediction and/or proj ection. Since that time, observational capabilities have improved considerably, our analytical tools such as regional and global mode ls have advanced, our computational power to examine the vast amounts of data from satellites other sources has become UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-44 Copyright National Academy of Sciences. All rights reserved.

135 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space understand this integrated system as a whole has exponentially better, and our ability to examine and rapidly accelerated. The convergence of advanced observation and anal ytical capabilities, as well as the three-decade evolution toward Earth system science, extends the in tegrated Earth system approach so it is capable of addressing new and more complex problems. It is now possible to examine the dynamic coupling between interactions, which are both necessary for a full parameters, as well as their direct and indirect understanding of the Earth system. These capabilities do not reduce our need to u nderstand the underlying processes that govern individual components of the Earth system. The need for that disciplinary knowledge expands, since we can explore these fundamental Earth system elements in a framework that incorporates other detailed process knowledge, along with enhanced understanding of the couplings and interactions among those processes, how they change with time, and ultimately how they affect the trends and behavior of the Earth system. Such a system framework allows us to unde rstand our Earth System at a level never before possible. As a result, we can better understand the m echanisms of change, the full range of impacts of change, and our role in the evolving behavior of the Earth system. The resulting understanding of the Earth system will position us to assess alternative ad aptation pathways to a more resilient future. ESAS 2017 OBSERVATION SYSTEM PRIORITIES Starting from the science and applications prioriti es presented in the prior section, the committee proceeded to define the observations needed to pur sue those priorities. As described earlier in this chapter, the observation needs arising from the science and applications priorities were first compared to the POR to identify those priorities served by the PO R. Observation needs for the unsatisfied priorities s. The resulting set of Targeted Observables—those were then aggregated and analyzed for commonalitie observations needed by SATM priorities but not satis fied in the POR—is summarized in the Targeted Observables Table in Appendix C. With limited resources, it was not possible to recommend all Targeted Observables from ed below, the committee identified those highest Appendix C for flight implementation. As describ priority observations that could be accomplished within the decade’s available budget and defined a programmatic approach to implementing them. The result is a comprehensive system of space-based observations, as appropriate for each sponsoring agency in accordance with the statement of task. The remainder of this section presents the proposed observation system, describes how it achieves the science and applications priorities within a realistic budget scenario, and defines existing and new agency program el ements that can be used to implement the system. A Comprehensive Observation System 4 The proposed observation system includes the Program of Record (POR) , which the committee be protected in the budget to do so), and the assumes will be implemented as planned (and must additional observations proposed in this chapter. The add itional observations are relevant to all three agencies - NASA, NOAA, and USGS - from various perspectives, but all are anticipated to be implemented as instruments or missions under NASA’s leadership. The extent to which NOAA and/or USGS participate in this NASA-implemented observing program is discussed in Chapter 4. 4 This system includes the ongoing operational satellite program of NOAA, to the extent that it contributes to the POR as documented in Appendix A. In accordance with the SOT, the committee did not consider changes or additions to NOAA’s expected operational satellite syst em, except as onramp opportunities to augment the capabilities of that system. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-45 Copyright National Academy of Sciences. All rights reserved.

136 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space esses the decade’s highest priority science The ESAS 2017 recommended Observing System addr 5 nt with specified budget assumptions and applications objectives, consiste . Additional observations needed in the observing system, beyond t Observing System hose in the POR, are summarized in the Priorities Table (Table 3.5). This table lists the committee’ s priority Targeted Observables, selected from among those in the Targeted Observables Table in Appendix C as highest priority for flight implementation. The methodology used to select th ese priority Targeted Observables has direct traceability from the panel guidance through the Science a nd Applications priorities in Table 3.3 (a more detailed description of the traceability methodology follo ws Table 3.3). Each is assigned to one of three flight program elements (identified in the last three columns of the table). These flight program elements are: 1. Designated Element. Funding for observations identified as requiring dedicated flight opportunities, directed or competed at the discretion of NASA. Competitive opportunity for selected priority observations 2. Earth System Explorer Element. (identified in Table 3.3), implemented through a new Earth System Explorer program element. 3. Incubation Element. Investment for priority Targeted Observables needing technology advancement, requirements refinement, or other advances prior to cost-effective implementation, implemented through a new Incubation program element . This also includes a new Innovation Fund to enable program-level response to une xpected opportunities that occur on sub-decadal time scales. The three new NASA flight program elements, along w ith the existing program elements referred to in this discussion, are summarized in Table 3.4, including their role in creating a comprehensive and robust overall observation program. 5 All budget assumptions, and the approach to establishing a credible budget profile, were described at the beginning of this Chapter 3. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-46 Copyright National Academy of Sciences. All rights reserved.

137 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space lements referred to in this ommended and existing program e TABLE 3.4 A summary of newly rec report. Program Element Description Purpose NEWLY RECOMMENDED PROGRAM ELEMENTS Addresses five of the highest-priority Earth Designated Cost-capped core elements of the program specifically observation needs, including three large recommended for missions and two medium missions. Elements implementation. Could be of this program are considered foundational elements of the decade’s observation competed or directed. Earth System Each competition seeks to Address three key science and applications Explorer address one of seven pre- needs. The seven candidate Targeted specified Targeted Observables are not prioritized by importance, Observables with medium rather competition is expected to drive sized cost-capped missions innovation (technical and/or programmatic). (<$350M); three competition opportunities are recommended for the decade Investments made in three Focus investments in key areas that are known Incubation Targeted Observables that are to be priorities, that are not sufficiently mature considered very high priorities for deployment at this time, but would benefit for the 2027-2037 decade, but from targeted investment. This differs from the that are not currently ready for standard ESTO model in that it is specifically competition or directed focused in three pre-determined areas. implementation. New strand of the Venture Provides opportunities for new and innovative Venture-Continuity program targeted at ways to continue exis ting measurements, and incentivizing low-cost seeks to address the tension between making new measurements vs. continuing existing continuity of existing measurements by bringing forward innovative measurements approaches to sustain measurements at lower costs. EXISTING PROGRAM ELEMENTS Earth Venture Provide opportunities in any area of Earth Unchanged from what was recommended in ESAS 2007, science without restriction; Can potentially be Suborbital, used to address Targeted Observables not three strands targeted at new Instrument, and recommended in the three above categories, or opportunities that emerge, with Mission (EV-S, no pre-specified science and EV-I, and EV-M any other topic that is sufficiently meritorious respectively) applications area. Wide open and viable, as deemed by the review process. competition for any idea of Allows for agile responses to emerging science merit. and applications topics. Program of Record Existing domestic and Provides many needed Earth system science measurements, providing the foundation on international program for which commitments are in which the recommended program is built. ESAS 2017 priorities were based on the place, with the full expectation that the missions contained in assumption that every mission in the POR will the program will fly. be deployed. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-47 Copyright National Academy of Sciences. All rights reserved.

138 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space In addition, the committee is propos ing an expansion of the Ventur e program elements to include a Venture-Continuity program element strand, focused on competitive opportunities for continuity observations with a goal of reducing cost. Candidat es for the Venture-Continuity element could come from among Targeted Observables in Appendix C and Ta ble 3.5, but are not explicitly identified as such. Each of these flight program elements is included in the funding wedge shown in Figure 3.11 and so can be implemented within anticipated resources. De tails on these program elements, and the Targeted Observables to be implemented through them, are provided in the follow text. The six Targeted Observables shown at the end of Table 3.5 were not prioritized within these three program elements, but they are still considered important observations to implement in support of SATM priorities. These observables are strong candidates for Ventur e, for international collaboration, or for or Designated missions when measurement needs opportunistic inclusion in Earth System Explorer associated with unallocated Target ed Observables can be accommodated within noted cost caps. For Ocean Ecosystem Structure Targeted Observable has the potential to be example, the unallocated addressed opportunistically through implementation of th e Aerosols Targeted Observable or as part of a multifunction lidar proposed in response to an Earth System Explorer opportunity. The committee also recognizes that the focus on targeted observables as opposed to missions and pre- specified measurement approaches, does not lend itsel f to the consideration of multiple objectives being served by a single measurement technique. While this h as the advantage of allowing for greater flexibility in implementation, it does not take full advantage of both the scientific and technical synergies that can develop from a more interdisciplinary framing. In particular, measurement technologies that address a particular Targeted Observable may likely also address others, and a careful evaluation of these nning for flight program elements while respecting opportunities should be built into the thinking and pla 6 cost caps and guarding against mission creep leading to significant cost inflation. As NASA proceeds with planning for the implementation of the recommende d flight program elements, the effort will benefit from input from a wide interdisciplinary community to help identify benefits and trade-offs among isciplinary opportunities can be identified where different measurement approaches. Possible interd observing techniques are common among Targeted Observables in Table 3.5. Lidar is highlighted as a clear example. Recommendation 3.2: NASA should implement a set of space-based observation capabilities based on this report’s proposed program (which was designed to be affordable, comprehensive, robust, and balanced), by implementing its portion of the Program of Record and adding observations described in this report’s Observing System Priorities Table . The budgetary considerations and decision rules implemented program should be guided by the contained in this report and accomplished through five distinct program elements : Program of Record. The series of existing or previ 1. ously planned observations, which must be completed as planned. Execution of the ESAS2017 recommendation requires that the total cost to NASA of the Progra m of Record flight missions from FY18-FY27 be capped at $3.6B. 2. Designated. A program element for ESAS-designated cost-capped medium- and large- size missions to address observables essential to the overall program, directed or competed at the discretion of NASA. 6 An example of both synergistic benefits and cost growth challenges is available through the lessons learned from the interdisciplinary scientific planning that emer ged for the ESAS 2007 recommended Tier 2 Decadal Survey lidar mission known as the Aerosol/Cloud/Ecosystems (ACE) profiling lidar mission (ACE Science Study Team, 2016). UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-48 Copyright National Academy of Sciences. All rights reserved.

139 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space 3. Earth System Explorer. A new program element involving competitive opportunities for cost-capped medium-size instruments and missions serving specified ESAS-priority observations. A new program element, focused on investment for priority observation Incubation. 4. -effective implementation, including an capabilities needing advancement prior to cost Innovation Fund to respond to emerging needs. Earth Venture 5. . Earth Venture program element, as recommended in ESAS 2007, with the addition of a new Venture-Continuity component to provide opportunity for low-cost sustained observations. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-49 Copyright National Academy of Sciences. All rights reserved.

140 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space TABLE 3.5 Observing System Priorities—Observati ons (Targeted Observables) identified by the ecade, beyond what is in the Program of Record, allocated as noted committee as needed in the coming d in the last three columns (and color-coded) to th ree new NASA flight program elements (Designated, Earth System Explorer, Incubation; as defined in the a ccompanying text). Within categories, the targeted observables are listed alphabetically. Targeted Candidate Measurement Science/Applications Summary Approach Observable Explorer Incubation Designated Backscatter lidar and multi- Aerosol properties, aerosol vertical profiles, and cloud properties channel/multi-angle/polarization to understand Aerosols X their effects on climate and air quality imaging radiometer flown together on the same platform Radar(s), with multi-frequency Clouds, Coupled cloud-precipi tation state and dynamics for monitoring global hydrological passive microwave and sub-mm Convection, X radiometer cycle and understanding contributing and processes including cloud feedback Precipitation measured by Large-scale Earth dynamics Spacecraft ranging measurement of the changing mass distribution within and gravity anomaly Mass Change X between the Earth’s atmosphere, oceans, ground water, and ice sheets Hyperspectral imagery in the visible Earth surface geology and biology, Surface and shortwave infrared, multi- or ground/water temperatur e, snow reflectivity, Biology and X hyperspectral imagery in the active geologic processes, vegetation traits Geology thermal IR and algal biomass Earth surface dynamics from earthquakes Interferometric Synthetic Aperture Surface and landslides to ice sheets and permafrost Radar (InSAR) with ionospheric Deformation X correction and Change CO and methane fluxes and trends, global Multispectral short wave IR and 2 and regional with quantification of point thermal IR sounders; or lidar** Greenhouse X sources and identification of sources and Gases sinks Lidar** Global ice characterization including elevation change of land ice to assess sea level contributions and freeboard height of X Ice Elevation sea ice to assess sea ice/ocean/atmosphere interaction Doppler scatterometer Ocean Coincident high-accuracy currents and vector winds to assess air-sea momentum Surface X exchange and to infer upwelling, upper ocean Winds and mixing, and sea-ice drift Currents UV/Vis/IR microwave limb/nadir Vertical profiles of ozone and trace gases (including water vapor, CO, NO , methane, sounding and UV/Vis/IR Ozone and 2 X O) globally and with high spatial solar/stellar occultation and N Trace Gases 2 resolution Radar (Ka/Ku band) altimeter; or Snow Depth Snow depth and snow water equivalent lidar** including high spatial resolution in mountain and Snow X areas Water Equivalent Lidar** Terrestrial 3D structure of terrestrial ecosystem including forest canopy and above ground Ecosystem X biomass and changes in above ground carbon Structure UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-50 Copyright National Academy of Sciences. All rights reserved.

141 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space stock from processes such as deforestation and forest degradation Active sensing (lidar, radar, 3D winds in troposphere/PBL for transport of pollutants/carbon/aerosol and water vapor, scatterometer); passive imagery or Atmospheric X X wind energy, cloud dynamics and convection, radiometry-based atmos. motion Winds vectors (AMVs) tracking; or lidar** and large-scale circulation Microwave, hyperspectral IR Diurnal 3D PBL thermodynamic to properties and 2D PBL structure sounder(s) (e.g., in geo or small sat understand the impact of PBL processes on constellation), GPS radio occultation Planetary weather and AQ through high vertical and for diurnal PBL temperature and X Boundary humidity and heights; water vapor temporal profiling of PBL temperature, Layer profiling DIAL lidar; and lidar** for moisture and heights PBL height Radar; or lidar** Surface High-resolution global topography including bare surface land topography ice Topography X topography, vegetation structure, and shallow and water bathymetry Vegetation ** Could potentially be addressed by a multi-function lidar designed to address two or more of the Targeted Observables Other ESAS 2017 Targeted Observables, not Allocated to a Flight Program Element Aquatic Biogeochemistry Radiance Intercalibration Magnetic Field Changes Sea Surface Salinity Ocean Ecosystem Structure Soil Moisture The Targeted Observables in Table 3.5 are respons ive to the Science and Applications priorities 3, and ultimately to panel guidance addressing the (as measured by the Sci/App Importance) in Table 3. science and applications as well as the needed obser vables. Table 3.3 was developed largely by the the steering committee without any direct panel panels, but Table 3.5 was the responsibility of consultation. Development of Table 3.5 from Table 3.3 involved two fundamental steps. 1. Identify those gaps in the Program of Record corresponding to Identify Observation Gaps. observables needed to address highest priority sci ence and applications objectives. In practice, large tables of data. The first is the Program of Record this meant working directly with two Table Science and Applications Traceability Matrix (Appendix (Appendix A). The second is the B). By comparing the two tables, a list was developed detailing gaps in observing capability (i.e., needed observations not available in the next d ecade’s Program of Record) that are anticipated during the coming decade. The r esult of this effort was the Targeted Observables Table (Appendix C), which lists the 22 key unmet observation needs (referred to as Targeted Observables) in the next decade’s Program of Record, as identified by the committee. 2. Prioritize the Observation Gaps. Prioritization of the 22 Targeted Observables in Appendix C to derive the priorities in Table 3.5 was accomplished through extensive deliberation by the committee, with consideration of the following two factors:  Scientific and Applic ations Priority. Scientific and application priority is summarized in the “Sci/Apps Priorities” column of the (Appendix C). The Targeted Observables Table entries in this column refer to lines in th e Science and Applications Traceability Matrix (Appendix B). At a simple level, priority can be discerned by the total number of Sci/App priorities, along with the preponderance of MI and VI priorities. While the committee used this simple view for guidance, decisions were made through direct review of the original science/applications priorities in Appendix B. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-51 Copyright National Academy of Sciences. All rights reserved.

142 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space With the assistance of The Aerospace Corporation,  Cost and Technical Feasibility. various implementations were assessed to un derstand the potential cost and feasibility of measuring each proposed Targeted Observab le. Because the committee is recommending observations, rather than missions, the assessment performs the role of an existence proof rather than an implementation plan. Nevert heless, a comprehensive range of feasibility factors was considered, from technical readiness to flight heritage. this process is traceable and grounded in the guidance of the The committee focused on ensuring panels. While endeavoring to achieve objectivity to the extent possi ble, the process inevitably involves subtle and often subjective tradeoffs among the ab ove factors. The committee held discussions involving complex considerations to arrive at the priorities in Table 3.5. An important consideration for the committee was the extent of interdisciplinary (i.e ., Earth system) benefit obtained from a Targeted Observable, as measured by the degree of cross-pa nel prioritization. In addition, the committee considered programmatic balance (see Chapter 4 for a description of balance considerations) to be a desirable aspect of the recommended program. While an inherent subjectivity exists in any prio ritization, the multiple diverse perspectives from as the consideration of panel input as informed by the steering committee that led to Table 3.5, as well that is appropriately balanced, informed, and RFI responses, has produced a set of recommendations reflective of societal and scientific needs As also discussed elsewhere in this section, the committee chose to not simply list the prioritized Targeted Observables but to group them according to three candidate programmatic implementations (Designated, Earth System Explorer, and Incuba tion), based on the suitability of each Targeted Observable for each approach. In a general sense, the si ze of the resource commitment is justified by the value of the science and applications, as summari zed for each Targeted Observable in the Sci/App Priorities column of the Targeted Observables Table (Appendix C). The committee’s overall goal was to maximize the amount and quality of the science and app lications that can be achieved within constrained resources. An example of the process is shown in Figure 3.11, illustrating the use of each of the key tables in development of the Aerosols Targeted Observable within Table 3.5. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-52 Copyright National Academy of Sciences. All rights reserved.

143 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space FIGURE 3.11 Example of traceability from science and applications prioritie s (Table 3.3) to observing system Targeted Observable. Each candidate Targeted priorities (Table 3.5), shown for the example of the Aerosols Observable (in Appendix C) represents an observation needs (as documented in Appendix B) not adequately addressed in the Program of Record (Appendix A). For each Targeted Observ able in Appendix C, the steering able 3.5), by prioritizing the Appendix C Targeted committee then identified Observing System Priorities (T Observables based on consideration of the science and application importance and the implementation feasibility. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-53 Copyright National Academy of Sciences. All rights reserved.

144 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Fitting within a Realistic Budget that guided its partitioning of the funds The committee had two fundamental strategic goals anticipated to be available for the Earth science progr am. The first goal is to achieve and preserve balance in the program portfolio by maintaining approximate ratios between each of the program elements. This is to be accomplished by application of the Decision Rules, which provide guidance on the cadence that elements in each program line should be implemen ted, as well as the possibility, if necessary, of modulating the schedule for development of the larg er missions. The second goal is to control carryover into the next decade such that implementation of the large mission pipeline over the decadal boundary is maintained without deeply impacting the future funding wedge. Figure 3.12 shows how the recommended program elements accomplishes these two objectives 7 and fits within the assumed profile of available funding. This budget assumes spending of $3.4B on new programs (beyond the already allocated POR r unout budget) during the coming decade, and a lien on the following decade of $1.7B due to flight programs starting late in the decade. Such liens are an accepted consequence of achieving continuity from one decada l study recommendation to the next, but minimizing the lien is desirable (NAS, 2015). The lien proposed by ESAS 2017 is less than half of the $3.6B lien currently estimated to complete POR flight programs this coming decade that were started last decade. As described in more detail in the remainder of this chapter, the recommended program includes funding for: Three large (two expected to be <$800M FY18, one expected to be <$650M) and Designated.  two medium (one expected to be <$300M and the other expected to be <$500M) cost-capped projects, directed or competed at the discretion of NASA.  Earth System Explorer. Three competitively-selected cost-capped (<$350M) projects.  Venture-Continuity. Two projects that are <$150M each, selected competitively with a goal of reducing costs as compared to prior projects for the same observable. Incubation. Funded at $20M per year to advance identified priorities for concept and technology  maturation, including the Innovation Fund to enable program-level innovation. Given the breadth of observations needed to a ddress priorities in Earth system science and the expectation of comparatively austere budgets, the recommended program relies on competition as the core approach to controlling individual mission costs, through the use of cost ca ps combined with trades between performance and risk during the formulation and implementation stages of system development. 7 Recognizing the natural latency that occurs at the deca dal transition, it was assumed that the first three years of the next decade’s wedge will be encumbered to complete the 2017 decade’s slate of implementation recommendations, with most of the available flight funding used in the first post-transition year, half in the second year, and less than one quarter in the third year past th e transition. This approach, if followed for each successive decade, will ensure a full mission pipe line while also facilitating an earlier start for subsequent Decadal Survey priorities. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-54 Copyright National Academy of Sciences. All rights reserved.

145 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space FIGURE 3.12 ESAS 2017 estimated costs (colored wedges), broken down by element, as compared to the anticipated flight budget (black line), show ing how the ESAS 2017 costs fit within the available budget. The total NASA Earth Science Division budget for flight elements assumes growth at the rate projection. Budgets to support ESAS 2017 flight of inflation for years beyond the current budget recommendations are shown for the Designated, Ea rth System Explorer, Venture-Continuity, and Incubation Program elements described in Recommendation 3.2. ESAS 2017 estimated costs are phased to match the expected available unallocated funding wedge of $3.4B over the decade of 2017- 2027 with a carryover of $1.7B into the followi ng decade, as discussed further in the accompanying text. Only the investments related to ESAS 2017 r ecommendations are shown; the remainder of the ESD budget is allocated to existing projects and pr ograms (see Figure 3.3). All values of in real-year dollars. The gap between the estimated costs and th e available budget represent funds that have been committed to other mission-related activities. An Aspirational Program The committee intended the proposed observing syst em to be realistically accomplished within nominal budget growth, with recommended investment s stated in terms of the “maximum recommended 8 NASA development cost” levels to ensure program balance is maintained. For each program element, the included Targ eted Observables were drawn from the non- prioritized list of Targeted Observables identif ied in Appendix C, and intended to address the science/applications priorities listed in that table. In order for implementations of Targeted observables to remain within their respective cost caps, however, it is not expected that every science and applications 8 The maximum recommended NASA development cost is wh at is allocated from NAS A’s budget to cover the pre-launch (development phase) and launch vehicle cost fo r a mission to address the stated science and applications objectives. Development phase costs include reserves. Opera tions cost and associated R& A funding are not included in this total. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-55 Copyright National Academy of Sciences. All rights reserved.

146 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space for each relevant Targeted Observable. Rather the priority identified in Appendix C will be achieved currently unmet science and applications priorities, implementation is expected to first target the highest and address others as feasible. That should not preclude NASA from trying to acco mplish more, and the committee believes that NASA can and should do so. Indeed, the new Earth System Explorer Program includes more than twice ves will be implemented in this decade, and as many candidate observations as the committee belie Appendix C suggests there are many more good candidates for flight opportunities such as Earth Venture. With an increased budget, an aspirational program co uld be pursued. More importantly, however, it is possible to be aspirational within the nominal budget—to get more done with the same amount of resources. NASA is quite adept at ma ny of the ways this could happen: a. International partnerships that provide the same capability with reduced cost to NASA. Interest by several viable international partners was expressed to the committee regarding potential collaboration opportunities. Several of the priority observations included in the system are known to also be high priorities of these internationa l partners, leading the committee to believe that partnering opportunities are quite promising. b. Technology innovation with the potential to reduce co st for all aspects of space-based observation. The commercial sector (in addition to NASA and academia) is presently introducing a wide range of technology innovation capabilities with the potential to be leveraged by NASA for cost reduction. c. Programmatic innovation, including public-private partnershi ps and spacecraft block buys, that accomplish the same goals with reduced resources. d. of programs being implemented. cost management and requirements control Aggressive e. for continuity observations that delay the need for new missions. Extended operations on these issues and reduce cost can generate A comprehensive effort within NASA to focus the additional resources needed to expand the Eart h System Explorer program element, complete these high-priority observations more rapidly, and/or increase opportunities for continuity of key Earth system observations. Doing so would enable additional high priority science beyond the baseline recommended by this committee. The key to achieving this aspirational program relies on managing costs through the approaches outlined in Items (a)-(e) above, and incentivizing mission development to be accomplished as far below the cost cap as possible. Recommendation 3.3: NASA should manage development costs for each flight program element (including the Program of Record co mmitted to prior to this report), and for each project within the Designated program element, so as to avoid impact to other program elements and projects.  rammatic or technological advances and Innovative cost reduction, through prog partnerships, should be sought and incentivized where possible.  By the time of the Midterm Assessment, NASA should report on steps it has taken (e.g. use of innovative approaches and/or partnershi ps) to ensure cost-effective development in each program element, and if/how these steps translate to increased science opportunity across the program.  NASA should consult its standing scientific advi sory committees if the project cost of the Program of Record is expected to grow to consume more than $3.6B in the FY18-FY27 decade, if more than one mission in this Decadal Survey is delayed more than 3 years, or upon premature loss of a mission in the Program of Record or one required to make the measurements of this Decadal Survey. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-56 Copyright National Academy of Sciences. All rights reserved.

147 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space When appropriate, cost-effective, and c  onsistent with recommended cost caps, NASA nd mission designs that can increase science/applications should consider instrument a return by combining Targeted Observables having common measurement technologies. Program Element: DESIGNATED The Designated program element represents a gr oup of Targeted Observables believed by the committee to be of sufficiently high value to the Ea rth system science and applications communities to This implementation could occur through directed warrant designated implementation during the decade. guidance to the NASA Centers, through internal or external competition, or other means chosen by NASA ESD to achieve the most cost-effective solution. Within the Designated program element, five Targeted Observables are recommended for implementation during the 2017-2027 decade. Table 3.6 lists these and indicates the maximum recommended NASA development cost, with science and implementation considerations discussed for each in the sections below. The maximum recomme nded development costs are not to be taken as mmittee expects NASA to identify implementation expected development costs. Instead, the co approaches which achieve the recommended objectives for less than the identified maximum. TABLE 3.6 Targeted Observables to be addressed th rough the Designated program element, and their respective CATE-confirmed or estimated cost caps. These are listed in alphabetical order, with proposed development sequencing discussed in the text. TARGETED COST CAP BASIS FOR BEING FOUNDATIONAL ($FY18) OBSERVABLE Aerosols CATE Cap Essential for air quality forecasting; provides critical insights into key $800M radiative forcings, both direct and indirect (from cloud hydrometeor size Clouds, Convection, and and optical depth). When combined with P recipitation of aerosol effects on clouds observations, enables assessment and precipitation. Addresses many of the “Most Important” objectives of the Climate Panel and Weather and Air Quality Panel, along with key components of the water and energy cycle integrating theme. Clouds, Convection, CATE Cap Critical for assessing low and high cloud feedbacks, seasonal and and Precipitation interannual climate variability and its pr ediction, processes that are at the $800M core of severe weather and extremes. Fundamental observations for water cations. When combined with Aerosols resource and hydrological appli observations, aerosol indirect effects can also be substantially advanced. Addresses many of the “Most Importa nt” objectives of the Climate, Weather and Air Quality, and Hydrology panels, along with key components of the Water and Energy Cycle and Extreme Events integrating themes. Mass Change Estimated Cap Ensures continuity of measurements of ground water and water storage $300M mass change, land ice contributions to sea level rise, ocean mass change, with altimetry), glacial isostatic ocean heat content (when combined adjustment, and earthquake mass movement. Also important for operational applications, including drought a sting, hazard ssessment and foreca response and planning water use for agriculture and consumption Addresses various “Most Important” objectives of the Climate, Hydrology, and Solid Earth panels and key components of the Water and Energy Cycle Integrating theme. anges (eruptions, landslides and Surface Biology and CATE Cap Key to understanding active surface ch $650M Geology evolving landscapes); snow and ice accumulation, melting, and albedo; hazard risks in rugged topography; eff ects of changing la nd-use on surface energy, water, momentum and carbon fluxes; physiology of primary UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-57 Copyright National Academy of Sciences. All rights reserved.

148 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space producers; and functional traits and health of terrestrial vegetation and inland and near-coastal aquatic ecosystems. Further contributes to managing agriculture and natural habitats, water use and water quality, and urban development as well as understanding and predicting geological natural hazards and land-surface inter actions with weather and climate. the Targeted Observable may also Depending on implementation specifics, contribute to hyperspectral ocean observation goals. Addresses many “Most Important” objectives of the Ecosystem, Hydrology, and Solid Earth Panels, and addresses key components of the Water and Energy Cycle, Events integrating themes Carbon Cycle, and Extreme Estimated Cap Surface Deformation Critical for assessing ice sheet stability and the potential for ice sheets to and Change $500M make large rapid contributions to sea level rise. Key to mapping surface strain rates in order to understand ear thquakes, volcanoes, landslides, and sea level rise. Enables monitoring of tectonic plate deformation, changes in groundwater and subsidence, and thawin g of permafrost. Directly addresses six “Most Important” and four “Very Important” objectives of the Earth as one or more “Most important” Surface and Interior panel, as well objectives in Hydrology and Climate panels. Provides important insights into various components of the Water and Energy Cycle, the Carbon Cycle, and Extreme Events integrating themes. Consistent with other NASA sp ace science decadal surveys, The CATE Cost Confirmation. ll concepts expected to cost more than $500M, Aerospace Corporation CATE process was applied to a 9 excluding operations costs . For those expected to be less than $500 M, a cost analysis was performed to estimate a cost cap but no formal CATE was completed. For the three large (two<$800M, one <$650M) Targeted Observables in Table 3.6, notional proof-of-concept missions with the recommended capabilities were evaluated to ensure top-level t echnical and programmatic risks were understood. Aerospace found each of these proof-of-concept missions to be implementable within the listed cost cap. The CATE process is by its very nature conservative (NAS, 2015). It does not, for example, account for partnership opportunities which have incorporate innovative implementation approaches or the ability to significantly lower the cost of im plementation to NASA. For large (>$500M) Targeted entified this cap as a cost category limit within Observables listed in Table 3.6, the CATE process id which the observable could be implemented at acceptable risk. The actual estimated price was always below the cost cap, suggesting the realistic expecta tion that each implementation can be completed for e 3.6 (potentially considerably less). less than the cost cap values in Tabl Implementation Sequence. Based on the relative costs and risks of the three higher-cost Targeted Observables, and the substantial synergy ga ined by obtaining overlap between the Aerosols and Convection, and Precipitation Targeted Observables, the comm ittee suggests that the order of implementation be: 1) Surface Biology and Geology , 2) Aerosols , and 3) Clouds, Convection, and Precipitation . However, if opportunities present themselves th at in NASA’s judgment allow for a more effective implementation (through lower costs, bette r technologies, etc.), this implementation sequence such opportunities. One reason for choosing this should be flexible in order to be responsive to sequencing is that cost-reducing opportunities, such as international partnerships, are particularly viable d late in the survey interval. for Targeted Observables implemente Cost Control. To further protect program balance, th e committee expects that NASA will manage the Targeted Observables within the Designated prog ram in such a way that the implemented missions adhere to their costs caps and do not adversely affect other elements of the flight program. To achieve this, the progress with developing and implemen ting these missions should be reviewed during the 9 As discussed earlier in this chapte r, the Cost AND Technical Evaluation (C ATE) process is a formal cost and technical readiness evaluation, performed by The Aerospace Corporation. It is mandated for the Decadal Survey. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-58 Copyright National Academy of Sciences. All rights reserved.

149 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space cost-effective investments, to NASA and its Midterm Assessment. The advantage of ensuring science/applications programs, is clear. When implem entation can be achieved below the stated allocation , program breadth may be nerships, or other means) (e.g., through innovative program implementation, part increased to address other identified priority observati ons consistent with the decision rules provided in Chapter 4. Each of the Targeted Observables in Tabl e 3.6 recommended for the Designated program rgeted Observables presented element is discussed in the following text, with Ta in alphabetic order. For each, the related Science/Applicatio ns Objectives (from Appendix C) are listed, with those ranked as portant in blue. Noteworthy is the extent of cross- Most Important in red and those ranked as Very Im the number of different panels represen ted for each Targeted Observable, disciplinary value, indicated by and the extent of science/applications Objectives served by each. It is possible that not all listed Science/Applications Objectives can be add ressed within a feasible implementation. DESIGNATED — Targeted Observable: Aerosols [ H-1c , 2b ; W-1a , 2a , 5a , 6a, 9a, 10a; S1a ; C-2a , 2g , , 7b, 9a] 2h 5a , 5b, 5c , 5d, 7a , 3d, 4d, The Aerosols Targeted Observable corresponds to a co mbination of TO-1 and TO-2 in the Targeted Observables Table (Appendix C). The IPCC 2013 Climate Change Assessment Report determined that uncertainties associated with aeros ols and aerosol-cloud interactions are the largest radiative forcing uncertainty. Because of this, knowle dge of their characteristics and processes is critical climate change. As a result, aerosol observations are to reducing the uncertainty of future projections of one of the higher priorities from the Climate Panel. In addition, the health effects from pollution are the e one of the most important boundary layer properties largest environmental risk and therefore aerosols ar identified by the Weather Panel as both essential for air quality forecasting and for connecting health effects with pollution. Finally, aerosols, through their alteration of direct and diffuse radiation change the amount of radiation available to ecosystems. The co nsidered implementation of a backscatter lidar and multi-angle, multi-spectral polarimeter would addr ess important inputs for aerosol direct radiative at partially addresses indirect effects through aerosol effects on cloud forcing, and provide information th hydrometeor size and cloud optical depth. These radiative forcings are a significant source of uncertainties in climate model proj Clouds, Convections, and ections. In combination with the Targeted Observable, it would also address th e additional aspects of the aerosol indirect Precipitation effect (on clouds and precipitation formation), thus accomplishing key objectives of the ESAS 2007 recommended ACE mission. Science Considerations. Aerosol measurements are essential elements for understanding climate forcings and feedbacks. The types of systems that wo uld contribute profiling (lidar) would also be capable of detecting height and optical properties of high thin clouds and cloud properties of lower thicker clouds, the climate system. In addition, such observations providing important information for cloud feedbacks in forecasting, about aerosol profiles in the boundary would provide information, critical for air quality layer. The information on clouds and aerosols woul d yield additional insights into cloud processes and processes that affect precipitation, particularly when is flown simultaneously with a system for Aerosols observing cloud, convection and precipitation. As such, this observing system spans climate, and weather, and air quality and directly maps to the wa ter and energy cycles Integrating Themes. Candidate Measurement Approaches. The candidate measurement a pproach considered includes a lidar and a polarimeter. Expected lidar measurem ent implementations will provide aerosol extinction profiles and cloud top heights as well as vertical profiles of cloud occurrence in thinner clouds. The polarimeter provides column integrated information on aerosols that could be used to constrain lidar extinction profile estimates as well as cloud size in th e upper one to two optical depths of clouds as well as total cloud optical depth. The polarimeter can also provide aerosol particle size, optical depth, and some information on speciation. The polarimeter can provide other aerosol/cl oud properties information UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-59 Copyright National Academy of Sciences. All rights reserved.

150 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space cloud optical depths and cloud droplet size in the supporting science and applications objectives related to upper one or two optical depths of the cloud and aeroso l/particulate matter optical depth, particle size, and nguish between spherical cloud drops and non-spherical some information on speciation. It can also disti ice particles. Depending on implementation specifics, a lidar may also contribute to aquatic ecosystem structure, ocean mixed layer depth, ice sheet topogra phy, land topography, and PB L height. In particular, many of the scientific and technical opportunities a nd challenges for a joint aerosol-ocean measurement system using backscatter lidar have been mapped out in some detail as part of the planning for the ESAS 2007 Aerosol/Cloud/Ecosystems (ACE) mission (ACE Science Study Team, 2016). The recovery of l Lidar with Orthogonal Polarization (CALIOP) upper-ocean plankton profiles from the Cloud-Aeroso the Aerosol Targeted Observable should also be able sensor indicate that, with appropriate consideration, rgeted observable TO-10 (Appendix B) and the to address key aspects of the Ocean Ecosystem ta associated high-priority Science/Applications objectives from the Ecosystem, Weather, and Climate panels (W-3a, E-1b, E-3a, C-2d, and C-8d). The Ae rosol targeted observable instrument and mission design, therefore, should seek to address these inte rdisciplinary objectives while recognizing that the primary mission focus is meeting the aerosol scien ce objectives as described and remaining within the cost cap.Opportunities should be assess ed to determine the extent these additional science goals can be achieved while also meeting the aerosol science objec tives and maintaining overall costs at or below the recommended cost cap. CATE Evaluation. The CATE evaluation considered a reference concept consisting of a backscatter lidar and polarimeter. It found that th e concept is based on mature technology and is consistent with a cost cap of $800M or less (e xcluding operations). High Spectral Resolution Lidar (HSRL) was a desired capability as part of Aerosols , but cost and technical readiness considerations precluded its consideration. Descope Options. In the event development costs are exp ected to exceed the $800M cost cap, the TO-1 and TO-2 Target Observables (Appendix C) could be instead included separately as candidates within the Earth System Explorer competition, usin g the available budget to fund two additional Earth System Explorer solicitations. This is preferred over descoping Aerosols to just one of the two (lidar and polarimeter) measurement techniques or to pro ceeding with a higher cost for the combined implementation. POR Assumptions. There are no dedicated atmospheric lidars in the POR after EarthCARE, which is expected to end in about 2022. The POR includes polarimetry from the European 3MI by the MAIA but this measurement has limited instrument. Multi-angle polarimetry will be provided coverage. Partnerships. ial and/or international partnership NASA is encouraged to seek commerc on costs and enabling overlap between the Aerosol opportunities with the goal of reducing implementati and Clouds, Convection, and Precipitation efforts. In keeping with the guidelines of the Designated program element and Budgetary Guidance. this report’s Recommendation 3.3, the Aerosols Targeted Observable has a maximum recommended development cost of $800M (in FY18$). DESIGNATED — Targeted Observable: Clouds, Convection, and Precipitation [ H-1a, 1b , 1c , 3b, 4b; , W-1a, 2a , W3a 4a , 9a, 10a; S-1c , 4b; E-3a ; C-2a , 2g, 2h , 3f, 5d, 7e, 8h] Clouds, Convection, and Precipitation Targeted Observable corresponds to TO-5 in the The Targeted Observables Table (Appendix C). This targ eted observation provides measurements essential to advancing our understanding and pred iction of cloud feedbacks, moist convection and its influence on weather and extremes, and on the processes of preci pitation as well as measuring important modes of precipitation not addressed within the POR. This ta rgeted observation addresses priorities that emerged across multiple panel recommendations and from consider ations of broader Earth science integration as, for example, described in the prior discussion of integrating themes. Precipitation measurements and UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-60 Copyright National Academy of Sciences. All rights reserved.

151 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space and precipitation processes to be addressed by the more notably the combined information on cloud the high priorities of the hydrology panel, cloud proposed measurements are fundamental for addressing feedbacks identified in the climate panel, and importa nt topics on weather extremes. When combined with other measurements, such as those proposed to a ddress the Aerosols Targeted Observable, other important objectives such as those associated with aer osol indirect effects can also be advanced. critical challenges within Earth Sciences. Clouds TO-5 is motivated to address widely recognized ons of climate change (IPCC 2013 Assessment). These are a principal source of uncertainty in projecti uncertainties are associated with low and high cloud feedbacks as described in the climate panel report, and to the process of moist convection which is a central theme of the cloud-climate sensitivity grand challenge of the WCRP (Bony et al., 2015). Moist convection is also central to two high importance weather priorities, being the essential building block of our major storm systems found throughout the tropics and mid-latitudes. Convective storms are the sole source of precipitation in many regions of our planet and are major source of severe weather and extreme precipitation. Measurements of the vertical distribution of cloud and precipitation properties, incl uding measurements of water and ice contents, and ng the processes that underp in these challenges. A microphysical information are essential for quantifyi missing piece of critical information that would be provided by this proposed measurement for both addressing cloud feedback processes, moist convection a nd for better predicting the intensity of storms and extreme weather is the measurement of the vertical motions in clouds and storms. Science Considerations . This observation will address science a nd applications goals related to cloud cover (with the assistance of the POR) and cloud property profiles (with the assistance of the POR and/or the Aerosols Targeted Observable), precipitati on profiles, precipitation (including light rain and snowfall), diurnal cycle of precipitation (with the assistance of the POR), convective vertical motion or proxies of it, and cloud water and ice contents. When cloud and precipitation observations overlap with the aerosol measurements above, they can better address th e aerosol indirect effect priority called out by the climate panel as high importance. This measurement is also a central component of the ACE mission quency radar could help of operation of higher fre identified in ESAS 2007. A short pulse altimeter mode measure snow depth on the ground, which is an important priority of both hydrology and to ice mass measurement objectives, though this capability should not be allowed to drive mission cost or complexity. The expected radar measurements will provide joint cloud Candidate Measurement Approaches. Doppler motions within clouds and convection. The water and ice contents, rain and snow amounts, and radar will also provide profiles of shallow and deep clouds and some information on microphysical properties through the combination of frequencies and Doppler. The radar is expected to be a dual frequency W/Ka band system. Depending on implem entation specifics, a W-band radar could also considered a priority for implementation and thus is contribute to surface snow depth; however, this is not not recommended if it drives cost. Cloud optical properties and diurnal cycle information will come from the POR observations of clouds from available advan ced geostationary imaging radiometers. Time and spatial coverage of precipitation similarly relies on the microwave radiances data within the POR, and diurnal cycle coverage of precipitation could be an ticipated if this POR is augmented in decade. CATE Evaluation. The CATE evaluation of a reference c oncept consisting of a dual-band radar building on CloudSat heritage found that the concep t is based on mature technology and has the potential to be executed within a cost cap of $800M or l ess (excluding operations). Recent technological advances have led to either miniaturized or greatly simplifie d microwave radiometers and radar systems, now being realized in the example of Raincube, in the demons tration of COWVR (Brown et al., 2016) and in the implementation of the TROPICS and TEMPEST EV-I mi ssions, suggesting the possibility of leveraging technology innovation and miniaturization to achieve the selected goals at reduced cost. Descope Options. In the event development costs exceed the maximum NASA development cost level recommended here, the radar can be descope d to a single band, retaining Doppler capability. POR Assumptions . Addressing this Targeted Observable requi res fully leveraging missions in the POR and augmenting them with new measurements that offer a dimension of time and motion as a way of quantifying and understanding processes. New obser vations of the dynamical aspects of cloud, convection, and precipitation processes will place those processes within the context of global mapping of UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-61 Copyright National Academy of Sciences. All rights reserved.

152 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space es (such as cloud amount, optical depth and particle precipitation and its diurnal cycle. Cloud properti size) are available from the spectral radiance measurements provided by MODIS and VIIRS on polar orbiting satellites that are assumed within the POR. The same information can now be extracted from the advanced imagers available from the current and planned constellation of operational geostationary are assumed within the POR but higher latitude satellites. Precipitation radar measurements from GPM r CloudSat and EarthCARE. Cloud profile data from precipitation is not addressed within the POR afte CloudSat and CALIPSO will not be available in the decade, while EarthCARE will be available only in the early part of the decade. Precipitation informa tion especially over ocean relies heavily on microwave imaging radiometer measurements and the time/space cove rage of this information will not be available in the coming decade (Box 4.4). Partnerships. ial and/or international partnership NASA is encouraged to seek commerc opportunities with the goal of reducing implementati on costs and enabling overlap between the Aerosol and Clouds, Convection, and Precipitation efforts. In particular,NASA is encouraged to assess the extent to which the three series of operational microwave radiom eters that will be flying concurrently later in the coming decade - EUMETSAT/EPS-2G-B, USAF/WSF and CMA FY-3 - can contribute toward meeting the objectives of this mission. Budgetary Guidance. In keeping with the guidelines of the Designated program element and this report’s Recommendation 3.3, the Clouds, Convection, and Precipitation Targeted Observable has a maximum recommended development cost of $800M (in FY18$). 1b , DESIGNATED — Targeted Observable: Mass Change [ H-1a , 2c , 3b, 4c; S-1b , 3a , 4a , 5a , 6b; C-1a , 1c , , 7d, 7e] 1d The -9 in the Targeted Observables Table Mass Change Targeted Observable corresponds to TO isture, groundwater, snow, ice, ocean water, etc. (Appendix C). The movement of mass, whether it is mo represents exchanges within and across elements of th e Earth system. As such, the Mass Change Targeted Observable provide an integrated view of the entire physical Earth system, and allows the relating of changes in one system component to changes in another. By providing continuity of the GRACE measurement record, it addresses Most Important and Very Important objectives for three panels (Climate, Hydrology, and Solid Earth) and contributes to several integrating themes. . This Targeted Observable will ensure continuity of information on Science Considerations ground water and water storage mass change, land ice mass change, ocean mass change, glacial isostatic ined with altimetry, additional information on adjustment, and earthquake mass movement. When comb heat storage is obtained. Gravity-derived measureme nts of mass change, as have been shown by GRACE, address key objectives related to sea level rise, ocean heat content (Box 3.7), terrestrial water storage, etc. Consequently, monitoring changes and movement of mass throughout the Earth integrates objectives of the climate, solid Earth, and hydrology panels, as we ll as addressing key integrating themes such as water or not systems may be approaching thresholds or and energy cycle with linkages to assessing whether observed, especially given the continuity of mass tipping points, and assessing trends in the parameters change and gravity measurements since the launch of GRACE in 2002. Candidate Measurement Approaches. The GRACE gravity observation can be accomplished through either of two ranging techniques (microwave a nd optical) that are being evaluated as part of the GRACE-FO mission. Either approach provides bulk meas urements of mass fluctuations that are primarily associated with water changes within the Eart h system. A down-select to a single measurement technology is likely required to fit within the allocat ed investment level. These measurements provide a way to track bulk changes in terrestrial water, cha nges to ice mass and ocean water mass. Although these measurements are coarse in scale, making their use in water resource management challenging, these bulk measurements provide valuable insights on how water in bulk form cycles through and the changes within the Earth system. When this information is augmente d with the ocean altimeter observations of the POR and in situ data from ARGO floats, then additional but important inform ation about ocean heat storage is UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-62 Copyright National Academy of Sciences. All rights reserved.

153 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space is fundamental for assessing the state of the Earth obtained. Monitoring this energy change over time system and its future evolution. Cost and Technical Evaluation . Cost and technical evaluation of a notional mission concept similar to the original GRACE mission found that the c oncept to be technically mature with costs that are well-understood, supporting a low risk implementation recommendation. . The POR includes mass change measurements from GRACE-FO and POR Assumptions continued altimetry measurements from the Jason and Sentinel-6 missions. Descope Options. No descope options have been identified. Partnerships. nal partnership opportunities to implement NASA is encouraged to seek internatio this mission, and to phase implementation to ensure flight readiness prior to the end of the GRACE-FO mission. Budgetary Guidance. In keeping with the guidelines of the Designated program element and this report’s Recommendation 3.3, the Targeted Observable has a maximum Mass Change recommended development cost of $300M (in FY18$). , DESIGNATED Surface Biology and Geology [ H-1c Targeted Observable: 2a , 2b, 3a, 3b, 3c, 4a , 4c, — 4d; W-3a; S-1a , 1c , 2b , 4b, 4c, 7a; E-1a , 1c , 1d, 2a , 3a , 5a, 5b, 5c; C-3a , 3c, 3d, 6b, 7e, 8f] Targeted Observable, corresponding to TO-18 in the Surface Biology and Geology The Targeted Observables Table (Appendix C), enables improved measurements of Earth’s surface characteristics that provide valuable information on a wide range of Earth System processes. Society food, fiber and many othe is closely tied to the land surface for habitation, r natural resources. The land surface, inland and near-coastal waters are changing rapidly due to direct human activities as well as natural climate variability and climate change. New opportunities arising from enhanced satellite remote sensing of Earth’s surface provide multiple benefits for managing agriculture and natural habitats, water use and water quality, a nd urban development as well as understanding and predicting geological natural hazards. The Surface Bi ology and Geology observa ble is linked to one or more Most Important or Very Important science objectives from each panel and feeds into the three ESAS 2017 integrating themes: water and energy cycle, carbon cycle, and extreme event themes. Science Considerations. This Targeted Observable will likely be addressed through hyperspectral measurements that support a multi-disciplinary set of science and applications objectives.Visible and shortwave infrared imagery addresses multiple objectives: active surface geology (e.g., surface deformation, eruptions, landslides, and evolving landscapes); snow and ice accumulation, melting, and albedo; hazard risks in rugged topography; eff ects of changing land-use on surface energy, water, momentum and carbon fluxes; physiology of primary producers; and functional traits of terrestrial ecosystems. Thermal infrared imagery provides vegetation and inland and near-coastal aquatic canopy, and water surface temperatures as well as complementary information on ground, vegetation ecosystem function and health. Depending on implem entation specifics, the Targeted Observable may also contribute to hyperspectral ocean observation goal s. However, such goals are met to a large degree by POR elements, in particular the hyperspectral PA CE mission, and are not considered a priority for additional implementation (and thus are not recommended if they drive cost). Observations of the Earth’s surface biology and geology, with the ability to detect detailed spectral signatures, provide a wide range of opportunities for Earth system science parameters ac ross most of the panels and integrating themes. As such, this Targeted Observable maps to some of the highest panel priorities as well as the Integrating Themes. Candidate Measurement Approaches. High spectral resolution (or hyperspectral) imagery provides the desired capabilities to address important geological, hydrological, and ecological questions, building on a successful history of past and ongoing multispectral remote sensing (e.g., MODIS). UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-63 Copyright National Academy of Sciences. All rights reserved.

154 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space with moderate spatial resolution ( 30-60m) is identified as a priority Consequently, hyperspectral imagery for implementation. The CATE evaluation considered the Hy spIRI concept, which was developed CATE Evaluation. by SMD following a recommendation from the 2007 ESAS d ecadal survey, and found that the concept is technically mature and costs are well-understood, su pporting a recommendation for early implementation. In the event costs exce ed the maximum cost level recommended here, relaxed Descope Options. instrument requirements and/or eliminating TIR instrument are advised. It is assumed that the Sustainable La POR Assumptions. nd Imaging program continues to provide the measurements described here. Landsat-class land imagery to complement Budgetary Guidance. In keeping with the guidelines of the Designated program element and this Surface Biology and Geology Targeted Observable has a maximum report’s Recommendation 3.3, the recommended development cost of $650M (in FY18$). DESIGNATED — Targeted Observable: Surface Deformation and Change [ H-1c , 2c , 4a , 4b; S-1a , 1b , 3b 1c 2a , 2b , 2c , 3a , , , 4a , 4b, 5a , 6a , 6b, 6c, 6d, 7a; C-1c , 7b, 8f] Targeted Observable corresponds to TO-19 in the Surface Deformation and Change The Targeted Observables Table (Appendix C). By monitori ng the physical dynamics of the Earth’s surface, ologic hazards, and monitoring progressive surface we increase our ability to anticipate devastating ge deformation can reveal how the Earth’s systems are ch anging either naturally or through human activity. oving our ability to anticipate future Earth states. Such monitoring is key to avoiding surprises and impr This Targeted Objective is cited by nearly all of the Earth Surface and Interior objectives as well as matching needs in Hydrology and Climate. Science Considerations . The Targeted Observable will provide surface deformation measurements including surface change monitoring, ice sheet dynamics, the Antarctic grounding line subsidence. The measurements support science and migration, and permafrost thaw derived from canoes, landslides, sea level, plate tectonics, the applications objectives related to earthquakes, vol le to examining terrestrial ecosystem structure cryosphere, and groundwater. InSAR also is applicab (Treuhaft et al., 2004). Measurements of displacement and surface deformation capture many of the Most Important and Very Important objectives of the Solid Earth, Hydrology, and Climate Panels. Moreover, the more than 25-year history of InSAR observations of deformation and displacement establishes a long history of displacement observations, enabling detections of trends in behavior of land and ice processes. Candidate Measurement Approaches. The presumed measurement implementation involves Synthetic Aperture Radar (SAR) and Interferometric SAR (InSAR). SAR and InSAR have wide application across Earth Science, including: detecti on and monitoring of ice-sheet motion and grounding of ice sheets and their potential to cause rapid sea line locations, which are critical for assessing stability kes; detection of surface deformation and eruptive level rise; detection of ground motion from earthqua products of the active volcanoes; and mapping of landslides. Consequently, providing surface deformation measurements with improved space-time coverage post-NISAR was identified as a priority for implementation. Cost and Technical Evaluation . The recommended cost level does not support InSAR continuity through a re-flight of a NISAR-like mission. Instead, con tinuity is to be pursued through consideration of international partnerships and/ or a constellation of small sat ellites with relaxed performance characteristics to provide for the desired continuity cost level. The decade’s within the specified NASA priority science and applications objectives suggest implementation should consider reduced spatial resolution in favor of improved temporal resoluti on, which further enables innovative implementation approaches (i.e., smaller aperture requirements imply possibility of smaller satellites and lower cost). Descope Options. In the event development costs exceed the maximum NASA development cost level recommended here, a higher cost impl ementation is not desirable. Instead, the Surface Deformation UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-64 Copyright National Academy of Sciences. All rights reserved.

155 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Targeted Observable becomes a good candidate for the Earth System Explorer competition, and Change itional Earth System Explorer solicitation. using the available budget to fund one add POR Assumptions . The POR includes NISAR which is curren tly planned to launch in late 2021 and is designed to operate for 3-5 years, w ith no planned follow-on. NISAR’s primary science requirements are: crustal deformation, glacier and ice sheet motion, biomass structure, and sea-ice left to view Antarctica, for ice-sheet monitoring. dynamics. NISAR operates at L-band, and can also look enable improved detection of slow solid Earth It will have a dual frequency ionospheric correction to signals. Partnerships. NASA is encouraged to seek internatio nal partnership opportunities to implement the NISAR mission. If the NISAR mission launch this mission, and to phase implementation to follow date slips, implementation of this mission may also move to the right. Budgetary Guidance. In keeping with the guidelines of the Designated program element and this report’s Recommendation 3.3, the Surface Deformation and Change Targeted Observable has a maximum recommended developmen t cost of $500M (in FY18$). Program Element: EARTH SYSTEM EXPLORER To improve programmatic responsiveness while al so maximizing the role of competition in implementing flight recommendations, a new medium -class (<$350M FY18) cost-capped solicitation is recommended. The Earth System Explorer would be aimed at addressing the specific list of observing system priorities identified in Table 3.5 and summarize d in Table 3.7 (no relative priorities are assigned to the candidates in the list). The new Earth System Explorer line consists of a set of competitively-selected PI-led missions intended to mirror the proven success of the Astroph ysics and Heliophysics Medium Class Explorer 10 (MIDEX) lines. This program is designed to acco mplish high-quality Earth system science investigations addressing one or more priority Targ eted Observables in Table 3.7, utilizing innovative, streamlined, and efficient management approaches th at seek to contain mission cost through commitment tions costs. Analogous to MIDEX, specific mission to, and control of, design, development, and opera objectives are defined by the PIs in their proposals and approved by NASA through confirmation 11 review. These missions seek to conduct scientific inves tigations of modest and focused programmatic scope, and can be developed relatively quickly (generally in 40 months or less) and executed on-orbit in 3 years or less. The program does not maintain a budget reserve to which investigations exceeding their cost commitments may have access for cost overruns. If, at any time, the cost, schedule, or scientific performance commitments of a selected mission appear to be in peril, and descope options are not available, the mission can be subject to a cancellation review by NASA. Each Earth System Explorer mission is cost-ca pped at $350M, including the launch vehicle and three-years of operations. Cost capping the missions at $350M leaves ample room for instrument costs, operations costs, reserves, and a range of mission- unique trades (from use of larger launch vehicles to constellations, and/or inte gration/coordination with more sophisticated payloads to multiple-spacecraft existing or new ground/suborbital assets). The program element line opens Earth system science to the 10 MIDEX spacecraft generally include selective redunda ncy with a cost in the range of $60M to $85M, depending on attitude control performance, communication requirements and the need for propulsion. Launch costs are generally in the range of $55M. MIDEX missions typically consists of a 250-kg payload (approximately 100-kg for instruments and 150-kg for the spacecraft) la unched on a Pegasus-class vehicle to orbit. 11 While Table 3.5 lists the observing system priorities to be addressed in proposals to Earth System Explorer solicitations, the scope and technical requirements, and thereb y extent to which any partic ular objective associated with a Targeted Observable is met, depend on the propose d implementation approach, which will be assessed as part of the competitive selection process. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-65 Copyright National Academy of Sciences. All rights reserved.

156 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space but flight-ready, technology alte rnatives including novel spacecraft bus benefits of innovation and to new, stributed launch options. concepts, miniaturized instrumentation, small sate llites, constellations and di eting for these medium-class missions will likely Competition is an excellent motivator, and comp stimulate an already creative and motivated community to be even more so. Moreover, the health of the ey see opportunities to compete for their priorities. diverse scientific communities is strengthened when th t is expected, as Earth Venture has done, to drive As such, the Earth System Explorer program elemen act and retain talented scientists and engineers who engagement across the scientific community and to attr are motivated by opportunity. The Earth System Explorer program element is recommended in part because the science priorities identified are of sufficien tly similar importance that the key discriminators addressing cost, scope, on what should go forward are those that will emerge through competition technical performance, technical read iness, and programmatic capabilities. By identifying seven science ar eas for three competitions, community members associated with different science areas and different measurement a pproaches will be more inclined to seek innovative 12 echnologies so they can compete successfully. and creative approaches, partnerships, and t The selection among them should be made on the basis of compe titive peer review, and in the context of the international POR as it stands at the time of comp etition. The recommended program includes funding to support three solicitations in the decadal period, w ith a goal of supporting additional solicitations if additional funding is made available per the decision rules outlined in Chapter 4. The Earth System Explorer program element differs significantly from Earth Venture, in terms of both underlying philosophy and scope, which is why it is introduced as a new class of missions distinct from Earth Venture. The Earth System Explorer program element is confined to addressing any of seven priorities in Table 3.7. In recognition of the ty pes of missions required to address these objectives, it carries a $350M cost cap that is significantly greater than that of the Earth Venture. In contrast, the intent levant observing systems without any prescribed focus of Earth Venture is to present an opportunity for re or science and applications objectives and which is sm aller in scope than the observing systems in Earth System Explorer. TABLE 3.7 Targeted Observables to be addressed through the competitive Earth System Explorer program element. All rows are of equal ESAS 2017 prio rity, and are shown here in alphabetical order. TARGETED OBSERVABLE IMPLEMENTATION CONSIDERATIONS Address all or part of TO-4 . Active sensing (lidar, radar, scatterometer); passive imagery or radiometry-based atmospheric Atmospheric Winds* motion vectors (AMVs) tracking; or lidar** Address all or part of TO-6 . Can be active or passive, global or Greenhouse Gases regional; or lidar** Addresses all or part of TO-7 . Lidar**; if CryoSat-3 not approved Ice Elevation then highest priority function for multi-function lidar Ocean Surface Winds and . Doppler scatterometer Address all or part of TO-11 Currents . UV/Vis/IR microwave limb/nadir Address all or part of TO-12 Ozone and Trace Gases sounding and/or UV/Vis/IR solar/stellar occultation 12 nder (ESSP) program, which was a cost-capped program in the late NASA’s Earth System Science Pathfi 1990s and early 2000s, produced such successful missions as GRACE, Cloudsat, CALIPSO and Aquarius. Currently the ESSP program is limited to Earth Venture concepts, which include open solicitations for suborbital strand (EV- S; $30M each) competitions with five se lections every four years, an instrument strand (EV-I) with one selection every 18 months, and missions (EV-M; up to $150M each) with one selection every four years. Providing opportunities with higher cost-caps that are similar to thos e of the previous ESSP progr am (in today’s dollars), will likely produce similarly succe ssful concepts that can substantially advance important science and applications objectives. Unlike ESSP, however, the recommended Earth System Explorer solicitation is designed to solicit proposals responsive to a specific set of id entified science and applications priorities. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-66 Copyright National Academy of Sciences. All rights reserved.

157 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Snow Depth and Snow Water Address all or part of TO-16 . Radar (Ka/Ku band) altimeter or lidar Equivalent Terrestrial Ecosystem Address all or part of TO-22 . Lidar** Structure * Indicates Incubation program investment is also recommended to ensure comp etitiveness prior to the end of the decade (see description of Incubation Program in following section). ** Could potentially be addressed by a multi-function lidar designed to address two or more of the Targeted Observables. e 3.7 recommended for Earth System Explorer Each of the Targeted Observables in Tabl competition is discussed in the following text, lis ted without priority in alphabetical order. EARTH SYSTEM EXPLORER — Targeted Observable: Atmospheric Winds [ H-2a , 4a , 4b; W-1a , 2a , 7c 4a , 4b, 5a, 5b, 7a , 7b, 4a , 7d, 7e, 8i] , 9a, 10a; C-3f, The Atmospheric Winds Targeted Observable corresponds to TO-4 in Appendix C. It is included both in ESAS 2017 recommendations for the Earth System Explorer and the Incubation program element. The committee believes Atmospheric Winds is not yet ready for immediate implementation with acceptable risk, but could be during the decade with pr oper technology advances. The expectation is that Incubation investment could reduce risk sufficiently to accomplish that. A detailed description is included in the Incubation section. 3a , 4a, 5a, EARTH SYSTEM EXPLORER — Targeted Observable: Greenhouse Gases [W-8a; E-2a , 5b, 5c; C-2d 3a , 3b, 3c, 3e, 3g, 4a , 4d, 7b] , The Greenhouse Gases Targeted Observable corresponds to all or part of TO-6 in Appendix C. ) and methane are the two most important anthropogenic greenhouse gases Carbon dioxide (CO 2 (Hofmann et al, 2006; Montzka et al., 2011; IPCC, 2014) but their atmospheric budgets are still poorly understood, limiting our ability to predict futu re concentrations. A central question for CO is the role of 2 the terrestrial biosphere as a sink to moderate the rise in atmospheric concentrations. This terrestrial sink regions are highly uncertain, and the environmental is poorly quantified, the contributions from different e very large uncertainties in the factors controlling controls are largely unknown. For methane, there ar wetland emissions and the magnitudes of differe nt anthropogenic source sectors and regions. Observational Approach. Observations of CO and methane from space can provide unique 2 information to constrain surface fluxes of these gases at the continental/regional level and down to the scale of point sources, considerably enhancing cove rage relative to the sparse network available from surface sites. Inverse analyses e xploiting the satellite observations can guide improvements in process- sion inventories that provide the based biogeochemical models and emis basis for enabling projections of future concentrations. Space-based measurement approaches include: Global observations of CO  and methane at horizontal resolution of a few km and daily revisit 2 with sufficiently high precision to constrain re gional budgets of surface fluxes on a weekly time scale. This might be achieved with SWIR spectro meters that observe the atmospheric column with sensitivity down to the surface, compleme nted by TIR spectrometers that provide information on vertical distribution as well as da ta over the oceans and at night. Lidars may provide complementary information with sensitiv ity down to the surface over the oceans and at night.  Geostationary continental-scale ob servations with sub-km horizontal resolution and revisit of at most a few hours. This could involve SWIR spectrometers with high precision, possibly complemented by TIR spectrometers. Staring ove r selected regions with large surface fluxes may provide unique insights into daily variations of these fluxes and sporadic high emissions. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-67 Copyright National Academy of Sciences. All rights reserved.

158 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Low Earth Orbit observation of plumes from point  sources using SWIR spectrometers with very limited viewing domains. Precision should be high spatial resolution (less than 50 m) over sufficient to quantify source magnitudes on th e basis of a single pass of the satellite. Science and Applications Value. Atmospheric levels of CO and methane play a critical role in 2 driving climate change. They are controlled by bi ogeochemical cycling and anthropogenic emissions in ways that are presently not well understood. As a resu lt, we lack a sound basis to interpret current future trends. Coupling of atmos pheric changes with biogeochemical atmospheric trends and to project cycles could lead to important climate feedbacks. Air quality is also expected to respond significantly to meteorological variables and directly through the changes in greenhouse gases, both indirectly through role of methane as a precursor of ozone pollu tion. Improved measurements of atmospheric CO and 2 methane from space, combined with mapping of surf ace properties, would allow us to better understand and quantify the sources and sinks of CO and methane. This spans the interests of multiple panels and is 2 central to the carbon cycle integrating theme. EARTH SYSTEM EXPLORER — Targeted Observable: Ice Elevation [H-4b; W-3a ; S-3a ; C-1c , 8a , 8b , 8c , 8h] The Ice Elevation Targeted Observable corresponds to all or part of TO-7 in Appendix C. Land ice and sea ice are both important components of the cryo sphere that play different roles in the Earth’s climate system, a fundamental parameter that should be monitored for both of them is surface elevation. For land ice, the surface elevation measurement is used to determine Observational Approach. glacier and ice sheet mass balance. The largest uncertain ty in future sea-level rise is the contribution from increasing. The ice sheet contribution (Greenland and melting land ice (glaciers and ice sheets), which is Antarctica) likely will soon surpass thermal expans ion as the dominant component, and they have the l rise (tens of cm per decade). Observations of potential to cause rapid and large amounts of sea leve dramatic changes in the ice sheets have made us realize the complexity of ice sheet response to atmospheric and oceanic forcing on various timescales, challenging our traditional view of ice sheets that evolve slowly. Improved understanding of processes drivin g ice sheet changes is vital for predictions of continual monitoring of land ice provides a multi- future ice sheet mass loss and sea level rise. Only decadal record of change, and the continuous nature of these observations is critical, so that we can learn observed changes. This allows assessment of the contributions of which processes are contributing to the seasonal, inter-annual and inter-decadal variability in snow accumulation, surface-melt and ice flow dynamics and their impact on ice-sheet mass balance. For sea ice, the freeboard (height of the ice su rface above the sea surface), enables estimates of sea-ice thickness. The shrinking sea-ice area in the Arctic is one of the most striking manifestations of climate change since the satellite record began and the resulting albedo reduction is an important climate ce age, as older ice is thicker. Estimation of sea-ice feedback. The ice thickness is an indication of the i anges of energy, mass and moisture between the ice, ocean, and thickness enables is to examine exch ickness is more challenging, as there is significant atmosphere. In the Antarctic, estimation of sea-ice th snow on the sea ice and often the freeboard is negative. Measuring land ice surface elevation and sea-ice freeboard height by satellite or radar laser altimeter along repeated ground tracks provides an estimate of the volume change of land ice and sea ice over time. ICESat-2 is planned for launch in Septem ber 2018. The planned lifetime of the mission is 5 years. After that, there will be a gap on our observi ng capabilities for ice surface elevation and freeboard, and it is critical that this gap be filled by a sate llite system. Operation IceBridge has been successful filling the gap between ICESat and ICESat-2, but only provides one measurement per year for for a very limited subset of the ice sheet. Space-based measureme nts of ice surface elevation would include a polar- orbiting satellite (to 88  ) carrying a scanning laser or radar alti meter, as a follow-on to ICESat-2 and CryoSat-2. Over land ice, the spatial sampling should be at least 1-km over the central parts of the ice UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-68 Copyright National Academy of Sciences. All rights reserved.

159 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space sheets, with 0.1 km sampling around the ice-sheet margins, and be accurate to 10-20 cm. Repeat period should be weekly. Over sea ice, the spatial sampling s hould be at least 1-km, and be accurate to 10-20 cm. Repeat period should be weekly. Land and sea ice play critical roles in the areas of climate, Science and Applications Value. weather, energy balance and the water cycle. Sea ice insulates ocean water from overlying polar air, with As sea ice forms and ages, it loses some of the salt direct impacts on atmospheric and ocean circulation. in the seawater to the surface, altering the density struct ure of the underlying water, which in turn impacts her, climate, and the energy cycle. The Greenland ocean circulation. These processes directly affect weat ective frozen water containing the equivalent of 7 m and Antarctic ice sheets are vast stores of highly refl 13 and 58 m of sea level respectively . The topography of these ice sheets, which rise to several km in elevation, and the energy, mass, and momentum ex changes with the atmosphere affect regional and global weather patterns, climate, sea level, and the cy cling of water. Moreover, the hydrology of mountain glaciers directly contributes to timing and amount of water availability for rivers, reservoirs, and consumption throughout the world. Implementation Contingency. In the event that ESA implements the Cryosat-3 radar altimetry mission, which has a different implementation but si milar goals to the ice altimetry mission described here, this priority can be changed to a multi-purpose a ltimeter. The altimeter need not be optimized for ice be designed for any relevant geophysical parameter sheet observations (although it can), but rather can commitment to Cryosat-3, however, ice altimetry addressed through altimetric measurements. Absent a should be the mission driver. — Targeted Observable: Ocean Surface Winds and Currents [H-4b; EARTH SYSTEM EXPLORER W-1a , 2a , 3a ; C-3d, 4a , 4b, 5a , 6a , 7a , 7b, 7d, 7e, 8d , 8i] Targeted Observable corresponds to all or part of TO- The Ocean Surface Winds and Currents 11 in Appendix C. Ocean surface winds are important to the Earth system for a number of reasons. These ocean and atmosphere strongly influencing the fluxes winds are critical elements in the coupling between (e.g. questions/goal W-3, C-9). Ocean surface winds of heat and momentum transferred at the interface and thus the interaction between winds and currents are also a central driver of upper ocean currents, e atmosphere and ocean. Small-scale variations in provides a measure of momentum exchange between th sea surface temperature modulate heat and momentum ex changes, which can vary on time scales of hours to days. Observational Approach. Advancing our understanding of the coupling between the atmosphere and ocean will require coincident swath measurements of surface winds, near-surface atmospheric properties, and surface currents. Space-based measurement approaches include:  Vector surface winds from scatterometers. Passive microwave radiances can be used to infer vector winds but require full polarimetric capability and have high uncertainties, particularly at low wind speeds. (These observations will also provide passive sea surface temperature and radiance data from passive sensors also pr ovide atmospheric water vapor, cloud and precipitation.)  Surface currents from Doppler anomalies measured by scatterometer, using a larger antenna and re interferometric phase. A system that can higher pulse repetition frequency in order to measu to infer sea ice drift to show pathways by measure surface winds and currents can also be used which freshwater can propagate through Arctic and Antarctic regions. Doppler Scatterometer measurements are a new technology tested in aircraft measuremen ts conducted in 2017. 13 P. Fretwell; H. D. Pritchard; et al. (31 July 2012). “Bedmap2: improved ice bed, surface and thickness datasets for Antarctica” (PDF). The Cryosphere. Retrieved 1 December 2015. “Using data larg ely collected during the 1970s, Drewry et al. (1992), estimated the potential sea-level contribution of the Antarctic ice sheets to be in the range of 60-72 m; for Bedmap1 this value was 57 m (Lythe et al., 2001), and for Bedmap2 it is 58 m. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-69 Copyright National Academy of Sciences. All rights reserved.

160 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space single band scatterometers, as noted in the POR, but no systems in International partners operate operation are capable of observing wind and currents. Science and Applications Value. Ocean surface winds are critical elements that couple the ocean to the atmosphere, driving oceanic circulation and exerting a momentum drag on the atmosphere. They ntum across the air-sea interface. Jointly measuring strongly influence the fluxes of heat, gas, and mome of the momentum transfer between the ocean and winds and currents will provide a direct assessment atmosphere. In the short term, these processes have significant impact on weather, and in the long term, they affect regional and global climates. Observi ng and understanding ocean surface winds and currents together will provide key insights into the Earth’s weather, climate, and energy cycles. EARTH SYSTEM EXPLORER — Targeted Observable: Ozone and Trace Gases [ W-2a , 4a , 5a , 6a, 7a, 8a; C-2g , 3f, 3g, 6c, 9a] The Ozone and Trace Gases Targeted Observable corresponds to all or part of TO-12 in Appendix C. Observational Approach. The UV shield from stratospheric ozone is critical to life on Earth. NASA satellites have played a central role in mapping ozone depletion over the past decades, and are now poised to observe the ozone recovery expected in response to the Montreal Protocol. Satellite observations are needed to monitor the ozone recove ry at different latitudes and altitudes, and examine whether it is consistent with our understanding of the underlying chemical processes. An improved understanding of how meteorological variability and ot her natural factors such as volcanic eruptions affect the ozone layer is also essential. Tropospheric ozone is of separate interest as a greenhouse gas, a surface air pollutant, and a precursor of the hydroxyl (OH) radical, the main atmospheric oxidant. The factors controlling tropospheric ozone are poorly under stood, including the effect of human activity on a global scale, and multidecadal trends have been challenging to explain. Anthropogenic emissions d satellites offer a unique perspective for observing affecting tropospheric ozone are rapidly changing an these trends. Observations of tropospheric ozone pr ecursors can also advance understanding of the controlling ozone concentrations. sources, chemistry, and transport Space-based measurement approaches include: Solar/stellar occultation and TIR/microwave limb  observations of the stratosphere and upper troposphere with ~1 km vertical resolution for ozone and related chemical species including H O, 2 CH O, NO , CO, halogens, and aerosols. Characte rizing the relationships between these , N 4 2 2 different species and ozone in different regions of the stratosphere will provide important information for understanding the factors controlling ozone.  Combined nadir/limb observations of the global tr oposphere in the UV/Vis/IR with nadir pixel resolution of a few km and daily return time for ozone and related species including CO, NO , 2 will allow improved quantification of the factors and HCHO with some vertical information. This controlling ozone on scales ranging from global to urban, and enable understanding of the connections between those scales. Science and Applications Value. NASA’s history of mapping stratospheric ozone depletion and its role in mapping the expected recovery resulting from the implementation of the Montreal Protocol, provide an excellent example of direct connecti ons between scientific observations, and life-saving policies. In addition to its response to destructive an thropogenic chemicals, the behavior of stratospheric ozone is also linked to meteorological variability and other natural factors such as volcanic eruptions in ways that are not yet well understood. In the tropos phere, ozone is a greenhouse gas and a pollutant, and as such has direct linkages to climate, weather, and air quality. It and other trace gases, even though they exist in relatively small quantities in the atmosphere, ha ve direct implications for the Earth’s energy cycle from the UV to the thermal infrared by influencing the radiative exchanges among the sun, atmosphere, and the Earth’s surface. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-70 Copyright National Academy of Sciences. All rights reserved.

161 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space — Targeted Observable: Snow Depth and Snow Water Equivalent EARTH SYSTEM EXPLORER H-1a , 1c , 2b, [ ; W-3a ; S-4a, 4b, 4c; C-8c , 8f] 4a The Snow Depth and Snow Water Equivalent Targeted Observable corresponds to all or part of TO-16 in Appendix C. Snow cover is the second largest area component of Earth’s cryosphere, 2 . Half of the Northern Hemisphere is covered by snow in winter. Most snow covering 1.9 to 45 M km eme cold does not allow much moisture in the air. falls outside the high Arctic and Antarctica because extr Two important properties of snow as a climate variab le are its high reflectivity, with a high albedo (90% ’s high albedo means that decreases in snow-cover when fresh), and that it is a good insulator. Snow extent act as a positive feedback to climate change by changing the global albedo. Its insulating properties mean that a snow layer over the Earth’s surface h as a major effect on the energy exchange between surface and atmosphere, which prevents soil freezing and slows down ablation of glaciers, ice sheets, and sea-ice. Only a few decimeters of snow cover can insulate underlying ground or ice from atmospheric temperatures. Insulation increases with snow layer thic kness, thus it is important to know its depth and how it changes over time. As snow ages, its density increases, and its albedo decreases. Observational Approach. Snowmelt plays a major role in water resources, affecting soil moisture, evapotranspiration, and runoff. Snow in mountain re gions contributes to water supplies for almost one- sixth of the world’s population (e.g. snow melt supp lies 85% of Colorado river). Changes in snow cover are having a dramatic impact on water resources. The important parameter for hydrology and water quivalent (SWE; how much water is contained in snow, equal to snow supply forecasting is snow water e depth multiplied snow density). SW E is important for hydrological modelling and runoff prediction; nt in hydrology models and in monitoring climate snowfall as a fraction of total precipitation is importa change. Snow area is mainly monitored by sate llites, including Landsat, NOAA satellites, and MODIS. Snow depth is monitored with passive microwave AMSR-E and SSM/I (passive microwave since 1978), nd also is affected by melt. Ground measurements are as the ground emissivity changes with snow cover a used to calibrate satellite data and constrain snow models and are also assimilated in NWP and reanalysis systems. In the western US, JPL’s Airborne Snow Obser vatory has been flying since 2013 and carries an imaging spectrometer to measure albedo and laser a ltimeter to measure snow depth before and after a snow fall event. Combination of albedo (to estimate ag e, and therefore density) and snow depth yields an ng snow depth is a high-frequency (W- or Ka-band) estimate of SWE. An alternative to lidar for measuri radar altimeter or interferometer. A Ka-band inte rferometer has been flown as an airborne sensor as GLISTIN, the Glacier and Land Ice Surface Topography Interferometer. Science and Applications Value. Snow cover, which spans half of the land area of the Northern Hemisphere in winter, directly affects climate through its high albedo (reflecting as much as 90% of incident sunlight) and its strongly insulative properties. The albedo in particular plays an outsized role in the surface energy balance, because of the strong diffe rence in reflectivity between fresh new snow and old wet snow as well as the difference between snow -covered land and land that is not snow-covered. r as the radiative and thermodynamic properties of These albedo differences also directly impact weathe -covered surfaces are dramatically differ ent and have a substantial impact on snow covered and non-snow near surface energy, mass and momentum exchanges. Fi nally, snow plays a critical role in hydrology and the water cycle by modulating the delivery of freshwa ter to streams and reservoirs. This is because snow serves as a storage for water in winter and releases that water relatively slowly over time through the spring and summer. — Targeted Observable: Terrestrial Ecosystem Structure [H-3c; S- EARTH SYSTEM EXPLORER , 3c, 8f] E-1b , 1e, 3a , 4a, 5a, 5b, 5c; C-2d 4c; UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-71 Copyright National Academy of Sciences. All rights reserved.

162 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Terrestrial Ecosystem Structure to all or part of TO-22 The Targeted Observable corresponds in Appendix C. Characterization of the three-dimensi onal structure of land-based vegetation, particularly for forested ecosystems, provides utility for multip le research, resource management and conservation perspectives. Canopy and understory structure re flects the species and functional composition of the nd nutrients across the landscape. Measurements of ecosystem as well as competition for light, water, a ecosystem structure inform on rates of primary production, ecological functioning, carbon storage, and changing land-use. A measurement approach using satellite-based lidar would build on Observational Approach. successful airborne experimentati on and the two-year pilot Global Ecosystem Dynamics Investigation (GEDI) instrument in the POR to be flown on the Inte rnational Space Station beginning in 2018. Vertical structure of plankton biomass and mixed layer depth across the upper ocean is also of considerable scientific interest and accessible vi a lidar approaches, though requiring different technical constraints than land vegetation structure (see TO-10 in Appendix C). Recovery of useful oceanographic data from the IOP) sensor motivate the inclusion of marine Cloud-Aerosol Lidar with Orthogonal Polarization (CAL ecosystem structure as an opportunistic measurem ent within the Aerosol Targeted Observable. Science and Applications Value. Observations and characterization of the three-dimensional structure of land-based vegetation provide critical in formation on ecosystem structure primary production, ecological functioning, carbon storage, and changing land-use. In addition to the obvious linkages between vegetation structure and ecosystems and the carbon cycle, the changes over time of these characteristics have direct connections to climate and hydrological processes which influence vegetation growth and health, as well as the water and energy cy cle, through evapotranspiration. Trees, shrubs, and other land plants compete for space, light, and othe r resources, resulting in the complex landscapes in forests and other land ecosystems. The three-dimensional structure of vegetation strongly influences ecosystem dynamics and carbon cycling but is difficult to decipher from standard satellite imagery alone. pes, and carbon storage vary substantially from the For example, primary production, plant functional ty canopy top, through the canopy and understory to the ground surface. Therefore, new observational tion structure will provide critical information on approaches to characterize three-dimensional vegeta In addition, geographic and temporal variations ecosystem fluxes, carbon cycling, and changing land use. in vegetation structure have direct connections to climate and hydrological processes which influence vegetation growth and health, as well as the water and energy cycle, through evapotranspiration. ********************************************************************************* a case study for the oceans community BOX 3.9 Achieving programmatic balance: ESAS 2017 established priorities based on an Earth system science perspective, but recognized the importance of achieving programmatic balance su ch that individual disciplines are supported. The committee viewed such support as the combination of observations already available through the Program of Record and any new observations proposed in this report. The oceans community presents a good example for t esting disciplinary balance in this approach. 14 At the time of ESAS 2007, the oceanographic community wrote a community letter to highlight key ocean variables that the community valued and that had been identified as priorities by GCOS and by the U.S. Oceans Commission Report as candidates for space- based observation. This baseline of six needed observations, and their disposition in ESAS 2017, includes: 1. Ocean surface vector winds. Ocean surface vector winds, which provide key information about the transfer of momentum between the atmosphere and ocean, are part of the Program of Record of international partners, as part of ME TOP-A, METOP-B, CFOSAT, and SCATSAT. The community has noted that a multi-satellite appro ach with appropriately timed equator crossings 14 The letter is available at http://cioss.coas.oregonsta te.edu/CIOSS/Documents/Oceans_Community_Letter.pdf. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-72 Copyright National Academy of Sciences. All rights reserved.

163 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space -frequency winds and diur nal variations. The would facilitate better investigation of high community letter emphasized the importance of achieving the coverage and accuracy of esence of rain. With this in mind, ESAS 2017 QuikSCAT with good measurements in the pr recommended the Ocean Surface Winds and Currents Targeted Observable which will provide critical insights into the coupling and ex changes between ocean and atmosphere. All-weather sea surface temperature (SST). Sea surface temperature has been assumed to be part 2. Program of Record of the ith international partners; however, , as part of ongoing collaborations w assive microwave time series (Box 4.4) this with the potential loss or interruption of the p capability is currently at risk. The opportunity for avoiding losses lies in the successful competition in the Earth Venture-Continuity strand. Alternatively, international partners could ner with a demonstrated capability to do so. fulfill the need, but there is currently no part Sea surface height (SSH). Sea level has been identified as a priority measurement that is 3. contained within the Program of Record through the international partnership provided by the Jason-CS/Sentinel-6 mission and the ongoing Sentin el-3 series. In addition, ESAS 2017 supports Mass Change Targeted Observable , continuation of the GRACE satellite series through the ved sea level rise to its thermal expansion and which will enable appropriate attribution of obser mass gain components. 4. A wide-swath altimeter. This objective will be addressed by the US-French Surface Water and Ocean Topography (SWOT) mission within the Program of Record , which was recommended in ESAS 2007 and is scheduled to launch in 2021. In addition, the steering committee has recognized the importance of wide swath ocean altimetry (as Targeted Observable TO-21 in Appendix C), identifying it as a candidate for Earth Venture opportunities. 5. Ocean color. For the open ocean, the Program of Record includes a number of sensors (MODIS, Landsat, VIIRS, PACE) that will help to meet ocean color objectives. In the coming decade, the hyperspectral radiometer on PACE is likely to ocean color capabilities for provide more advanced addressing key science priorities addressed in this survey’s SATM including marine ecosystem and biodiversity. The global ocean ecosystem data from PACE fluxes and structure, function and terrestrial ecosystem information that complements the near-shore coastal, aquatic inland, Surface Biology and Geology would be derived from the . High Targeted Observable spatial/temporal resolution coastal and inland aqua tic ocean color also h as been identified as a variable of interest (see Aquatic Biogeochemistry Targeted Observable , Ecosystem Panel, Appendix C), and the NASA/Moore Foundation Pr oof of Concept mission (the HawkEye Ocean Color Sensor on a CubeSat) will help to address this. 6. Sea surface salinity (SSS). Salinity is a desirable variable, wh ich helps to identify the impact of the water cycle and together with temperature de termines density of surface waters, which in turn impact circulation. In addition to the now-comple ted Aquarius mission, sa linity is measured as Program of Record part of the ongoing SMOS and SMAP missions in the . ESAS 2017 recommended Sea Surface Salinity Targeted Observable candidate for an Earth Venture as a ogy development to reduce costs and better mission opportunity, as well as for continued technol address the accuracy and cold-temperature limita tions inherent in microwave salinity sensing. The RFI submissions concerning salinity identif y a number of promising options worthy of research and competition for further technology development. Thus, each of the six priorities identified by the ocean science community in their 2007 community letter is addressed in some way over the next decade by the Program of Record, and potentially with an Earth System Explorer or Venture mission. ESAS 2017 also offers additional opportunities to expand beyond these capabilities. These opportunities are:  The Atmospheric Winds and Planetary Boundary Layer Targeted Observables to measure atmospheric winds and profiles in the atmospheri c boundary layer, which will enable examination of air-sea exchanges. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-73 Copyright National Academy of Sciences. All rights reserved.

164 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Lidar systems with the potential to measure mixed-layer depth, upper ocean turbulence, or  Targeted biological productivity in the upper ocean (such as the Ocean Ecosystem Structure ) which will offer a possibility of building better understanding of the ocean side of Observable air-sea exchange. ********************************************************************************* END OF BOX Program Element: INCUBATION The Incubation program element provides investment funds to support maturation of mission, instrument, technology, and/or measurement concepts to address specific high priority science and applications Targeted Observables as needed to enable cost-effective implementation. Three observing system priorities are recommended for maturation via Incubation program funding (see Table 3.8). These are observations that, despite their high priority, lack sufficient technical maturity to be considered ready for low risk implem entation. Each of the identified Targeted Observables establish and mature their associated prospective would benefit from focused and sustained attention to user communities to make material progress to wards maturing both measurement requirements and implementation concepts within this decade. To foster program-level innovation, the co mmittee also recommends that NASA establish an Innovation Fund within the Incubation Program to enable responses to unexpected opportunities that occur on sub-decadal scales. Such responses could include leveraging new technologies; responding to international, commercial, or private partnership oppo rtunities; or providing seed investments to evaluate or demonstrate new approaches (e.g., alternative procurement models, novel launch services concepts, data buys, leveraging unco nventional data sources, block buys, exploiting available multi-instrument platforms) to implementing priority Targeted Observables The committee has included an additional $20M /year from the budget wedge (Figure 3.11) to cated to programs such as ESTO, and notes that the support these activities, some portion of which is allo maturation of mission, instrument, technology, and/or measurement concepts (described below) further requires the coordinated use of existing resources. TABLE 3.8 Targeted Observables selected by the Committee to be addressed through the Incubation program element. TARGETED CANDIDATE INCUBATION PROGRAM GOALS OBSERVABLE Atmospheric Improve understanding of me asurement needs through advanced Earth system  Winds modeling representative of winds in coupled atmosphere-ocean-land-ice models with realistic planetary boundary layer (PBL)  Explore the best combination of active (lidar, radar) and passive (radiometry) technologies that can leverage ESTO inv estment in active technologies and POR AMVs from GOES-R and international GEO and LEO satellites  h measurement needs can be addressed Mission concept studies to define whic with state-of-the-art technology via Venture and/or Earth System Explorer opportunities and which requi re further development Strategic technology development investments to ensure flight maturity of  needed measurement technologies by end of decade Planetary  Improve understanding of me asurement needs, through modeling and mission Boundary Layer concept studies, to define which can be addressed with state-of-the-art technology and which require further development.  Identify needs which can be addressed through ground-based or airborne UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-74 Copyright National Academy of Sciences. All rights reserved.

165 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space ace-based component. Identify any needed mechanisms rather than requiring a sp technology developments. Identify any elements which are mature and suited to Venture-class  opportunities. Identify any proposed compone nts that could be ready for Earth System Explorer opportunity, for consideration by Midterm Assessment. Identify elements which may be appropriate for NOAA consideration as “on-ramps” described in Chapter 4.  Consider suborbital observations of temperature/humidity and modeling needs to complement atmospheric winds and PBL height measurements. Surface  Improve understanding of me asurement needs, through modeling and mission Topography and concept studies, to define which can be addressed with state-of-the-art Vegetation technology and which require further development.  obtained through suborbital means and Identify which measurement needs can be Identify those ready to compete in which require a space-based component. Venture-class opportunities.  Identify any proposed components that could be ready for Earth System Explorer opportunity, for consideration by Midterm Assessment.  Consider appropriate split between global observations from space and potentially less expensive and higher resolution airborne measurements  Look into obtaining commercial data to meet needs; define a pathway to ensure any identified spaceborne component matures towards flight in the following decade. For each Targeted Observable in the Incubation prog ram element, a coordinated program of strategic investments in technology, research, modeling, and/or data system development would be developed by ts. This would entail strategic coordination of NASA towards maturing the overall measurement concep resources and support from the Technology, R&A, and Flight program elements to support concept ovide funding to mature individual technologies and maturation. Several existing programs already pr to be implemented as individual open calls and do instrument concepts. However, those programs tend rm progress toward a defined objective as is called not provide a mechanism to make coordinated long-te expected to develop an understanding of measurement for here. A team of scientists and engineers will be needs through modeling and mission concept studies to address the specific goals outlined in Table 3.8. Activities might include:  Trade Space Examination. Formally define and explore the trade space of implementation options.  Solutions Brainstorming. Explore means to achieve breakthroughs and alternate sources to obtaining the needed measurements. Consider commercial, ground, airborne, and partnership opportunities  Impact Evaluation and Sensitivity Assessment. Establish a quantitative understanding of the impact of the observations on science and appli cations, including sensitivity analysis showing echniques such as OSSEs when appropriate which aspects are most important, using t  Evaluate the observations’ impact parametrically, through mechanisms Requirements Refinement. such as OSSEs when appropriate, to assess the most important observational requirements with the objective of relaxing less important requirements.  State-of-the-Art Evaluation. Evaluate the current capability of technologies, models, and data systems to achieve and utili ze the considered observations.  Evaluation of Opportunities. Evaluate the identified needs to determine which are candidates to be addressed via open-solicitations such as Eart h Venture (all strands) and ROSES, and which (if UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-75 Copyright National Academy of Sciences. All rights reserved.

166 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Explorer solicitations to be considered by the any) might be candidates for future Earth System Atmospheric Winds as a potential Midterm Assessment. The co mmittee has identified only candidate for this decade’s Earth System Explor er opportunities based on its current level of technical maturity. Should substantia l development advances be made in Surface Topography and or Planetary Boundary Layer by the Midterm Assessment, Vegetation their suitability for the Earth System Explorer competition can be reassessed. Identify specific shortfalls in the state-of-the-art of  Identification of Gaps and Investment Needs. technologies, models, and/or data systems that are barriers to achieving or utilizing the observation. Identify and invest in needed ground, aircraft, or suborbital instrument, subsystem flight readiness of these mission concepts. and/or mission technologies to increase the Each of the Targeted Observables in Table 3. 8 recommended for the Incubation program element is discussed in the following text, listed without priority in alphabetical order. INCUBATION — Targeted Observable: Atmospheric Winds [ H-2a , 4a , 4b; W-1a , 2a , 4a , 9a, 10a; C-3f, 4a 7a , 7b, 7c , 7d, 7e, 8i] , 4b, 5a, 5b, The Atmospheric Winds Targeted Observable corresponds to a ll or part of TO-4 in Appendix C) Measurement of atmospheric winds was identified as a recommendation in ESAS 2007 (Table 2.1), and ithin ESAS 2017. The technology readiness of this this observation again appears as a high priority w measurement and apparent high cost of considered approaches, however, presen ts challenges for near- uded within the Earth System Explorer candidates and term implementation. For this reason, the TO is incl tion is that Incubation investment could achieve also within the Incubation candidates. The expecta etition within the Earth System Explorer program sufficient risk reduction to achieve readiness for comp element during the decade. Science and Applications Value. One of the most pressing science and application priorities in the coming decade is to better observe the properti es in the PBL and lower troposphere and improve prediction of high-impact natural hazards such as sev ere air pollution outbreaks and tropical and winter storms, renewable wind energy applications, transpor t and distribution of global water and carbon in 15 hydrological and energy cycles of the Earth system. Observing 3D winds is key to addressing these priorities to meet societal needs. Measurement of atmospheric winds is not only im portant to weather and air quality forecasts but also is fundamental to other components of the Eart h system. Wind is a central driver for ocean currents and essential for determine air-sea-land-ice surface fluxes. Atmospheric 3D winds are an essential and the coupling between expression of the circulation of the atmosphere clouds and the general circulation is central to address cloud and climate grand challenges (Bony et al., 2015). Large-scale winds and, together with vertical motions of convection, also transport energy and water through the atmosphere of trace gases and other constituents around the globe. are a principal input in quantifying transports Transports by winds are critical inputs to methodologi es that invert concentration of trace gases into eco- system fluxes. Winds are also fundamental to unde rstanding the hydrological cycle and related water resource applications. For example, the narrow ribbo ns of water-laden tropospheric winds of the subtropics act like rivers of moisture bringing heav y rains and snows to the southwestern United States. Observations of winds in the PBL are critical fo r better understanding and forecasting of extreme high winds in winter storms, tornadoes, hurri canes and wind-induced storm surge. Observational Approach, Tec hnology Readiness, and Risk. The importance of global measurements of the evolution of atmospheric wind v ectors is highlighted as an urgent need in the NASA Weather Research Community Workshop Report (Zeng et al. 2016). Measurement of the atmospheric 15 3D winds here refer to vertical profiles of horizon tal wind vectors and vertical velocity in convective precipitation, which can be observed from space. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-76 Copyright National Academy of Sciences. All rights reserved.

167 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space winds was identified as a priority in ESAS 2007, and this observation again is a priority in ESAS 2017. Yet, progress in advancing observation of three-dime nsional (3D) winds has been relatively slow. The technology readiness of this measurement and appa rent high cost of current approaches presents challenges for near-term implementation. ine where wind information will most impact A detailed assessment is required to determ 16 forecasts quired for the upper tropospheric and lower and the temporal and spatial resolution re applications such as extending weather and air quality forecasting troposphere/PBL winds for various and prediction on subseasonal-to-seasonal and longer from hours to 2 weeks and Earth system modeling scales. 17 Multiple active and passive technologies currently receive ESTO investment. A number of OSSE studies have been performed to evaluate poten tial impact of specific Doppler wind lidar (DWL) approaches (Baker et al. 2014). Some OSE studies ha ve evaluated the impacts of atmospheric motion vectors (AMVs) measurements on numerical weat her predictions (NWP) (Warrick 2016). The long- mics Mission (ADM) Aeolus (planned for January anticipated launch of the ESA Atmospheric Dyna 2018), designed to produce line-of-sight winds, ma y offer some partial assessment when it becomes available. Trade studies may still be needed to design the most cost-effective strategy for wind and AMVs) from satellites and airbor ne flights and the benefits of measurements (based on lidar, radar, combinations of approaches. As Zeng et al (2016) stat es, “it is important to avoid all-or-nothing strategies for three-dimensional (3D) wind vector measuremen ts, as important progress is possible with less than comprehensive observing strategies.” For these reasons, the TO-4 is included within the Earth System Explorer candidates and also within the Incubation Program. The expectation is that Incubation investment could achieve sufficient risk reduction to achieve readiness for competition within the Earth System Explorer program element during the coming decade. Incubation Goals. Particular incubation goals are described in Table 3.8. , ; INCUBATION — Targeted Observable: Planetary Boundary Layer [ H-2a C-2b W-1a , 2a , 3a , 10a; 4a , , 7b, 7c , 7d, 7e] 7a Planetary Boundary Layer Targeted Observable corresponds to all or part of TO-13 in The Appendix C. Science and Applications Value. The planetary boundary layer (PBL) literally couples the surface of the Earth to the atmosphere above. The im portance of the PBL to the next generation global prediction system (NGGPS) which requires bette r understanding and modeling of the coupling among the atmosphere, ocean surface, sea ice, and land in th e integrated Earth system is now recognized (NRC, 2016a). Boundary layer wind and thermodynamic informa tion together with air quality measurements are needed to improve understanding and prediction of severe air pollution outbreaks that affect human health (NRC, 2016b). The boundary layer is also a critical element in understanding the role of biospheric feedbacks in the Earth system as well as air-sea exchanges. Processes within the PBL and osed as new emergent constraints on understanding how the PBL mixes with the air have been prop 16 For example, it is expected that wi tropical regions than the extra-tropics nd information more directly impact where available strong atmospheric mass constr aints serve to constrain large-scale winds. 17 Each measurement approach has advantages and disadvantages, and the optimal approach varies depending on application. Passive sensing AMVs use indirect measur ements of atmospheric water vapor and clouds to derive winds, which have large error in assigning a height of re trieved wind in the atmosphere and wind speed (Forsythe 2007, Maschhoff et al. 2015). Good temporal coverage is provided by geostationary satellites; however, the low vertical resolution of AMVs is a limiting factor for observing winds in the PBL. Active sensing using a 2-μm aerosol backscatter DWL (Kavaya et al. 2014) may be best suited for observing winds in the PBL/lower troposphere where aerosol is abundant, though a 355-nm or 532-nm molecular backscatter DWL (Tucker et al. 2015) may have advantage in observing upper tropospheric winds. The combination of lidar winds and AMV winds might also provide some advantages where one is used to calibrate the other. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-77 Copyright National Academy of Sciences. All rights reserved.

168 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space lation to climate sensitivity arises from the influence climate sensitivity (Sherwood et al., 2015). This re of these mixing processes on boundary layer clouds thus making PBL processes central to low cloud feedbacks. hnology Readiness, and Risk. The PBL is the lowest layer of the Observational Approach, Tec with the Earth surface. The PBL includes the air we atmosphere and is directly influenced by its contact breathe and the weather we experience. Yet, this near-s urface layer of the atmosphere is relatively poorly moisture and pollutants between this layer, the observed and modeled, as is the exchange of energy, ese exchanges are critical to weathe surface, and the free atmosphere. Th r and climate because the bulk of ation that drive the atmosphere and ocean take place the interactions with solar heating and surface evapor within the PBL rather than the free atmosphere. Fo r forecasts longer than a few days, errors in these growing errors in weather forecast models. In order to adequately exchanges lead to substantial and represent the key boundary layer processes, high resolu tion, diurnally resolved, 3D/2D measurements of the PBL are required. While the POR and other elements of the Designated program provide measurements in the PBL, the global temporal (3 hourly) and vertical resolution required by the SATM study is needed to quantify the limitations of POR for thermodynamic profilers is not achieved. Further s through technology development and strategic combination of the and determine appropriate investment the Designated program) to fill the gap. elements of the POR (and other parts of PBL profiles include measurements of three-dime nsional (3D) temperature, water vapor, aerosol and trace gas (e.g., ozone) concentrations. They also include two-dimensional (2D; in the horizontal direction) PBL height, cloud liquid water path, clou d base, precipitation, and surface fluxes of water and energy. 3D horizontal wind vector measurements, which are part of the Targeted Atmospheric Wind Observable are also essential to understanding PBL processes and thus consideration of the Atmospheric Wind and Planetary Boundary Layer Targeted Observables together is warranted. A number of the 2D variables can be measured by existing ground-based networks (mostly over land) and by a variety of instruments onboard polar-o rbiting and geostationary satellites within the POR. the Designated program element) will provide The recommended Aerosol TO investment (part of measurements of aerosols in the boundary layer and the height of the PBL. GNSS measurements in the ght measurements. The recommended Clouds, Convection, and POR will also contribute PBL hei Precipitation Targeted Observable (part of the Designate d program element) will contribute to PBL cloud and precipitation properties. Microwave radiance me asurements within the POR provide cloud liquid water path and precipitation. Although thermodynamic pr operties of water vapor and temperature are also contained within the POR, much higher vertical reso lution and diurnally resolved information is needed to advance understanding of the role of the PBL on Earth system processes. The PBL processes that are important to weather prediction and to the Earth system more broadly exhibit a strong diurnal cycle. For instance, the PBL height can increase by an order of magnitude from near sunrise to mid-afternoon over land. While curre nt observations from geostationary satellites can fully resolve the diurnal cycle and provide useful information on cloud properties (refer to the Cloud, Convection, and Precipitation Targeted Observable), temperature and humidity soundings with sufficient capability to resolve the PBL does not yet exist le t alone from GEO platforms. A combination of geostationary, polar and suborbital profiles is need ed to obtain diurnally resolved PBL observations. orbital platforms are capable of providing high Active and/or advance hyperspectral sensors on the vertical resolution, but investment in each technol ogy is needed to achieve the required vertical resolution. For example, previous study has dem onstrated the readiness of prototypes such as the Geosynchronous Imaging Fourier Transform Spectr ometer (GIFTS) developed through the NASA New Millennium Program. Further, there are internationa l efforts with sensors that nearly match the capabilities of GIFTS; these include a Chinese plan to fly a series of Geostationary Interferometer Infrared Sounder (GIIRS) with capab ility approaching that of GIFTS, and a European advanced IR sounder (IRS) similar to GIFTS will be a key part of Meteosat Third Generation. Incubation Goals. There are a number of challenges that n eed to be examined via the Incubation program element, some of which will be nefit from cross-coordination with the Atmospheric Wind incubation effort described earlier: UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-78 Copyright National Academy of Sciences. All rights reserved.

169 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space SATM entries call for high vertical and tempor  al resolution to resolve a number of related ly temporal resolution). Analysis is required objectives (i.e., 0.2 km for 3D variables and 2-3 hour to assess the feasibility of meeti ng these resolutions using existing elements of the space and ground POR, and which specific augmentations to the POR would be optimal for meeting unmet needs An assessment of the state-of-t  he-art of passive and active technologies to provide advanced thermodynamic profiling of the PBL including wa ter vapor profiling and thermodynamic profiles identify emerging new capabilities technology readiness in the clear and cloudy PBL is needed to of existing capabilities such as DIAL measurements of water vapor versus identified needs to y investment may be required. determine where additional technolog  Methods to resolve the diurnal cycle of important PBL properties need to be further explored by considering: • Exploitation of existing geostationary and GNSS assets in the POR Role of suborbital (ground or airborne) observations • Capabilities of hyperspectral instrument protot ypes for geostationary application (e.g., GIFTS • or the Hyperspectral Environmental Suite; HES) • Technology development investments to mature measurement needs not met with existing technologies Novel concepts involving PBL-capable sensors alone or in constellations to meet identified • needs. INCUBATION — Targeted Observable: Surface Topography and Vegetation [H-2b, 2c, 3c, 4b, 4d; W- , , 8f] 3a ; S-1a , 1b , 1c , 1d, 2b C-1c 2c , 3a , 3b , 4a , 4b, 4c, 6b, 7a; E-1b , 1e; The Targeted Observable corresponds to all or part of TO- Surface Topography and Vegetation 20 in Appendix C. Science and Applications Value. Characterizing surface topography with contiguous rtical resolution will allo w for detailed understanding measurements at 5 m spatial resolution and 0.1 m ve of geologic structure and geomorphological process es, which in turn can provide new insights into surface water flow, the implications of sea level rise, and storm surge in coastal areas, the depth of off- shore water in near coastal areas, and more. In a ddition, assuming a lidar-based system, the implications for ecosystem structure, and the associated cycling of carbon will be significant, as described above under the Terrestrial Ecosystem Structure Targeted Observable. Observational Approach, Tec hnology Readiness, and Risk. Space-based lidar offers the possibility of simultaneously mapping at high spatial resolution the vegetation structure and underlying “bare earth” topography across globe. Such data would revolutionize our capability to understand how Earth’s surface works, and greatly enhance our ability to predict hazards and anticipate the effects of surface change. Although increased topographic resolution from 30 m (SRTM) to 12m (TanDEM-X) using synthetic aperture radar has been accomplishe d, much higher resolution is needed. Vegetation height from radar also involves much analysis. Optic al methods such as that provided by DigitalGlobe have reduced the resolution to 2 to 5 m, but such me thods track canopy heights, not the ground surface in vegetated environments. In the 2007 Decadal Survey, the Lidar Su rface Topography (LIST) mission was proposed to obtain a 5 m global topographic survey with decimeter precision. Although the mission did not go forward, NASA commissioned a LIST study. This study identified major challenges in detection efficiency, imaging technology, data rate and th roughput, and high average power and long lifetime lasers. There have been advances is all of these area s. Substantial progress in lidar technology has been made since 2007 through significant funding from NASA, other US agencies and the commercial sector. NASA has supported technology advancement for the LIST program through airborne programs (LVIS, SIMPL, MABEL, ALISTS) and space-based missions (ICESat-2 and GEDI). A lidar would have significant synergy with the recommended Terrestri al Ecosystem Structure Targeted Observable, UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-79 Copyright National Academy of Sciences. All rights reserved.

170 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space lution, spatial footprint, and repeat time. Higher depending on choices and tradeoffs among vertical reso temporal resolution may be needed for some ecological science objectives. Although perhaps now practicable, a 5 m resolution lidar program of the entire Earth would likely have a cost in the flagship mission range. Interna tional collaborations could reduce the cost, and some combination of high-altitude airborne (where f lights are permitted) and optical systems (e.g., reduce the area needed to DigitalGlobe) where vegetation density is low may be surveyed by a space- based system. For example, NASA’s LVIS system provides 10 m footprints, but can do 5 m in a swath several kilometers wide. The greatest challenge for airborne deployment ma y be obtaining permission to fly in many areas. The Solid Earth community expressed the goal of reaching 1 m spacing at 0.1 m vertical precision (the common standard in airborne lidar surveys) from spac e. Whether the spacing is 5 m or finer, the data collect needs to be spatially continuous, not a ser ies of swaths separated by large distances. This Incubation program should encourage active collabor ation between those who advance the technology and those who seek applications such that compromi ses may be found to move forward in reaching the mapping (and vegetation structure) from space. long-held goal of high resolution topographic Incubation Goals. Particular incubation goals are described in Table 3.8. Targeted Observables Not Allocated to a Flight Program Element A number of Targeted Observables were identif ied by the committee but were not specifically allocated to a flight program element (Appendix C). These are:  Aquatic Biogeochemistry (more details in Ecosystem Panel)  Sea Surface Salinity (more details in Climate Panel)  Ocean Ecosystem Structure (more details in Ecosystem Panel)  Radiance Intercalibration  Magnetic Field Changes Soil Moisture  and Ecosystems Panels) (more details in Hydrology Each of these Targeted Observables is tied to science and applications in the SATM (Appendix B), and none are adequately satisfied by the POR. More detailed descriptions are included in the individual panel reports. These six unallocated Target ed Observables are candidates for Earth Venture mission opportunities. In addition, some may also have opportunistic synergies associated with implementation of one or more of the Designated, Earth System Explorer, or Incubation observables. For example, a multi-function lidar developed to implem ent the measurements for one of the Earth System Explorer candidates could also serve one or more of these unallocated Targeted Observables, depending on the implementation approach. Each of these non-allocated Targeted Observables ha s strong reasons for inclusion in one of the flight implementation elements. For example, the Radiance Intercalibration Targeted Observable has ESAS 2007 and the CLARREO Pathfinder scheduled heritage to the CLARREO recommendation from for ISS implementation in 2021. The committee felt th e long-term value of this calibration facility was best achieved by seeking lower-cost options, such as Venture-Continuity, th at motivated multi-decade continuity of this important measurement. Program Element: VENTURE-CONTINUITY The Venture program is considered a critical element of the ESAS 2017 observing system program, though the open competitive selection process involved meant the committee did not specify candidates for the Venture program. In its statement of task, the committee was char ged with evaluating whether the present 3-strand Venture-Class competed program should be expanded or modified, including whether ESD should initiate UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-80 Copyright National Academy of Sciences. All rights reserved.

171 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space different cost caps. As described in Chapter 4, additional or different Venture Class strands, possibly with the committee notes that the Venture program appear s to be working well overall, serving its intended onstration of innovative and to facilitate the dem purpose of restoring more frequent launch opportunities nd Venture-class program responds directly to the ideas and higher-risk technologies. The current 3-stra ESAS 2007 recommendation, which suggested the program include “stand-alone missions...more complex instrument of opportunity...or complex sets of instruments flown on suitable suborbital platforms to address focused sets of science questions.” Similar innovation is warranted oviding for long-term, sustained specifically in the context of pr and therefore the committee proposes that a new observations Venture strand be established to incentivize innovation to enable sustained observations in a mo re cost-effective way. Sustained observations are identified in the balance discussion in Chapter 4 as a priority area for achieving programmatic balance. The Venture-Continuity strand would specifically seek to lower the long-term carrying cost of providing for continuity observations, rewarding innov ation in mission-to-mission cost reduction through technology infusion, or programmatic efficiency, or other means. Box 4.5 provides a recognized example for the significance of this need. Budget for this pr oposed expansion of Venture opportunities is included in the budget wedge. Both the international and national communities c ontinue to call for the creation of a sustained global satellite-based Earth monitoring system (NRC, 1999; NRC, 2008). The need for such monitoring for the purpose of understanding the Earth system is se lf-evident to scientists. However, such endeavors are costly undertakings and justification for it re quires clear, important, societal objectives with well- articulated, achievable goals in addition to the need to understand the behavior of the Earth system and predict its change over time. The evaluation of space-b ased continuity measurements in the context of the quantified Earth science objectives was an important recommendation of the 2015 NRC report Continuity of NASA Earth Observations from Space: A Value Framework (NRC, 2015). Limited resources force an inherent tension be tween continuity of measurements and the it is imperative that as technologies that were introduction of new observation capabilities. As a result, once groundbreaking in enabling observations of new va riables become more routine, a shift in emphasis hnical innovation, is needed. Otherwise, either the toward reduced cost through programmatic and/or tec sustained monitoring of critical variables will be put at risk, or innovati on and new observations will stagnate as the need to fund long-term measurement records further strains an already resource-limited budget. The Venture-Continuity strand provides a much-needed opportunity to incentivize development of cost-efficient means to provide for sustained ob servations of those critical parameters for which development and implementation costs can be brought down significantly by leveraging innovation to reduce costs rather than improve performance. The Vent ure-Continuity strand is envisioned to be similar to the Venture-Mission strand, including full missi on implementation costs whether for instruments, spacecraft, and launch vehicles or hosted payloads with hosting services included. It challenges the ll use of technical advances and programmatic science and engineering communities to make fu opportunities in order to develop low-cost capabilities in order to enable sustained monitoring. ACCOMPLISHING INTEGRATED SCIENCE WITH THE OBSERVING SYSTEM The four elements of the ESAS 2017 observing program - Designated, Earth System Explorer, Record to address a broad range of topics within Incubation, and Venture - will augment the Program of Earth system science and applications, spanning both the panel priorities and ESAS 2017 Integrating Themes. Taken collectively, these ongoing and new ob servations will advance our understanding of the Earth as a system and the interaction of its various co mponents in ways that directly affect the way we live. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-81 Copyright National Academy of Sciences. All rights reserved.

172 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space To illustrate the ability of the program to address integrated science, it is useful to consider the contributions of the Targeted Observables in the context of the three integrating themes examples discussed previously: Water and Energy Cycle, Carbon Cycle, and Extreme events. Table 3.9 shows how system themes. In most cases, each observable the various Targeted Observables map to these Earth addresses elements of multiple integrating themes. It is clear from this mapping that each Targeted science, and that collectively they can advance Observable will contribute substantially to integrated progress in high priority Earth System science challenges. vables in addressing ESAS 2017 Integrating Themes. TABLE 3.9 Potential roles of Targeted Obser Integrating Targeted Observable Contribution Theme  Aerosols: Radiative forcing and feedbacks, aerosol cloud interaction : Forcings and feedbacks, thermodynamic and Precipitation Clouds, Convection  processes  Mass Change : Movement of water throughout the Earth, ocean heat content Role of winds in energy transport and evapotranspiration  Atmospheric Winds :  : Contributions of various greenhouse gases in Earth’s energy Greenhouse Gases balance  Ice Elevation : Ice sheet contributions to the water cycle; modulation of ocean/atmosphere energy exchanges by sea ice Water and : Ocean/Atmosphere Energy and moisture  Ocean Surface Winds and Currents Energy exchanges, and ocean energy transport Cycle Snow Depth and Snow Water Equivalent : Storage and distribution of water, Latent  energy associated with snow melt, insulation modulating land/atmosphere energy exchanges, surface radiative balance associated with snowcover Terrestrial Ecosystem Structure  : Cycling of water and energy through evapotranspiration, carbon uptake, soil moisture  and Vegetation: Cycling of water and energy through Surface Topography evapotranspiration, carbon uptake, soil moisture  Planetary Boundary Layer : Energy and moisture exchanges in the boundary layer, cycling of water through evaporation and precipitation  Atmospheric Winds : Surface fluxes, vertical and horizontal transport of CO and CH 2 4 and CH : Emissions and uptake of CO  Greenhouse Gases and contributions to 2 4 greenhouse warming : Inhibition of vertical transport of greenhouse gases  Planetary Boundary Layer Carbon Cycle  Surface Biology and Geology : Carbon uptake by terrestri al and marine ecosystems  Surface Deformation : Methane release from thawing permafrost.  Surface Topography and Vegetation : Carbon uptake from te rrestrial vegetation Terrestrial Ecosystem Structure : Carbon uptake from terrestrial vegetation  UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-82 Copyright National Academy of Sciences. All rights reserved.

173 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space  Aerosols : severe outbreak of air pollution in the boundary layer  Atmospheric Winds : Dynamic forcing of severe convective storms, transport of water vapor for heavy rain events and flash floods, extreme winds in severe storms, hurricanes and winter storms : Intense convective storms, floods,  Clouds, Convection and Precipitation precipitation induced landslides  Ice Elevation : Increased risks of storm surge associ ated with sea level rise; episodic events associated with coastal erosion Extreme : Storm surge, hurricanes and severe storms  Ocean Surface Winds and Current Events induced maritime wind/wave hazards Ozone and Trace Gases : Fire plumes, volcanic plumes, industrial disasters, surface  ozone smog events, stratospheric ozone depletion events Surface Deformation : Hazards related to landslides, earthquakes, volcanic eruptions,  and both coastal and river erosion and flooding.  Snow Depth and Snow Water Equivalent : Flooding associated with rapid melt  Planetary Boundary Layer : Processes that affect severe weather In addition to the contributions of individual observations to advancing Earth System science these parameters offers an opportunity for more listed above, simultaneous combined observation of comprehensive insight into critical Earth system processes. One example is the combination of the observable, which targets direct aerosol radi ative forcing and feedbacks by observing aerosol Aerosols observable, which targets the effects of optical properties, with the Clouds Convection, and Precipitation aerosol forcings and feedbacks by observing clou d thermodynamics and optical properties. The combination of the two produces a far more complete understanding of components of the energy cycle that have the strongest forcings and feedbacks. Similarly, combining the observable with the Terrestrial Ecosystem Structure Greenhouse Gases and Surface Biology and Geology observables enables a more comprehensive tracking of sources of carbon dioxide as inferred from the qua ntity and locations of observed CO and the sinks as inferred from 2 biomass and ocean primary productivity. Observing the sources, sinks, and transport between them provides much more insight into the dynamics of carbon dioxide than any of the individual observations could. ted program with concurrent observations in These examples illustrate the value of a coordina ence in which the whole of the observations is much advancing an integrated approach to Earth system sci greater than the sum of its parts. DISPOSITION OF ESAS 2007 MISSIONS IN THE ESAS 2007 OBSERVING SYSTEM The missions identified in priorities by ESAS 2007 reflected the highest priority science at the time. It is instructive to assess how those priorities are reflected within the ESAS 2017 observing system, including the Program of Record. Table 3.10 su mmarizes how the ESAS 2007 missions map to this committee’s proposed observing system. TABLE 3.10 Disposition of ESAS 2007 priorities within the observing system proposed by ESAS 2017. ESAS 2007 ESAS 2017 DISPOSITION MISSION UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-83 Copyright National Academy of Sciences. All rights reserved.

174 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space NASA-DIRECTED SMAP Implemented by NASA, in POR ICESat-II Implemented by NASA, in POR DESDynI Key objectives addressed in NASA POR via NISAR (in partnership with ISRO) and GEDI Objectives would be met with HyspIRI Surface Biology and Geology Objectives partially addressed in POR and recommended as a candidate for Earth ASCENDS System Explorer ( Greenhouse Gases ) SWOT Implemented by NASA in partnership with CNES, in POR Aerosols in the Designated program Partially addressed by TEMPO in POR and by GEO-CAPE element ACE Key objectives could be pr ovided by a combination of Aerosols and Clouds, Convection, and Precipitation Recommended under Incubation ( LIST ) Surface Topography and Vegetation PATH Recommended under Incubation ( Planetary Boundary Layer ) GRACE-FO, implemented by NASA in part nership with GFZ, meets key objectives, GRACE-II in the Designated Program element, which seeks to ensure as does Mass Change continuity SCLP Recommended as a candidate for Earth System Explorer ( Snow Depth and Snow Water Equivalent) Recommended as a candidate for Earth System Explorer ( Ozone and Trace Gases ) GACM 3D-Winds Recommended as a candidate for Earth System Explorer and for Incubation (Demo) Atmospheric Winds ( ) NOAA-DIRECTED GPSRO Implemented by NASA, NOAA, NSF and international partners, in POR XOVWM Recommended as a candidate for Earth System Explorer ( Ocean Surface Winds and Currents ) JOINT NASA-NOAA CLARREO Partially implemented by NASA, in POR ACHIEVING AN INSPIRATIONAL PROGRAM lly inspire innovation. As we look forward to The challenges of observing Earth from space natura the decade ahead, we seek to harness such inspiration to provide the bol d and credible program the nation needs for making rapid progress in space-based Eart h observation. This report’s proposed program establishes a realistic, structured framework within wh ich inspired progress be made over the next decade. The committee recognized that numerous impedi ments to success exist (see Chapter 2 for the specific discussion on Programmatic Impediments and Vu lnerabilities), each of which must be addressed in order to harness this inspiration. In res ponse, the recommended program embraces the role of competition, strengthens the critical leveraging of international and commercial partnerships, ensures UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-84 Copyright National Academy of Sciences. All rights reserved.

175 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space have been languishing through existing programmatic focused incubation of needed measurements that ence priorities, and more. Through such changes, channels, introduces robust guidance for maintaining sci it seeks to break the mold of “business as usual” a nd achieve new, more effective ways of pursuing science and applications. In particular: Through the new competitive Earth System Expl  orer program element, the NASA Earth Science Program will be able to address pre-identified high priority target observables leveraging the latest innovations and ideas available to propo sers at the time of solicitation, and in full consideration of the domestic a nd international program of record at the time. The committee believes this represents a significant shift that will create opportunities for new competitors and new innovations in Earth observations from spa ce, capitalizing on the full potential of the aerospace and Earth science community. It will also enable the programmatic flexibility NASA needs to optimize its flight program throughout the decade as domestic and international programs evolve.  The Venture-Continuity line will incentivi ze reducing cost of maintaining long-term measurement records, as will the recommended increase in ESTO funding for game-changing technology development (Chapter 4). Lower co st capabilities for measurement continuity will help reduce the inherent tension between long-t erm continuous measurements, and the emergence of new observations, creating more fertile ground for Earth observation capabilities. These elements provide a much-needed programmatic me chanism to incentivize innovation in favor of cost efficiency rather than improved performance.  The establishment of an Incubation Program will allow real progress to be made on large and nd investment which cannot be made through a important challenges through focused attention a series of one-off competitive calls and should not be made by starting a mission that isn’t yet well defined.  The establishment of decision rules (Chapter 4) en sures that community guidance with respect to science priorities is well understood when adjustme nts to the NASA flight program are required due to budgets that are greater or less than anticip ated, or when unanticipated events alter plans. The committee believes this program changes the existing programmatic paradigm, enabling innovation while constraining cost and ma naging risk. The program rises to the Community Challenge presented in Chapter 1, ensuring effective use of r esources to accomplish outstanding science and enable valuable applications over the coming decade and beyond. REFERENCES Andela, N., D. C. Morton, L. Giglio, Y. Chen, G. R. van der Werf, P. S. Kasibhatla, R. S. DeFries, G. J. Forrest, G. Lasslop, F. Li, S. Mangeon, J. R. Collatz, S. Hantson, S. Kloster, D. Bachelet, M. Melton, C. Yue, and J. T. Randerson. 2017. A human-driven decline in global burned area. Science 356:1356-1362. Asner, G. P., G. V. N. Powell, J. Mascaro, D. E. Knapp, J. K. Clark, J. Jacobson, T. Kennedy-Bowdoin, A. Balaji, G. Paez-Acosta, E. Victoria, L. S ecada, M. Valqui, and R. F. Hughes. 2010. High- resolution forest carbon stocks and emissions in the Amazon. Proceedings of the National Academy of Sciences 107:16738-16742. Asner, G., R. Martin, D. Knapp, R. Tupayachi, C. Anderson, F. Sinca, N. Vaughn, and W. Llactayo. 2017. Airborne laser-guided imaging spectroscopy to map forest trait diversity and guide conservation. Science 355:385-389. Asrar et al., “The World Climate Research Program Strategy and Priorities: Next Decade,” chap. 1 in Asrar, G. R., and Hurrell, J. W. (eds.) (2013) . Climate Science for Serving Society: Research, UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-85 Copyright National Academy of Sciences. All rights reserved.

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179 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Workshop. Hobart, Tasmania. renfeld, R. Ferrare, and G. Mace ACE Science Study Team, A. da Si lva, R. Swap, H. Maring, M. Beh ber 2016, 154 pp., (2016) ACE 2011‐2015 Progress Report and Future Outlook, Septem ted.pdf https://acemission.gsfc.nasa.gov/documents/ACE_5YWP‐FINAL_Redac S. Bony, B. Stevens, D. M. Frierson, C. Jakob, M. Kageyama, R. Pincus, T. G. Shepherd, S. C. Sherwood, A. P. Siebesma, and A. H. Sobel, “C louds, circulation and climate sensitivity,” Nature Geoscience, vol. 8, pp. 261-268, 2015. Tapley, Byron D. et al. “GRACE measurements of mass variability in the Earth system.” Science 305, 5683, 503-505 (2004). Fasullo, J. T., C. Boening, F. W. Landerer and R. S. Nerem, Australia’s unique influence on global sea level in 2010-2011. Geophys. Res. Lett. 40, 4368-4373 (2013). Rodell, M., Beaudoing, H. K., L’Ecuyer, T. S., Olson, W. S., Famiglietti, J. S., Houser, P. R., Wood, E. F. (2015). The observed state of the water cycle in the early twenty-first century. Journal of Climate, 28(21), 8289-8318. doi:10.1175/JCLI-D-14-00555.1 Rignot, E., Velicogna, I., Van Den Broe ke, M. R., Monaghan, A., and Lenaerts, J. (2011). Acceleration of the contribution of the Greenland and Antarctic ice sheets to sea level rise. Geophysical Research Letters, 38(5). doi:10.1029/2011GL046583 Johnson, G. C., and Chambers, D. P. (2013). Ocean bottom pressure seasonal cycles and decadal trends from GRACE Release-05: Ocean circulation implications. Journal of Geophysical Research: Oceans, 118(9), 4228-4240. doi:10.1002/jgrc.20307 Houborg, R., M. Rodell, B. Li, R. Reichle, and B. Zaitchik, Drought indicators based on model assimilated GRACE terrestrial water storage ob servations, Wat. Resour . Res., 48, W07525, doi:10.1029/2011WR011291, 2012. Chen,J.L., Wilson,C.R., Tapley,B.D., Grand S., (2 007), GRACE detects coseismic and postseismic deformation from the Sumatra-Andaman earthquake, Geophys. Res. Lett. , 34(13), L13302, doi: arXiv:10.1029/2007GL030356 . Han, S.-C., J. M. Sauber, and F. Polllitz. 2016. “Pos tseismic gravity change after the 2006-2007 great eartthquake doublet and constraints on the asthenosphe re structure in the central Kuril Islands.” , 42: [10.1002/2016GL068167] Geophys. Res. Lett. Ivins ER, James TS (2005) Antarctic glacial isostatic adjustment: a new assess ment. Antarctic Science 17(4):541{553 Velicogna I, Sutterley TC, Van den Broeke MR. Regi onal acceleration in ice ma ss loss from Greenland and Antarctica using GRACE time variable gravity data. Geophys Res Lett. 2014;41:8130-7. doi:10.1002/2014GL061052 . Erik R. Ivins, Thomas S. James, John Wahr, Ernst J. O. Schrama, Felix W. Landerer, Karen M. Simon, “Antarctic contribution to sea level rise obser ved by GRACE with improved GIA correction”, JGR Solid Earth, June 2013, doi: 10.1002/jgrb.50208 Riva, R. E. M., Bamber, J. L., Lavallée, D. A. and Wo uters, B. Sea-level fingerprint of continental water and ice mass change from GRACE. Geoph ys. Res. Lett. 37, L19605 (2010). Rousseaux, C.S. and Gregg, W.W., 2013. Interannual va riation in phytoplankton primary production at a global scale. Remote sensing , 6 (1), pp.1-19. Devred, E., Turpie, K.R., Moses, W ., Klemas, V.V., Moisan, T., Babin, M., Toro-Farmer, G., Forget, M.H. and Jo, Y.H., 2013. Future retrievals of water column bio-optical properties using the Hyperspectral Infrared Imager (HyspIRI). Remote Sensing , 5 (12), 6812-6837. Zhang, C, 2013: Madden–Julian Oscillation: Bridging Weather and Climate, Bulletin of the American Meteorological Society, vol. 94, issue 12, pp. 1849-1870, doi: 10.1175/BAMS-D-12-00026.1. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 3-89 Copyright National Academy of Sciences. All rights reserved.

180 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space 4 Agency Programmatic Context Chapter 3 described the committe e’s recommended decadal research strategy to advance Earth system science. Ultimately, our goal is to understand the Earth as a system in ways that provide benefit and value to society. Achieving such an understanding requires programs that translate Earth observation data into applications that meet user needs—the subject of the first part of this chapter. The “programmatic context” in which research at NASA, NOAA, and USGS is carried out and applied is the subject of the second part of this chapter. This chapter is not intended as a general revi ew of all programmatic elements within these agencies (which is beyond the committee’s statement of task). Instead, it focu ses on those programmatic elements that are specifically related to the sp ace-based observing system and that the committee felt required particular discussion. Many important programmatic elements, such as workforce, education, and outreach - and even some (such as product gene ration processes and computational advances such as machine learning) that are more closely related to the observing system itself - are not included. Programmatic topics not directly discussed within this chapter may be addressed to some extent by the guidance in the strategic framework presented in Chapter 2. MULTI-AGENCY CONTEXT Some contextual issues are specific to the im plementing agencies, but many are common. The topics presented in this section represent opportunities fo r each agency to advance individually as well as prospects for cross-agency sharing of best practices to advance together. Advancing the State of Applications Applications are often viewed in the context of “practical things that get done with scientific knowledge.” Gradually, this percep tion is expanding as we come to appreciate the intellectual and practical challenges of ensuring applied impacts from the fundamental science that has been the core of the community’s work. Applications challeng es are many, from how to understand the world’s multiplicity of use cases to more rapidly transiti oning knowledge into practical use. These challenges certainly reflect many practical problems, but they also embody a set of intellectual problems that are every bit as important as the foundational Earth science from which they are built. Importance of an Applications Perspective The first Decadal Survey for Earth Sciences and Applications (2007) promoted the proposition that the application of scientific knowledge about th e Earth system was as important as acquiring it in the first place. This was not a new insight about the Earth sciences, but it was important to articulate it clearly to NASA, NOAA, and the USGS. This Decadal Survey reinforces this view. The benefits to society of UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-1 Copyright National Academy of Sciences. All rights reserved.

181 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Earth science research are the partners of scientific discovery and progress; they are more than serendipitous by-products of basic research, but tance. Their impacts on are often co-equal in impor society, from safety to economics (Box 4.1) can be enormous. Conversely, scientific discoveries in the Earth sciences also generate important in sights for the use of that new knowledge. Each of the agencies whose programs and interest s we are examining in th is Survey has a long history of promoting the applications of their sci ence. For the USGS, applying their research to land management issues within the Depa rtment of Interior, and to mapping for exploration, planning, and management is part of their intrinsic miss ion. NOAA has both research and policy/operational components under one roof, from the National Weather Se rvice to the National Marine Fisheries Service, and has experience with transitioning knowledge fro m research to operations and policy. NASA has no formal y its formal role in ozone monitoring), but it is role in operations (with specific exceptions, notabl actively involved in promoti ng applied uses of its research and in providing policy-relevant information for global environmental issues, such as tropical defor estation, stratospheric ozone depletion, and climate change. ********************************************************************************* BOX 4.1 The Financial and Non-Moneta ry Value of Earth Observations The breadth of use of Earth science for societal benefit is well documen ted in National Science 002, 2007, 2012, 2013; National Aeronautics and and Technology Council 2014; NAS 2001, 2003, 2 Space Administration 2014; Macauley 2009; Macau ley and Laxminarayan 2010; CCSP, 2008, among it as, for example, “the data helped in deployment other studies. These studies tend to frame societal benef data were useful, such as “the availability of data of disaster relief” but provide less insight into why the within two hours at 10 km resolution allowed deployme nt of disaster relief twice as fast as without the data and enabled an estimated x% more lives to be saved.” But defining the counterfactual is very challenging. , developed in the 1960s (see Sa vage 1954; Hirshleifer and Riley The Value of Information (VOI) 1979; McCall 1982; Bikhchandani et al. 2013), provides an answer to the question “by how much has information influenced a decision to take (or not take) an action?” The canonical example is a weather forecast, with the likelihood of rain influencing deci sions about when to harvest and the “value” of the information is derived directly from the value of the decision; in this case, the value of the forecast is derived directly from the value of the harvest. A la rge body of literature uses VOI for natural resource management, including weather and crop forecasting (A dams et al. 1995, Babcock 1990, Considine et al. 2004, Katz and Murphy 1997, Lazo and Waldman 2011, Nordhaus 1986, Nelson and Winter 1964, Roll 1984, Sonka et al. 1987, Bradford and Kelejian 1977, Roll 1984, Lave 1963) and geologic mapping (Bernknopf et al. 1997). A growing body of literature uses VOI specifically to value Earth science satellite data (Table 4.1), with applications from insurance, agriculture, forestry, water quality, drought, human health, disaster relief, and carbon pricing. VOI is not limited to fina ncially denominated values. VOI methods can also integrate nonmarket values including nonuse values, option values, and existence values (Freeman 2003; Bernknopf et al. 2016). UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-2 Copyright National Academy of Sciences. All rights reserved.

182 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space rmation (VOI) methodology to value Earth science TABLE 4.1 Examples of the use of Value of Info satellite data. VALUE OF INFORMATION EARTH SCIENCE SATELLITE DATA USE METHODOLOGY Weather data for weather insurance (Osgood and Shirley 2012) o Price- and cost-based o Drought and land use information for index insurance (Skees et al. 2007) o Losses averted from vector-borne disease (Hartley 2012) o Forest carbon sequestration (Macau ley and Sedjo 2011a, 2011b, Macauley 2010, Richardson and Macauley 2011, 2012) Probabilistic Bayesian belief networks (Cooke and Kousky 2012) o Regulatory cost- o Nonpoint source groundwater pollution (Bernknopf et al 2012) Monitoring water quality (Bouma et al. 2009) effectiveness and policy o evaluation Social cost of carbon (Cooke, Wielicki et al 2014, 2015, 2016) o Econometric modeling ang and Lowenberg-DeBoer 2008) o Productivity (agriculture; Tenkor and estimation o Forestation (Pfaff 1999) Land use and climate change (Fritz et al 2012) o Simulation modeling and estimation Valuing information for climate-related purposes is particularly challenging, given the extent to less, the community is making progress on economic which impacts occur in the far future. Neverthe tools. A major challenge for valuing information with in any global climate observing system is illustrated tion. Current climate studies most often rely on through the example of seasonal to decadal scale predic observations designed not for climate but for weathe r or basic research. The former often lack the accuracy needed for decadal time s cale climate change while the later struggle to achieve continuity of multi-decade climate change records (NRC 2015, 2007, Weatherhead et al. 2017, Trenberth et al. 2013). Lack of accuracy of observations on decade time scales has also been shown to delay detection of anthropogenic climate change trends by decades (Leroy et al. 2008, Wielicki et al. 2013). e for the international community to design and These delays indicate a substantial societal valu implement an observing system designed specifically to climate change requirements. Recent studies have estimated the economic value of a more accura te and rigorous global climate observing system at $10 Trillion to $20 Trillion U.S. dollars (Cooke et al . 2014, 2016b, Hope, 2015, Weatherhead et al. 2017). Return on investment of a tripling of the current globa l investment in climate research is estimated at $50 herhead et al. 2017). At these levels, even a factor to $100 for every $1 invested (Cooke et al. 2014, Weat of 5 uncertainty in the economic analysis does not ch ange the final conclusion: that development of a more accurate and complete global climate observing system, climate system analysis, and climate veral reports have also discussed improved methods modeling is a very effective economic investment. Se for the design of a more rigorous climate observi ng system (Dowell et al. 2013, NRC 2015, Weatherhead et al. 2017). These recent reports and studies suggest a need to co nsider the appropriate level of investment in Earth observations and suggest that the most cost-e ffective approach would be a much higher level of investment than current national and international levels. For climate cha nge in particular, society will be managing Earth’s environment indefinitely into the future. An international observing system designed for this purpose appears to be the most cost-effective approach. ********************************************************************************* UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-3 Copyright National Academy of Sciences. All rights reserved.

183 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Definition of applications in this report A variety of functions could be expressed by th e term “applications.” For the purposes of this report, we focus primarily on two of these: Direct use of remote sensing products in an operational context. This is probably the most commonly understood use of the term applications - da ta products from remote sensing are used more or less directly in an operational program. They may be used directly to initialize models for numerical weather forecasting, for example. But remote sensi ng data products can also be used as part of an operational program without necessarily being used for parameter estimation. The use of vegetation index 1 data products, for example, is an essential f eature of the Famine Early Warning System , which is a collaboration involving NASA, NOAA, and USGS. Remote sensing products are increasingly used in a ng by government agencies and non-government end- large variety of ways to improve decision-maki users, as well as individuals who make use of a reliable stream of this type of information in their daily lives. Using remote sensing information in support of decision-and policy-relevant issues. Objective measurements are of critical impor tance to build understanding of issues around which policy questions are debated and to support decisions made by indi viduals, businesses, and government organizations. The ability to measure the loss of humid tropical forest in an objective and replicable way through Landsat data, has become an extremely powerful tool for u nderstanding the magnitude of tropical deforestation, and more generally, rates of land-c over and land-use change on global scal es. In Brazil, this measurement rational program for enforcing laws forbidding capability has become part of the government’s ope deforestation in some areas of the Amazon. Measureme nts of the Earth’s radiation budget, total column ozone, sea-level rise, or ice extent and mass balance have similar roles. They are not necessarily immediately incorporated into operational models or used in operational programs, but they are crucial for a more complete understanding of Earth science issues that are actively discussed and debated in policy forums. One of the earliest examples is the ob servations of the ozone hole over Antarctica, which led to the 1987 Montreal Protocol. Measurements that fall into this category are also of high value from a purely scientific standpoint—there is little to differen tiate their scientific value from their applications value from an information perspective. This is a t ype of application that is very common in the NASA Earth Science portfolio. Operations and applications are not identical and it is often important to make a clear distinction between the two terms. For example, NOAA has a clear operations mandate that is quite different from NASA’s support for applications of data from its scientif ic satellites. In general, this report does not focus on that distinction. Both operations and applications are considered to be applied uses for observations, in contrast to scientific uses. This distinction is genera lly reflected in the report rather than that between operations and applications, except in specific instan ces when the latter is relevant to a particular discussion. Examples of current and potential applications Table 4.2 shows the wealth of both existing and potential future applications from the interdisciplinary panels’ responses to a survey fro m the steering committee. Several important points emerge that are relevant to NASA, NOAA, and th e USGS. All the panels have salient examples of products that could fit in each of the two categories ab ove. None of the panels perceived that the goal of being able to apply the measurements was in conflict with being scientifically interesting and important. 1 Information on FEWS-NET is ava ilable at https://www.fews.net/ UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-4 Copyright National Academy of Sciences. All rights reserved.

184 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space TABLE 4.2 Examples of Issues Addressed with Remote Sensing, as Responses to a Survey of the Panels y Societal Issue ualit Earth Q and Air Interior Climate Weather Hydrology Terrestrial Ecosystems Marine and Surface and Y Y Y Y Food Security Human Health Y Y Y Y GHG Management Y Y Y Intl Environmental Agreements and Treaties Y Y Y Y Markets for Ecosystem Services Y Y Environmental conservation, protection Y Y Y Y Y Y Y Y Extreme events and hazard prediction and response Y Y Urbanization and other demographic change Y Internet Applications Y Y Y Improved weather prediction Y Y Barriers to improving applications ESAS (2007), as well as many other studies, identified a number of barriers to improving the applications use of remote sensing data and science. The most studied examples are those in the research- to-operations challenges exemplified in the relationship between NASA and NOAA vis-à-vis measurements that eventually find themselves being used in operational forecast products. The National Academies of Sciences, Engineeri ng, and Medicine itself has examined these issues in many reports over the past decade. There are barri ers due to funding constraints, which force choices between new and sustained capabilities. These arise from the understandable and often justifiable conservatism of operational agencies taking on new r esearch products, from technical evolution being too rapid for operations, and from research not taking sufficiently into account the known applications. In from resources, and from not appreciating the many non-operational realms, much of the difficulty stems s might be applied. In addition, the technical different ways in which Earth system measurement ing remote sensing data can still be overwhelming to many user requirements for accessing and analyz are not often widespread. Lack of standard communities, because they require technical skills that products with simple documentation ca n also be a large impediment, requi ring users to be satellite experts in order to apply the data. If applications are viewed as an add-on requirement, to be satisfied only if all scientific requirements can be addressed, then they d when budget constraints will inevitably be curtaile are inevitably encountered. Opportunities for future investment NASA has chosen to create and fund a separate Applications program, although there are clearly applications of its remote se nsing throughout its program. NOAA has created process teams and collaborations with NASA and its own operational users, to improve communications and intake of UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-5 Copyright National Academy of Sciences. All rights reserved.

185 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space gh its sponsorship of the Landsat Science Team, has research data into operational contexts. USGS, throu seamless manner, although its task is substantially internalized applications and research in a more simpler by being primarily concer ned with only one data stream. All three agency programs would benefit fro m a longer-term, strategic view of how the applications perspective might be improved. Ther these three agencies for e is limited opportunity in research on the science of how to make applications easier and more effective to achieve. Individual projects and “case study” approaches can be successful, but a more structured assessment process is needed to ensure that lessons learned from one project are transferable to future initiatives. In the context of global change science, the National Academies has written a number of reports that are immediately relevant to this problem in the context of use- based science, decision support, co-production of knowledge, and similar issues. NASA, NOAA, and US GS can benefit from substantially improving research-to-operations, applications development, and other aspects of the general process for gaining applied benefits from science. Applications are often viewed as an engineeri ng problem—constructing an approach for using or disseminating knowledge generated thr ough scientific exploration. Increasingly, the applications field is becoming associated with a science of its own (D ozier and Gail, 2009) re lated to generating new knowledge about how to effectively apply scientific re sults, how to rapidly transition science to societal benefits, who potential users are and how to reach them , ways to achieve the broadest possible impacts of science, and much more. NASA, NOAA, and USGS can all benefit by embracing this deeper view of the academic challenges associated w ith effective applications. The final missing piece of applications research in the agencies is the very initial phase of creating applications—supporting studies that have an idea about how an application might work, and then attempting to create a community for it, and demonstrate its utility. To expand the potential neficial to support “proof-of-concept” application applications of Earth observations, it would be be studies. Investigators could propose research to ev aluate potential data applications, whether a to expand the use of remote sensing data. preliminary idea, or a more mature approach NASA, NOAA, and USGS applications-oriented programs have successfully Finding 4A: transitioned remote-sensing-based research into applications of societal, economic, and operational value, but much more is both needed and possible. The transition has recognized that is often justified but may also make barriers: a) conservatism by operational agencies them slow to adopt advances; b) lack of early involvement in the research component of the research-to-applications process by operational ag encies; c) a shortage of specific funds and well-defined responsibilities for ensuring the rapid and effective realization of applications from research, and d) insufficient academ ic focus on the science of applications. Recommendation 4.1: NASA, NOAA, and USGS should reduce barriers to applied uses of remote-sensing research and seek innovative ways to accelerate the transition of scientific research into societal benefits. End-to-End Information Systems Effective use of Earth information increasingly requires viewing that information within the context of an end-to-end system, involving many el ements beyond observations alone. This concept is widely understood but often still poorly implem ented, both for science and for applications. Technological advances, including those available thr ough commercial services, have enabled much of this just within the last decade. In many ways, this systematic connection of observing systems to intermediate data processing steps and ultimately to scientific and practical end-uses constitutes an information infrastructure. Elements of this infrastruct ure exist in isolation, but to a growing extent this infrastructure is integrated at local, national, and even international levels. Such integration presents UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-6 Copyright National Academy of Sciences. All rights reserved.

186 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space res its own investments by the nation, led by this exciting new opportunities as well as challenges. It requi e strategic framework outlined in Chapter 2. report’s sponsoring agencies, consistent with th The topic of end-to-end information is critical to both scientific and applications progress over the next decade. Breakthrough science will be done by vi rtual science teams collaborating through complex, will emerge as advanced data systems enable the multi-observation data sets. Important new applications d communication of decision-support information to fusion of multiple, diverse data sources and the rapi governments, businesses, and individuals. However, comp rehensive treatment of the topic is outside the statement of task of this committee. For that reason, discussion of this topic is limited within the report. Modeling and Prediction Satellite observations are instrumental to devel opment and continued improvement of numerical weather prediction (NWP) and Earth System Mode ls (ESMs). These models incorporate our best ailable satellite and in situ observations, help us understanding of the Earth system, integrate the best av interpret the observations to improve our understanding, and provide the best tools for making valuable em connecting weather to climate time scales is forecasts of the future. A seamless ESM prediction syst fast becoming a reality (Palmer et al., 2008; Hoskins et al., 2013; Bauer et al., 2015). The growing emphasis on prediction at subseasonal-to-seasonal (S 2S) scales increases the importance of resolving couplings within the Earth system. Increasing use of satellite data in NWP has improved weather forecast quality, and satellite observations provide critical data for the verification and improvement of ESMs. The spatial resolution of these models steadily increases, a nd the range of interacting Earth system variables that they describe steadily expands to ser ve scientific and applications needs. Satellite observations now provide more th an 90% of all data for global NWP model sources remain critically important as well. Sustained satellite initialization, though non-satellite observations have provided critical data for climate model evaluation. They include, for example, two decades of global precipitation (from TRMM-GPM) and surface winds (from NSCAT-ERS2-QuikSCAT- ASCAT) and three decades of sea-surface temperature measurements from infrared and microwave (following the launch of TRMM in November 1997) sat ellite observations (e.g., Banzon et al., 2016; ric composition from instruments on Terra, Aqua, and Buckley et al., 2014). Satellite data for atmosphe Aura, in combination with advanced atmospheric mo dels, have provided the basis for quantifying the global burden of disease from air pollution (Bauer et al., 2015). NASA GMAO has made major contributions to assimilate satellite data to improve the global mapping of ozone (Wargan et al., 2015) and polar stratospheric clouds (Stajner et al., 2007), a nd have provided analysis and reanalysis data that include serve atmospheric chemistry models in NASA’ s Modern-Era Retrospective Analysis for Research and Applications (MERRA and MERRA-2). The user community has been served well by NASA’s commitment to being an “end-to-end” agency where satellite observations are ca rried to their ultimate scientific applications using advanced numerical models. Earth system modeling. ESMs have evolved over the last 20-30 years from uncoupled atmospheric NWP and climate models to coupled atmo sphere-ocean-land-ice models with complex model physics (e.g., Puri et al. 2013). Recent advances in high performance computing and computer technology such as the Earth Simulator over the last decade have made it possible to experiment with global cloud- permitting (3-5 km grid resolution) simulations in the Japan Meteorological Agency (JMA) Nonhydrostatic Icosahedral Atmospheric Model (N ICAM) and the NCAR Model for Prediction Across Scales (MPAS), and the NASA Goddard Earth Observing System version 5 (GEOS-5). GEOS-5 has the capability of simulation of global weather at 1.5 km resolution and detailed simulation of atmospheric chemistry with c720 cubed-sphere resolution (~12 km ). Improvements in ocean modeling (e.g., Griffies et al 2016; Rocha et al 2016), in ice-sheet modelling (e.g., Larour et al., 2012), in the representation of the coupled ice-ocean system (e.g., Buehner et al, 2017), and in ocean state estimation (e.g., Forget et al 2015; Penny et al., 2015; Stammer et al 2016) have also played critical roles in advancing Earth system UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-7 Copyright National Academy of Sciences. All rights reserved.

187 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space in cloud-resolving (~1 km) coupled atmosphere- modeling. Research investments by NSF and ONR contributed to the development of global cloud- ocean-land regional models over the last 20 years have permitting coupled NWP and ESMs, which are expect ed to become operational at ECWMF, JMA, the UK Met Office, and other operational centers in the coming decade. The plan for the national Next-Generation Global Prediction System (NGGPS), initiated by A, focuses on improving model prediction that could NOAA, in collaboration with the US Navy and NAS extend weather forecast lead-time from 1-2 weeks to a month. NGGPS will include a global cloud- permitting capability in ESMs. Alt activities among agencies is a good hough coordination of distributed start (Carman et al., 2017), realizing the NGGPS vision requires strong commitment and funding support by NOAA and other agencies. Data assimilation. Data assimilation systems associated with NWP models now weave multiple threads of global satellite and in situ observations into the best available estimate of the detailed state of the Earth System for prediction and analysis. Assimi lation of satellite observations has played a leading role in extending the range of weather forecasts ove r the past two decades. Assimilation of chemical observations from satellites is being used to initialize air quality forecasts. The analysis fields produced through data assimilation, by merging satellite and in situ observations and model information, provide us with continuous global information on the state of the Earth system, which is used in a wide range of Earth system science and applications. Three important developments over the last two decades have led to significant improvements in model initialization and predictions (Bauer et al. 2015). First, the implementations of 4DVar data assimilation at operational centers, started at ECWM F in 1997 and followed by Meteo-France, the UK Met Office, JMA, the Environmental Canada, and the US Naval Research Lab, have set a milestone for NWP. Second, this approach is further improved by direct assimilation of satellite data in their native state by including a forward model to predict the na tive satellite data from the model state. Third, the mble-based estimates of background error covariances recent trend towards using flow-dependent, ense and hybrid ensemble and variationa l data assimilation have been the ma in advances of atmospheric data ), which is also used in current Observing System assimilation in recent years (Bonavita et al., 2014, 2015 Experiments (OSEs) for assessing satellite data impact on NWP. There are many examples of assimilations of new Earth observations in operational analysis systems. Assimilation of SMAP Tb observations in the ensemble-based NASA GEOS-5 land surface data surface and root zone soil moisture product for a assimilation system at GMAO has produced the Level 4 broad range of applications (Reichle et al. 2015) . Janiskova (2015) describes the assimilation of CloudSat MWF operational system and impact on the global and CALIPSO radar and lidar data into the EC analysis. NOAA directly assimilates satellite- and gr ound-based cloud data into its regional models supported the development of ocean models and (Benjamin et al., 2016). NASA, NOAA, and NSF have ocean data assimilation (e.g. Forget et al, 2015; the ECCO consortium, 2017), which will eventually provide the ocean component for S2S NWP and for ESMs. The next-generation ESMs will weave the coupled atmosphere-wave-ocean-land-sea ice components with data assimilation systems. Augm ented satellite observations of the atmospheric variables (e.g., moist physical and dynamic processes, atmospheric composition, wind, and PBL structure) together with observations of the ocean, land, biosphere, and cryosphere will be critical for development of physically based coupling of the components of the Earth System. The resolution and scope of ESMs will also continue to increase, resulti ng in more explicit represent ation of important Earth system processes and more effective coupled assimila tion of a wide range of satellite data. A truly integrated Earth system modeling and analysis syst em will make the seamless weather-climate prediction a reality. Reanalysis. R eanalysis products are widely used in Earth sciences (Kalnay et al., 1996). Numerous reanalysis projects have been undertak en to assimilate observations from a variety of sources—ground-based stations, ships, airplanes, and satellites—and forecasts from NWP models. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-8 Copyright National Academy of Sciences. All rights reserved.

188 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Reanalysis efforts began in the atmospheric co mmunity and have since been extended to oceans (Balmeseda et al., 2013) with steps developed toward consistent reanalyses of the coupled climate system (Bosilovich et al., 2015). Reanalysis products are commonly created using a stable data assimilation system that blends observations and model-based forecasts to produce gr idded fields representing hundreds of variables with synoptically consistent spatial and temporal coverage ex tending over multiple decades. This combination of space-time uniformity and long time-series of variables that include many not available from observations directly is attractive and makes the rean alyses relatively straightforward to handle. However, not observations it is important to note that reanalysis pr oducts are blends of observations and models — quated with “real” observations and measurements Some warn that reanalysis data cannot be e themselves. (e.g., Schmidt 2011; Bosilovich et al. 2013) while others argue the differences from actual observations are smaller than might be expected (Parker, 2016). The value of reanalysis versus observations is a complex issue, and for a given va riable it depends in part on how well relevant physical processes are represented in the models used. Box 4. 2 provides two examples of reanalysis. The observational program proposed connects to these ongoing reanalysis activities in several important ways:  Many of the observations proposed relate to processes whose representation today remains challenging in global model and assimilation systems. Advancing the representation of these processes will further advance the utility of reanalysis. portant independent source of data for assessing  Many of the observations proposed provide an im variables derived from reanalysis. ********************************************************************************* BOX 4.2 The important role of reanaly sis in understanding the Earth system. Figures 4.1 and 4.2 provide two examples of the use of reanalysis, one highlighting the advantages of reanalysis and the other underscoring its challenges. Figure 4.1 shows trends in 2m land surface temperature derived from reanalysis. Land surface temperature trends from direct in situ observations suffer from a number of complicating factors such as ng practices, urban effects, land cover, land use station siting, instrument changes, changing observi ll been hypothesized as introducing artifacts on trends variations, and statistical processing which have a presented by the Intergovernmental Panel on Climate Ch ange and others. Compo et al (2013) ignore all air temperature observations and infer the land surface temperature from observations of barometric pressure, sea surface temperature, and sea-ice concentration using a physically based data assimilation system referred to as the 20th Century Reanalysis (20CR, Compo et al., 2011). As the 20CR does not use temperature observations from land stations, it is entirely independent of those observations. Nevertheless, the time variations of TL 2m in the 20CR are very similar to those previously reported in the station-based data sets both over the 1901 to 2010 period and the more rapidly warming 1952 to 2010 period. Figure 4.2 shows the trends in the atmospheric column integrated water vapor over a 30-year period over oceans expressed as a sensitivity of % change in column water vapor per degree of SST warming. This sensitivity is thought to be a funda mental metric of the water vapor feedback that contributes the majority of the warming to forced changes of climate. Four independent observational records (in red) are shown being close to the theore tical guidance of Clausius-Clapeyron theory ranging between 6-7%/K (horizontal lines). This same trend is derived from six different reanalysis data records that are widely used in Earth science research. The tr ends in reanalysis vary over an order of magnitude from 2.5%/K to 25%/K. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-9 Copyright National Academy of Sciences. All rights reserved.

189 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space (90N-60S) 2 m air temperature anomalies between FIGURE 4.1 Temporal comparison of near-global land 20CR and station-temperature based estimates. Red curve: global anomaly series from in situ data (CRUTEM4, [Jones et al., 2012], black curve: the average of ve additional station-temperature data sets, fi and blue curve: the 20CR. 95% uncertainty ranges are shown for CRUTEM4 (yellow fi ll) and 20CR (blue fi fi ll) and their overlap (green ll). (Compo et al., 2013) Permission Pending ic water vapor from JRA55, ERA interim, MERRA2, FIGURE 4.2 The trends in column integrated ocean ERA20C, MERRA and CSFR reanalyses (Schroeder et al., 2017). ********************************************************************************* models provides significant opportunities Finding 4B: The integration of satellite data with to advance scientific understanding, predicti on skill, and applications. A key factor contributing to the success of global weather pr ediction over the last two decades is data of satellite data in Numerical Weather assimilation systems that optimize the impact enhanced modeling efforts for other aspect Prediction (NWP) models. Assimilation has also of the Earth system and could lead to advances in Earth system models (ESMs). Progress in on of scientific, observing, and computational modeling the Earth system requires a combinati each of these elements. With the expected advances, including a concerted investment in nd growing societal needs to develop continuing improvement in NWP and ESMs, a information on finer scales and for a broader suite of Earth system variables, the coupling UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-10 Copyright National Academy of Sciences. All rights reserved.

190 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space between the component models for the Earth system and the coupled data assimilation of satellite based observations will be a focus for advancing Earth system science and applications. Recommendation 4.2: To ensure continued advances in m odeling in conjunction with Earth observation:  for a strong sustained commitment to Earth NASA should develop a long-term strategic plan system modeling in concert with observations . Success in observation-driven modeling holds the key for maintaining the end-to-end capa bility that has served NASA well in its effectiveness and service to society.  NASA, in collaboration with NOAA, should take a leadership role in developing fully coupled ESMs that assimilate comprehensive satellite, aircraft, ground-based, and in situ observations to advance unders tanding of the Earth system. NOAA should develop a close partnership with NASA and other agencies to lead the Next-  Generation Global Prediction System (NGGPS) e ffort in developing the next-generation cloud-permitting, fully coupled ESMs with advanced data assimilation and NOAA’s sustained global ocean observing system for enabling subseasonal-to-seasonal (S2S) forecasting and seamless weather-climate prediction. Data and Computation in the Cloud Investments in data, data science, and computation are critical to enabling a future that allows faster development of knowledge and applications . New technologies are appearing rapidly, and the agencies and their supported research communities need to keep abreast. For example, cloud computing has the potential to benefit the community by avoi ding data downloading and lo cal management of data. nce the transparency of analytical techniques and Open source tools to analyze data can potentially enha attract users to cloud computing and analytics. Th ese benefits become particularly evidence when working with data at large scales. It is clear that large amounts of data can be analyzed efficiently and made available within a cloud computing environment, but there may also be different cost models for this service, and policy issues surrounding archiving and access to data will be important to get right. NASA and NOAA are both evaluating the use of big data in a manner that facilitates the development of new knowledge and applications, but in very different ways. The Europ ean Space Agency (ESA) is also investigating how it might proceed. NOAA has established Cooperative Research and Development Agreement (CRADA) contracts that enable partnerships with private contractors or universities, to enable greater access to its data. Among early successes was the provision of NE XRAD data on Amazon’s commercial cloud services. When Amazon established those services, usage of th e NEXRAD data increased by 2.3 times at no net cost to the US taxpayer. Data access that previous ly took 3+ years to complete now requires only a few days. Cost recovery strategies in the longer run are un clear, as is whether or not there would be a similar increase in usage if larger amounts of satellite data were to be made available or whether other types of data were available (e.g. ocean). Nevertheless, the success of the NEXRAD services makes clear that these types of opportunities and engagement with the private sector should be further explored. NASA is studying whether to put their data on the cloud, and how best to provide analytical tools and computational resources to facilitate their use. It has established the NASA Earth Exchange as a virtual collaborative to bring scientists together in a knowledge-based soci al network to provide computing tools, computing power and access to big data. NASA also has available a Climate Model Diagnostic Analyzer with web-based tools running on the Amazon cloud. This provides dataset and analysis services, allowing users to download original datasets or higher-level data products. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-11 Copyright National Academy of Sciences. All rights reserved.

191 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space NASA’s 2015 Technical Capability Assessment T eam (TCAT) review recommended developing prototypes to explore costs and benefits of usi ng private sector cloud environments, before moving forward. Longer term decisions will depend on outcome of shorter term studies. Issues under investigation include: a) getting locked in to a single vendor, b) unknown future storage cost, c) potentially uncapped costs for terminating a vendor, d) security restrictions, and e) trust in the network access technologies. By resolving th users should achieve benefits ese issues, academic and government similar to those obtained by many commercial users of cloud resources. Nevertheless, in the near-term NASA is ex amining the feasibility of operating DAACs and EOSDIS core services using cloud Services providers . But many questions remain, such as whether private cloud services offer a cost-effective method of ensuring that publicly owned data are curated and archived properly for future use, and whether repr oducing the DAAC architecture in the cloud is really the best model for the future. The present DAAC struct ure offers one feature that should, somehow, be maintained. Each DAAC is hosted in an institutional setting that has resident discipline experts committed to being good stewards of that data. They have assume d responsibility for maintaining the integrity of its data; this sense of ownership should not be lost, but rather nurtured when considering any move to the cloud. ESA is developing a new mode of operating in response to technological advances (e.g., cloud computing, citizen science). Starting from January 2017, ESA has designated that 25% of research funding will be oriented towards new research practic es, focusing on interdisciplinary work, pairing big data analytics experts with Earth scientists who can interpret the results. ESA has determined that it is necessary now to invest in training existi ng and future scientists to use big data. The majority of U.S. Earth science students a nd researchers do not have the training that they to entry can be overwhelming to Earth scientists need to use cloud computing and big data. The barrier not trained in data sciences and it would be valuable for data centers (NCEI/DAACs) lead efforts to train , and other venues. Moreover, innovation may come the community at major meetings, online meet-ups “citizen science,” but rather anyone with a network from places far outside academia. This is not just connection and computer will be able to acce ss and analyze enormous quantities of data. With GOES-R data having become availabl e in early 2017, a rapid engagement of the se the opportunity for leveraging advances in data (external) scientific community is needed to u ore open-source, version control, workflow science. GOES-R presents an opportunity to expl documentation, data provenance, security, and qualit y control details. This “experiment” can be possible at little cost/risk for NOAA but will help all agencies (both in the U.S. and international) define metrics for success. Recommendation 4.3: NASA, NOAA, and USGS should continue to advance data science as an ongoing priority within their organizations in partnership with the science/applications communities by: a) identifying best practices for data quality and availability; b) developing ective and agile; c) exploring new data data architecture designs that are eff storage/dissemination strategies to facilita te more interdisciplinary collaborations. Complementary Observations Investments in observations from space are considerably enhanced by complementary, and generally far less costly, observations from in situ, airborne, and other vantage points. These observations are used for a variety of purposes: a) complementing space-based measurements within model data assimilation, b) calibration/validation of space-based measurements, c) algorithm development/refinement, and d) providing fine-scale complements to more coarse space-based measurements for process studies, and more. Box 4.3 provides an example from the highly successful Operation IceBridge. Sensors on commercial aircraft already provide im portant contributions to the global observing system, with significant opportunities for further contributions.New technologies and methodologies UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-12 Copyright National Academy of Sciences. All rights reserved.

192 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space can make airborne measurements far cheaper and promise substantial advances in these areas. Drones more readily available than some ground-based obser vations or those from conventional aircraft. Their itizen science and community observing networks, such use for scientific campaigns is growing rapidly. C 2 as the Community Collaborative Rain, Hail and Snow Network (CoCoRaHS), have proven enormously valuable for filling space-time scale gaps—increasing th e space-time density of observations beyond what is available from institutional networks. tions are often forgotten or neglected during Reference systems that enable quality observa observing system development, as they generally play more of a supporting role to those missions built One critically important example is the Terrestrial Reference primarily to observe geophysical variables. , which provides essential information about Earth coordinates that enable a wide variety of Frame observing systems. It is a system-of-systems: a) Very Long Baseline Interferometry (VLBI) and Satellite Laser Ranging (SLR) are needed to provide center of mass, orientation, scale, and Earth rotation; b) a large and international Global Navigation Satellite System (GNSS) network is needed to provide accurate allow satellite and aircraft missions access to the orbits both for users of GNSS ground data, but also to International Terrestrial Reference Frame (ITRF); and c) GNSS is also used to measure Earth rotation and tide gauge datums in ITRF. is the key technique for defining A substantial amount of science reliant on the IT RF is at risk if the ITRF is not properly maintained and advanced. NASA should complete planned improvements to its Global Geodetic Recommendation 4.4: Observing System (GGOS) sites during the first half of the decadal survey period as part of its contribution to the establishment and maintenance of the International Terrestrial Reference Frame (ITRF). ********************************************************************************* BOX 4.3 Airborne observations, such as Operati on IceBridge, fill space-based observation gaps and enhance the value of space-based observations. 3 NASA’s Operation IceBridge (OIB) airborne mission (Figure 4.3) was implemented in 2008 to gap between NASA’s laser-altimeter carrying satellite, acquire surface elevation data across the nine-year ICESat (2003-2009) and the follow-on mission IC ESat-2 (planned for launch in 2018). Only uninterrupted monitoring of land ice provides a multi-d ecadal record of change, and the continuous nature of such observations is critical. Data acquisition pe riods are every spring for each hemisphere, and from multiple airborne platforms (P-3B, DC-8, B-200, HU-25, BT-67, DHC -3, G-V, C-130H), and will operate two more campaigns (one Greenland, one Anta rctica) after the ICESa t-2 launch in 2018. Because an airborne program cannot match the sp atial and temporal coverage of a satellite, the primary focus of OIB was on key regions of the i ce sheets that were already known to be changing rapidly, including the coastal regions of the Green land Ice Sheet, as well as outlet glaciers flowing into the Amundsen Sea, adjacent to the vulnerable West Antarctic and on the Antarctic Peninsula. OIB has series in these key regions. These data are useful for successfully added data to the surface elevation time interpolating between continuous time series, for cr eating DEMs, for cross-calibration between various altimeter missions, and validating th e data from the Cryosat-2 mission. 2 A more complete description of CoCoRaHS is available on their website https://cocorahs.org. 3 http://science.sciencemag.org/content/351/6273/590 , http://onlinelibrary.wiley.com/doi/10.1029/2011GL049216/full , http://onlinelibrary.wiley.com/doi/10.1029/2011GL049026/full , http://onlinelibrary.wiley.com/doi/10.1002/2013GL059010/full , http://www.nature.com/ngeo/jo urnal/v7/n6/abs/ngeo2167.html , http://science.sciencemag.org/content/341/6143/266 UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-13 Copyright National Academy of Sciences. All rights reserved.

193 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Complementing the altimetry, OIB’s airborne plat form also provided an opportunity to measure critical parameters of the ice sheet that cannot be measure d from space: ice thickness, stratigraphy and d magnetics have provided new bathymetry for many near-surface ice and snow properties OIB gravity an tic ice shelves and ice streams, providing data for Greenland fjords and some of the major West Antarc more accurate bathymetric maps near the grounding line, which are essential for estimating discharge of land ice. OIB uses different ice penetrating radar sy stems to investigate the subglacial environment and near surface snow and ice layers on both land ice and sea ice. On land ice, these data have proved to be invaluable to our understanding of ice-sheet mass balance and ice dynamics, resulting in more robust constraints for ice-sheet models that predict ice sh eet contributions to sea level rise, and have led to improved bedrock maps, grounding line positions, etc., providing critical information for improving estimates of Antarctica’s ice discharge. For sea ice, the radar data provide essential information for interpreting the satellite altimetry signals, which require knowledge of the overlying snow cover. FIGURE 4.3 Top panels: Flightlines of NASA’s Operati on IceBridge campaigns over the Arctic (left) and Antarctica (right) from 2009 to present (SOURCE: NASA https://icebridge.gsfc.nasa.gov/?page_id=1010. Bottom left panel: layers in radargram data collected by one flight made by Operation IceBridge across the Greenland Ice Sheet on May 2, 2011 (SOURCE: MacGregor et al., 2015). Bottom right panel: Three- dimensional representation of ATM data on rift in Antarctica’s Pine Island Glacier (SOURCE: NASA / Goddard Space Flight Center / Scientific Visualization Studio). ********************************************************************************* International Partnerships International partnerships have made, and continue to make, a significant contribution to the U.S. Earth science program (e.g., CNES/ESA/EUMETSAT/ EC and the Jason series, EUMETSAT and its polar and geosynchronous satellites, JAXA and TRMM/GPM, DLR and GRACE, ISRO and NISAR). Not only do they reduce U.S. costs, but they engage a larger and more diverse community of scientists (Box 4.3). The priorities of this report would be very different without the critical contributions to Earth measurement from our foreign partne rs. While partnerships pose a ch allenge in differing management styles and governance structures, one partner can support the other in challenging times. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-14 Copyright National Academy of Sciences. All rights reserved.

194 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space The implementation and impact of these partnerships is different for NOAA than for NASA, but joyed the benefits of numerous international they are no less important to each agency. NOAA has en agreements. These have included “accords with Japan for backup satellite coverage from geostationary orbit, with Europe for backup coverage from polar orbit, and with the international Coordination Group , whose members include Japan, Chin a, Russia, India, the European for Meteorological Satellites (CGMS) 4 the World Meteorological Organization.” Meteorological Satellite Organization and NOAA and EUMETSAT, in particular, have long ma intained a strategic collaboration in the field of operational meteorological satellite observations that has delivered full, free, and open data sharing essential to meeting NOAA’s commitment to protecti ng lives and property in the United States. On 2 December 2015, NOAA and EUMETSAT signed the Joint Polar System (JPS) agreement for the period 2020 to 2040. Building on the 2013 Agreement on Long Term Cooperation, the JPS follows the Initial (IJPS), EUMETSAT’s Metop and NOAA’s POES and Joint Polar-orbiting Operational Satellite System Suomi-NPP satellites, with assurance of observati ons from a pair of complementary morning and afternoon orbits to include each nation’s new ge neration of polar-orbiting satellites: the EUMETSAT 5 Polar System - Second Generation (EPS-SG) and the Joint Polar Satellite System (JPSS) . This is only a partial description of NOAA’s inte rnational agreements a nd partnerships, but it illustrates their critical importance. In today’s envi ronment of constrained resources, an issue shared by our partners, effective use of partnerships is more important than ever. Extending and leveraging these partnerships is central to NOAA’s progress. Ex panding them to include the larger life-cycle aspects of future capabilities, starti ng with the science that seeds futu re operational system priorities, is one possibility. Science has no boundaries, but policy constraints limit possible partnerships. China is one potential international partner whose capabilities are no t available to some US Federal agencies by law Toward a National Strategy (see the discussion of legislative guidance in the section of Chapter 2). As the world’s largest nation with a robust space program, China (notably the Chinese Meteorological Administration [CMA]) has the potential to fill gaps in our own program. As a specific example, in 2018 CMA is expected to shift at least one (FY-3E) of its polar orbiting meteorological satellites to an early morning orbit in response to international coordi nation at the WMO, thereby better complementing its ovide improved global coverage. In a time of counterpart satellites of NOAA and EUMETSAT to pr nations such as China, can enable a more robust constrained resources, access to all data, including from nities go unrealized when there are restrictions on U.S. program at lower cost to the U.S. Those opportu Federal agencies regarding engaging China and making use of their assets. An example of an opportunity concerning the continuity of microwave imagery a nd the multiple purposes it serves is provided in Box 4.4. Finding 4C: NASA, NOAA, and USGS have succe ssfully relied on international partnerships to enhance their programs. Partne rships potentially lower the overall cost to the of this Decadal Survey’s priorities than U.S of space-based observations and enable more current restrictions on potential international could otherwise be achieved. In certain cases, thus limit opportunities to reduce U.S. cost partnerships hinder access to observations and and/or enhance science and applications. Recommendation 4.5: Because expanded and extended interna tional partnerships can benefit the nation: 4 http://spacenews.com/42170gary-davis -former-noaa-satellite-executive-dies/ 5 It is important to note that the two systems operate in distinctly different orbit planes, each designed to preserve its relationship with the Sun such that every orbit for a given satellite passes over the Earth at the same local time every day. Due to the nature of these low-Earth orbits, a point on the Earth may experience an overpass of the EPS-SG in mid-morning, followed by the mid-afterno on JPSS overpass. Each of these satellites then flies over the same area some 12 hours later at night. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-15 Copyright National Academy of Sciences. All rights reserved.

195 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space existing partnerships and seek ing new partnerships when  NASA should consider enhancing implementing the observation priorities of this Decadal Survey.  strong international partnerships, by a) NOAA should strengthen and expand its already coordinating with partners to further ensu re complementary capabilities and operational backup while minimizing unneeded redundancy; an d b) extending partnerships to the more complete observing system life-cycle that includes scientific and technological development of future capabilities.  USGS should extend the impact of the Sustainable Land Imaging (SLI) program through further partnerships such as that with the European Sentinel program . ********************************************************************************* BOX 4.4 International Continuity of Microwave Imaging—A Potential Gap Microwave imaging typifies the issues, often comp lex, that must be faced when maintaining climate data records. Microwave imaging also typifies issues associated with th e transition from what has with the international been a U.S. endeavor for much of the past, to one that requires partnerships community in the future. Benefits. DMSP and its SSMI family of sensors—bu ilding on the record initiated by the SMMR ous, reliable source of global microwave imagery for aboard Nimbus-7 in 1978—have provided a continu benefit to the civilian Earth science community, serving almost four decades. This has been of enormous multiple interests:  NWP Assimilation. Microwave imagery is routinel y assimilated in weather forecast systems with demonstrable positive impact on forecast accu racy and skill (Cardinali and Prates, 2013; Kazumori et al., 2016; Geer, 2016).  Climate Data Records. This microwave imagery en ables: (a) climate data records of changing snow cover on land and the diminishing Arctic sea ice cover (Callaghan et al., 2011; Walsh et al., 2017); (b) Global Precipitation Climatology Proj ect climatologies of water vapor, cloud liquid water, and sensible and latent heating fluxes (S anter et al., 2009; O’Dell et al., 2008; Elsaesser et al., 2016; Clayson et al., 2013); and (c) when using AMSR or GPM with their lower frequencies, all-weather sea surface temperature (Wentz et al., 2000). Potential Gap. While the Earth science community has been served with continuous coverage up to the present, it faces a potential gap if and when th e last of the current microwave radiometers reaches the end of its life, assuming that happens prior to the launch of the next gene ration of radiometers in 2021-2022. Figure 4.4 illustrates when this gap might occur. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-16 Copyright National Academy of Sciences. All rights reserved.

196 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space FIGURE 4.4 A chronology of current and future microw ave radiometers; many are listed in the POR. As prior to 2021-2022 when EPS-2G-B is launched. noted, a potential gap in coverage may occur Status of Current Coverage. The three current DMSP radiometers are all well beyond their five- year design life. The spacecraft for the most recent mission, F-19, failed on orbit rendering the radiometer data unavailable; and a decision was made in November 2016 not to launch F-20, but to tear it down. At 14 years, Windsat is well beyond its design life. Megh a-Tropiques, with a 20-degree inclination, offers only low-latitude coverage. GPM, with a 65-degr ee inclination, lacks high-latitude coverage. Prospects for Future Coverage. Table 4.3 highlights the capabilities of the current and future microwave radiometers. For the future, only WSF, EPS-2G-B, and FY-3 are operational, continuing series with follow-on satellites planned for a specifi ed period of coverage. The frequency bands, and hence capabilities, of the WSF—th e follow-on to DMSP—are unknown at this time. The frequency bands of EPS-2G-B are not optimal for detecting sea ice cove r. While EUMETSAT is distributing FY-3 data via its EUMETCast and both ECMWF and the UK Met Offi ce are using them, policy issues have precluded access to Chinese satellite data (discu ssed elsewhere), hence the data qua lity and reliability have not been assessed in this country. COWVR is a technology demonstration with a minimal set of frequencies, hence minimal capabilities. The Japanese had been planning for NOAA to provide a scatterometer as an additional instrument to complement AMSR-2 in the GCOM-W 1 payload, but NOAA was unable to do so. JAXA is now unlikely to fly a GCOM-W2, and the next opportunity to fly an AMSR-2 sensor will be on GOSAT in 2022, but the prospect of AMSR being among its payload is uncertain. Looking to the Future. Improved coordination between the U. S. civil and military environmental satellite programs would be beneficial. Addressing pol icy issues that preclude access to Chinese satellite data, especially from the Chinese Meteorological Administration, would be helpful. Improved UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-17 Copyright National Academy of Sciences. All rights reserved.

197 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space tion Satellites, perhaps forming a Microwave coordination with the Committee on Earth Observa Radiometry Constellation, would be a step in the right direction. Helping the Japanese find a flight opportunity for an AMSR might be beneficial. TABLE 4.3 Projected capabilities of current and future microwave radiometers; the three Useful for columns reflect capabilities that are based on th e frequency bands of the particular microwave Observing Follow-on radiometer on each satellite; the Coverage until column indicates an operational series; the column indicates its planned period of coverage. Microwave Radiometer Capabilities Current and Future Polar-Orbiting Useful for Observing (First) Coverage Launch Follow-on Atmos. Until SST Sea Ice and Snow Satellite/Sensor ✓ ✓ DMSP/SSMI/SSMIS 1987 ? ✓ ✓ ✓ 2003 WindSat ✓ ✓ ✓ ✓ FY-3/MWI 2008 2023 ✓ MeghaTropiques 2011 ✓ ✓ ✓ 2012 GCOM-W1/AMSR2 ✓ ✓ GPM/GMI 2014 ✓ 2018 OR-6/COWVR ✓ ✓ EPS-2G-B/MWI ? 2022 2040 ✓ ? WSF/MWI ? ? ? 2022 NOTES: SST = all-weather sea surface temperature; Sea Ice and Snow = sea ice and snow-on-land cover; Atmos. = rain rate, columnar water vapor, cloud liquid water and wind speed over ocean. ********************************************************************************* Technology Innovation, Infusion, and Obsolescence hnological advances, both those funded within agencies and Understanding and leveraging tec progress in space-based observations (Box 4.4). This those occurring outside government, are central to holds for NASA, NOAA, and USGS. The ESAS 2017 recommended program is designed to provide the flexibility and responsiveness needed to leverage new opportunities and technological advances throughout the decade. Rather than locking in specific mission implementation recommendations based on technologies, implementation methods, and known oppor tunities available at the time of the decadal survey, the committee has provided a set of priority Targeted Observabl es for the decade. This approach allows for the program implementation to evolve and be optimized throughout the course of the decade at the time each mission is started and/or selected. Over the past few decades, the sp ace sector has evolved greatly. With an influx of ideas and flight demonstrations of disaggregation, small spacecraft and constellations, there are now multiple viable approaches to accomplishing many Earth science measur ement objectives. In addition to spacecraft and sensor technology, significant advances in software , data analytics, and advanced computational techniques offer the potential of extracting new knowledge and additional accuracy from space-based UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-18 Copyright National Academy of Sciences. All rights reserved.

198 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space cial and national security interests have resulted in measurements of the Earth. In these domains, commer significant investment in industry and academia. nnovation, and critical technology developments Cutting-edge Earth science relies on continuous i are now spread across small business, established indu stry organizations, new entrants, academia and the NASA Centers. As such, it is important that NASA (as well as NOAA and USGS, where appropriate) not just invest in technology, but identify and build partnerships that import as much innovation as possible into the Earth science enterprise. Mechanisms that offer the potential to improve the performance and efficiency of NASA flight programs include data buys, block buys, standard bus, public-private partnerships, crowdsourcing and citizen science, u se of commercial assets, and partnerships with 6 philanthropists, non-profits, and the defense community. Technology investment in sensors, low size, mass and power electronics, small satellites, small launch vehicles, and secondary payload and rideshare transportation elements remain critical. When possi ble, such technology investments should be made through competitive means, potentially in pa rtnership with NASA’s Space Technology Mission Directorate. Within NOAA flight programs, GOES-16 and JPSS-1 both benefited from block buys of life continuing through the timeframe covered by instruments and spacecraft, with an expected service this decadal survey. However, system replenishm ent in the following decade (2028-2037) will require decisions and investments in this decade in order to maintain and potentially im prove the quality of the data used for both research and operational forecasting. These systems have significant positive impact on U.S. economic competitiveness, national security and quality of life. The NESDIS plan is to “develop a space based observing enterprise that is flexible, r esponsive to evolving technologies and economically sustainable” (Volz, 2016) by moving away from sta ndalone space and ground programs and identifying low-cost and rapidly deployable space systems that m eet future needs. While this committee agrees with tal approach in which commercial system and data the NESDIS strategic goal, we suggest an incremen opportunities demonstrate an “equal or better” pe rformance baseline established by existing GOES and Review Team report (IRT, 2017). This risk of JPSS systems, as suggested in the NESDIS Independent moving to new commercial systems must be balanced against the technology availability risk of these legacy systems, particularly in areas related to critical sensor technologies. The continuity needs of Landsat data products al so suggest USGS implement a balanced strategy that weighs moving towards commercial systems and employing innovative approaches to advance system capability and reduce cost against the technology availability risk of legacy systems. As such, the committee suggestes that both NOAA and USGS make th e needed investments in both existing and new technologies to ensure the sustainment and improve ment of the measurements required for weather forecasting and continuation of critical climate measur ements. In the coming decade, it is expected that each of these critical Earth observing systems will move towards further use of commercial systems and data opportunities, while the importa nce and benefit of federal inv estment in space technology will continue to increase. ********************************************************************************* BOX 4.5 The promise of technology innovation The high cost of observing Earth from space has by necessity resulted in an observing strategy built around measurements of relativ ely few ‘essential’ variables (e.g . Simmons et al., 2016, Bojinski et al. 2014). This works against developing a more integrated observing strategy which is further exacerbated by rising costs of operational observing sy stems in times of flat or declining budgets. Technology innovation promises the potential to drive down costs of sensors, platforms and accessibility to space and in turn to change the way we currently think about Earth Observations. 6 One critical tradeoff is between “blo ck buys” (purchase of multiple instru ments or spacecraft to achieve cost savings) and technology advances. Block buys can reduce cost, but they constrain the ability to leverage newer technologies as they arise within the time duration of a block. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-19 Copyright National Academy of Sciences. All rights reserved.

199 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Much of the current discussion about technology innovation of spaceborne systems seems to revolve around discussion of cubesat capabilities (Nati onal Academies, 2016) and the affordable access to space that such capabilities offer. The cubesat development has forced a de facto standardization that, in concrete terms, provides design principles with specifications on the power/weight/volume into which sensors have to fit. Miniaturization of a number of ‘U-class’ sensors that potentially can address a range of important measurements is developing. For exam ple, Figure 4.5 highlights four U-Class sensors that have been developed under the NASA ESTO program th at are to be demonstrated in space in 2018. There are both opportunities and challenges associ ated with smallsats and cubesats. On the positive side, smallsats and cubesats have encourag ed development of smaller sensors and enabled creative alternative design solutions that can be very capable, even at lower co st. They also offer the possibility to explore the trade space between constella tions of small sensors (to provide higher temporal resolution) versus single, larger, more capable platfo rms. Smaller sensors and associated satellite systems onal vendors and providers within both the government and private have opened the door to non-traditi sector. The ability to produce small satellite systems at universities has positively impacted the vitality of the Earth observing enterprise. However, there are al so risks that must be acknowledged and accepted, as would be the case with any evolving technology in its early stages of realization. Risks in performance, stability, measurement availability, system reliab ility, and mission lifetime must be understood and weighed for each particular application. FIGURE 4.5 Sensor miniaturization and cost reducti ons are happening as a result of investments in key technologies (center). Four examples of U-class sensor miniaturization that are to be demonstrated in space in 2018 via NASA ESTO’s INVEST program are shown to the right of the figure. The sensors to the U-class defined to the right. The NASA INVEST the left are the counterpart sensors that map to program has been an important incubator of technology innovation and provides a pathway to demonstrate miniaturized sensor performance in space. ********************************************************************************* NASA PROGRAMMATIC CONTEXT UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-20 Copyright National Academy of Sciences. All rights reserved.

200 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space mmatic balance to technology innovation. NASA’s contextual issues range from progra l to effective implementation of the ESAS 2017 Successfully addressing each of these topics is essentia science and applications priorities and associated observation plan. NASA Programmatic Balance and Scope The NASA Earth Science Division (ESD) has a broad mandate to develop measurement measurements and science for societal benefit, and technology, to advance scientific discovery, to apply on of scientists. Within its budget, NASA ESD must to educate and inspire citizens and a next generati seek an optimal balance to achieve this broad missi on in the most effective and efficient way possible. NASA ESD must support a world-class scientific resear ch program that will both guide the development of the missions and will fully realize the valu e of the resulting data. While developing improved technology and addressing novel science questions, NASA al so must optimally utilize its existing fleet of satellites that continue to collect important data. In addition to scientific discovery, NASA has a congressionally directed mission to monitor the sible for continuing satellite measurements critical to stratosphere, and is also the de facto agency respon climate science (see the discussi Toward a National Strategy section of Chapter on of agency roles in the 2). It is also important for NASA to foster the transl ation of this information to societal benefit through applications of the data, partnering with operati onal agencies and transferring mature tools to these agencies. Robustness and Resilience A major purpose of striving for balance is to achieve programmatic robustness and resilience. To guide the balance discussion, th e committee identified characteristics of a robust and resilient observational program, including both flight and non-flight issues. : Finding 4D: A robust and resilient ESD program has the following attributes  to provide the community with regular A healthy cadence of small/medium missions ge advances in technologies and capabilities, and to rapidly flight opportunities, to levera respond to emerging science needs.  A small number of large cost-constrained mi ssions, whose implementation does not draw excessive resources from smaller a nd more frequent opportunities.  Strong partnerships with U.S. gove rnment and non-U.S. space agencies.  Complementary programs for airborne, in-situ, and other supporting observations.  Periodic assessment of the return on investment provided by each program element.  A robust mechanism for trading the need for continuity of existing measurement against new measurements. Elements of an Overall Balanced Program A properly balanced program needs to reflect multiple aspects of balance. In general, these aspects cannot be viewed in isolation. Doing so may r esult in optimal balance for that particular aspect of the NASA program, but sub-optimal balance for the program as a whole. The important aspects of NASA’s overall balance are discussed in this section, with specific topics regarding the flight program covered in the following section. Balance between Flight and Non-Flight Elements. Figure 4.6 shows the annual ESD expenditures for flight missions and mission support from 1996 to 2017. This figure shows actuals through 2016 and estimates, based on a simple inflation adjustment, during the decade 2017-2027. Total expenditures in UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-21 Copyright National Academy of Sciences. All rights reserved.

201 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space constant dollars are currently about 75% of expenditu res in the late 1990s. In recent years, the ratio of flight to non-flight expenditures has been about 60 to 40%. The number of beneficial Earth observations that NASA ESD can make has expanded, but the purchasing power of its budget has declined. FIGURE 4.6 The NASA Earth Science budget 1996-2016+ ($FY17), showing both mission and non- mission contributions. For the period following know n budget requests, a simple inflation-adjusted increase is used. Balance between ESD Program Elements. Fi gure 4.7 shows detail on how NASA-ESD expenditures (since 2007) are apportioned among six progr am element categories. The total ranges of these categories are given in Table 4.4. The proportion s have been fairly constant in recent years. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-22 Copyright National Academy of Sciences. All rights reserved.

202 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space ience Research, Systematic Missions, Earth System FIGURE 4.7 Percentage of ESD budget devoted to Sc Multi-Mission Operations, Technology and Applied Science Pathfinder missions (includes Venture), Sciences from 2007 to 2017. TABLE 4.4 Percentage ranges of expenditure categories since 2007. Low % High % Expenditure Category 24% 29% Earth Science Research Earth Systematic Missions 35% 52% Earth System Science Pathfinder 7% 14% Earth Science Multi-Mission Operations 9% 14% Earth Science Technology 5% 3% Applied Sciences 2% 3% Figure 4.7 and Table 4.4 show that since 2007 a large fraction of the budget has been spent on systematic missions. In 2016 about 47% of the budge t is for large missions and 12% for Earth System Science Pathfinder and Venture missi ons. Large directed missions are justified if they are needed to address a particularly difficult but important problem, or to collect the complement of measurements needed to address critical interdisciplinary problem s (NASEM, 2016). However, an appropriate balance for the broader community also requires a cadence of opportunity for PI-led and Venture class missions that is frequent enough to sustain a culture of innovation and creativity among the Earth observations from the space community. Between Mission Investment and Science Inv estment. As stated previously, a balanced Balance NASA program requires a strong scientific research and applications program to plan and utilize remote of the total ESD budget was directed toward Earth sensing measurements of Earth. In 2016 about 18% Science Research and Analysis, 3% to computi ng, including the across-NASA High-End Computing Capability (HECC) Project, and 3% to administratio n. Balance requires sufficient support for Earth Science Research and Analysis to effectively develop and utilize the space-based measurements. A balanced Earth science and applications program supports a robust community applying space-based measurements of Earth to benefit society for a br oad range of purposes including research, forecasting, public safety, and business. Balance of Responsibilities to Partner Agencies. NAS A ESD has a variety of responsibilities to other agencies. Three core responsibilities are:  NOAA Operational Satellite Development. NASA Goddard Space Flight Center (GSFC) has responsibility for developing and procuring sat ellites for NOAA NESDIS, under direct agreement between NOAA and GSFC going back many years. This is not included w ithin the ESD budget, and is not under ESD management authority. A separate NOAA partnership for the on-orbit Suomi-NPP mission, initiated during the NPOESS program, was carried within the NASA ESD development budget, though operations are now the responsibility of NOAA.  Sustainable Land Imaging (SLI). NASA ESD has responsibility for developing and procuring the Landsat satellite series, under its SLI partnership with USGS. Budget for this partnership project is included within the ESD systematic mission budget line.  Satellite Needs Working Group (SNWG). SNWG provides a means for multiple government agencies to provide input on national needs that could guide priorities for new NASA UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-23 Copyright National Academy of Sciences. All rights reserved.

203 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space use of current observations throughout the U.S. observations and ensure more effective applied 7 government . NASA’s obligation to the first two of these is we ll-defined, with clear expectations and budget obligations. The third is quite flexible, with NASA given discretion as to if and how needs from other l, this SNWG process has proven both less burdensome agencies get reflected in ESD priorities. In genera might be anticipated. The balance (between partner to ESD and more beneficial to partner agencies than of these areas appears appropriate, in that those needs and ESD’s own needs) achieved by ESD in each partner needs complement ESD’s missions without being disruptive and do not dominate ESD budgets. Balanced Applications to Society. NASA ESD measurements are critical to the advancing understanding and prediction of the Earth System, which carry tremendous benefit to humanity. The science program at NASA ESD is designed to pe rform this function. In addition, space-based measurements of Earth can be applied more loca lly and in other ways to benefit communities and businesses. The Applications Program at NASA ESD is designed to translate NASA Earth observations and science to the benefit of co mmunities and businesses. In a balanced program, measurements of Earth from space are translated into human benefit. Elements of a Balanced Flight Program Beyond general programmatic balance, the ESD Flight program has additional balance issues that are critical to address (Box 4.6 provides an example of the tradeoffs inherent in achieving balance): Balance Between Large and Small Flight Missions. A mix of large, medium and small flight missions will best advance progress in Earth remote sensing science at NASA. More expensive missions with more capable instruments or multiple instru ment packages may be the best option for addressing certain critical science questions. Smaller, less e xpensive missions can address many science questions ities to innovate and engage the science and engineering communities and provide more frequent opportun through a higher cadence of mission opportunities. Achieving the right balance among large, medium and small flight missions is critical. Large missions cannot be allowed to consume too much of the budget and opportunities for smaller, competed missions. Large thereby stifle the innovation fostered by frequent missions, especially, should be cost constrained (NAS 2012, page 5). The Balance Among Technology Development Phases. Inv estments in innovation are critical to ce and applications rely on long-term (sustained) the success of this new program. Earth system scien observations of many key aspects of the Earth system. Yet, there is at present currently no mechanism to ng the cost of providing for long-term observations. fund early-stage innovation that might lead to loweri 7 “The Satellite Needs Working Group (SNWG) was chartere d as an interagency working group by action of the National Science and Technology Council (NSTC), Committee on Environment, Natural Resources, and Sustainability (CENRS), U.S. Group on Earth Observations (USGEO) Subcommittee. The SNWG supports an annual Satellite Needs process by which Federal departme nts and agencies can communicate their Earth observation r providers of satellite observations. The SNWG federal satellite measurement or product needs to NASA and othe high-priority satellite needs collection was initiated in response to the President’s Budget for Fiscal Year 2016 which reflects the decision to make NASA responsible space segment for all U.S. for the acquisition of the Government-owned civilian Earth-observing satellites ex cept National Oceanic and Atmospheric Administration (NOAA) weather and space weather sate llites. The Administration further re cognized that user agencies will continue to need satellite data from NASA, and that thei to NASA decisions on which r needs should serve as input measurements to transition from experimental to sustai ned observations.” From: “USG EO Satellite Needs Working Group Reporting Federal High-Prior ity Satellite Needs,” available at: https://remotesensing.usgs.gov/rca- eo/documents/Satellite_Needs_Collection_Survey.pdf. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-24 Copyright National Academy of Sciences. All rights reserved.

204 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space state-of-the-art to qualify for consideration in Instead, teams are currently incentivized to improve upon competitive funding solicitations that are targeted to new scientific investigations. Put simply, there is no at ESTO establish a competitive call . The committee therefore proposes th incentive to drive for efficiency ogies to lower the cost and risk associated with to incentivize development of game-changing technol provision of sustained observations needed for Earth system science. The ESTO budget is currently at the low end of its historical range as a percenta ge of ESD’s budget. The committee recommends (Recommendation 4.6) that the ESTO budget be incr eased to 5% of the ESD budget, which remains 8 . within the historical range of ESTO funding (Table 4.4) The Balance of Mission-Enabling Investments versus Flight Missions. NASA must balance its Earth science technology efforts across broadly-b ased investments that reduce cost across multiple programs and focused mission technologies. In particul ar, broadly-based investments that reduce the cost or improve the resiliency of space launch are critical (including small launch vehicles, standard bus architectures and secondary payload and rideshare a pproaches). NASA also has the critical role of continuing to advance Earth system science sensor te chnology. As the Earth science community works to improve the accuracy of its measurement and pred iction capabilities and translate this knowledge into applications that impact U.S. economic competitiven ess, national security and quality of life, NASA must continue to keep the Earth science sensor community means to provide this at the cutting-edge. As a balance, new technology funds are included in the coming decade for both broadly-based investments and focused technology investments through the Incuba tion program element. In addition to focused technology investments for priority instruments a nd missions, this program element also includes an Innovation Fund to enable program-level response to unexpected opportunities that occur on sub-decadal scales. ********************************************************************************* BOX 4.6 Achieving balance in flight programs between performance, cost, and risk. A healthy Earth science flight program requires careful ppropriate balance between consideration of the a the three interrelated parameters of performance, cost, and risk. Increasing performance (i.e., through increasing scope or tightening technical requirements) generally implies an increase in cost and/or risk. Costs can be lowered by accepting more risk or reducing performance (e.g., by relaxing technical ilarly, low tolerance to requirements or reducing the scope of a mission). Sim risk can increase costs as funding is expended to, for example, improve parts selection, complete additional analyses, and hold in- depth reviews. Which of the three parameters are actively managed vers us allowed to vary as a function of the others has program-wide implications. Low tolerance to risk c oupled with tight technical requirements results in higher mission costs, which can limit the scope of the overall program as the number of missions implemented decreases in response to the increase in individual mission costs. Acceptance of high levels of risk may lower a mission’s cost, but it may also result in increased incidences of mission failures requirement for a mission’s success or its ability to and/or shorter mission lifetimes. The program-level tolerate reduced performance or limited mission duration should be used to determine its appropriate risk posture for a given mission. Higher levels of risk are for example, within the expected to be acceptable, Venture Program than in the Designated program element. 8 As noted in Chapter 3, a portion of the Incubation Program’s bu dget is expected to flow to ESTO commensurate with its role in the maturation of instrument and technology concepts. The remaining funding to support the recommended increase in ESTO’s budget is obtaine d through decreases in other program elements consistent with the report’s recommendation to maintai n those other program elements within their historical funding ranges. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-25 Copyright National Academy of Sciences. All rights reserved.

205 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space ESAS 2017’s recommended program includes a variety of program elements which serve to enable active consideration of the balance between cost, performan ce, and risk while providing flexibility throughout the decade to evolve as new opportunities emerge, te chnologies are developed, scientific discoveries are made, and the international contributions to the Program of Record evolve. ********************************************************************************* The Balance Between Heritage Technology and New Tec hnology. The development of advanced technology can provide novel measurement capabilities and the ability to make needed measurements at reduced cost. Less expensive measurement technolog ies are critically important if NASA ESD is to ithin its expected budget. In a balanced program, new innovate to obtain the most critical measurements w technology is continuously intr oduced into space-base d measurements, and innovation to improve of importance is implemented. New technologies do existing measurements and measure new variables require investment, however, and their successful a doption requires demonstrated capabilities to achieve measurement objectives. In the meantime, heritage technologies are essential for the achievement of observation objectives until such time as reliable transition to new capabilities can be accomplished. Balance . Valuable data can be collected from Between Extended Operations and New Missions missions that remain functional beyond their designed lifetime, but this data collection requires resources that might be used for other purposes. Extending the operational phase of successful space missions beyond their design lifetime generally provides valuable data at a low cost, relative to new instruments and launches. The recent NAS report (Extending Science—NASA’s Space Science Mission Extensions the present method of “senior review” to evaluate and the Senior Review Process (2016)) states that mission extensions is working well. Balance ting and Novel Measurements. Some satellite data records Between Continuity of Exis uity) of the record in time carries significant have been established for which continuation (contin be right if continuous measurements of key scientific and practical benefits. The balance will not variables needed to monitor and understand the Ea rth system are broken. The need for continuity ontinuity of NASA Earth observations from space: A measurements has been discussed in the report “C Value Framework” (NAS, 2015) and is illustrated in Box 4.5. ion Targeted Observable could be justified As an example, the recommended Surface Deformat on the basis of continuity, extending the record to be initiated by NISAR. However, this Targeted Observable’s new emphasis on temporal versus spatia l resolution implies some novelty to address needs for both continuity and new measurements, so it is hard to make a clear distinction. The committee emphasizes that many continuity measurements are provided by the national and international Program of Record (POR), especially the Europ ean Copernicus, perhaps making the proposed set of measurements appear skewed toward new measurements. However, if those POR measurement continuity capabilities mmended by the committee would have involved a did not exist, the proposed measurements reco different mix. Execution of the national and international Program of Record and the recommendations of this Decadal Survey, taken together, will provide for the continuation of many key satellite records through most of the next decade. The planning and prepara tion to continue such measurements beyond the next decade is urgently needed. International collaboration is required to ensure continuity, given individual agency resource constraints. The recent agreem ent between NASA, NOAA, ESA, EUMETSAT and the European Union (via its Copernicus program) to continue high precision ocean altimetry measurements via the Sentinel 6 program provides an example of international collaboration. As noted in Recommendation 2.2, NASA should continue to work with international partners to develop an international strategy for maintaining key satellite measurements and establish data-sharing agreements among the nations making the measurements. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-26 Copyright National Academy of Sciences. All rights reserved.

206 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Finding 4E: balanced Maximizing the success of NASA’s Earth science program requires investments across its program elements, each critically important to the overall program. flight program provides observations that the The program draws on to research and analysis perform scientific exploration, the applied sciences program transforms the science into real- world benefits, and the program accelerates the inclusion of technology advances technology in flight programs. The current balance ac ross these four program elements is largely Earth science program, and can be effectively appropriate, enabling a robust and resilient maintained using decision rules such as recomme nded in this Report. Some adjustment of warranted, as recommended in this report. balance within each program element is Recommendation 4.6: NASA ESD should employ the following guidelines for maintaining programmatic balance:  . Needed adjustments to balance shoul d be made using the decision rules Decision Rules included in this report.  Flight programs should be approximately 50-60% of the budget. Flight vs. Non-Flight.  Within Non-Flight : o R&A Program . Maintain at its current level of the ESD budget. o Technology Program. Increase from its current level of 3% to 5% of the ESD budget. o Applications Program. Maintain at its current level of the ESD budget. :  Within Flight o Ensure no flight program element is compromised by overruns in Program Elements. any other element. o New vs. Extended Missions. Continue to use the present method of “senior review”, consistent with NAS guidance (NAS, 2016). o New Measurements vs. Data Continuity. Lead development of a more formal continuity decision process (as in NRC, 2015) to determine which satellite r continuation, then work with US and measurements have the highest priority fo international partners to de velop an international strategy for obtaining and sharing those measurements. o Mission-Enabling Investments vs. Focused Missions. Other than additional investments in the Technology Program and the new Incubation program element, no change in balance is recommended. ********************************************************************************* BOX 4.7 The need for continuous measurements Satellite remote sensing of Earth grew rapidly in the 1970’s and many measurements became indispensable for Earth science and applications a nd are continuing. These measurements are used both for immediate applications and to establish long-te rm records that are essential for understanding Earth System behavior on longer time scales. The NAS Continuity Report (NRC, 2015) identifies key evaluation factors and puts forward a decision-ma king framework that quantifies the need for measurement continuity and the consequences of measurement gaps for achieving long-term science goals. It is important for Earth Science and App lications that these measurements be improved and continued, including observations that meet climate quality standards. A partial list of key variables for which continui ty of measurement is im portant is provided in Table 4.5. Table 4.5 is not a complete list of all rele vant measurements and it is not a priority listing. The committee has assessed the likely continuity of these and other measurements through the end of the coming decade, based upon the POR (Appendix A) an d the Observing System Priorities Table (Table 3.3), to identify potential gaps as it developed its recommended program. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-27 Copyright National Academy of Sciences. All rights reserved.

207 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Mass Change and ) are included in Two Targeted Observables ( Surface Deformation and Change continuity; several of the Targeted Observables the Designated program element specifically to ensure listed in Table 3.5 ( Greenhouse Gases Ozone and Trace Gases , and others) are recommended for , competition in the Earth System Explorer program elem ent in part to provide continuity; and others may competition strand described in Chapter 3. The be addressed via the recommended Venture-Continuity cal variables by the international community, which provision of continuous measurements of such criti w or missing observations, underscores the importance allowed the committee to focus its attention on ne ee’s recommendations were established. If the POR of the POR as the foundation upon which the committ and the program priorities recommended here are executed as planned by NASA, NOAA, USGS, and our y that many (but not all) of these international partners, then it is likel critical records will continue to be available for science and applications in the U.S. TABLE 4.5 Examples of observations associated with potential continuity needs, not in any priority order. These, and others not included in this sample, should undergo formal review, as through NAS Continuity Report (NRC, 2015), to plan for continuity needs. Purpose Start of Record Description Observation Globa Monitor land surface conditions 1972 - Landsat I Land Surface l visible and IR imaging of land at high spatial resolution. Conditions Ocean Color Measure near-surface ocean color for 1978 - CZCS Multi-wavelength visible imager fisheries and ocean biology and chemistry Sea ice concentration Sea ice concentration is important for 1978 - Nimbus 7 Multi-wavelength microwave imager. SMMR navigation, fisheries and climate monitoring. Precipitation Microwave imager provides estimate Multi-wavelength microwave imager. 1978 SMMR 1997 TRMM radar of precipitation over ocean. Precipitation radar. Temperature and Thermal IR and microwave sounders for all- 1978 - Microwave Temperature and humidity profiles are humidity profiles. and thermal IR needed for weather forecasting and weather data. sounding begins measuring change. Ocean Vector Winds Ocean vector winds useful for weather Radar scatterometry or polarimetric microwave 1978 - Seasat, but record not forecasting and seasonal prediction. radiometry . continuous Cloud cover and 1978 - AVHRR; Cloudiness is a key Multi-wavelength visible and IR imaging weather variable, and clouds are also a key climate optical properties Geostationary variable imaging 1979 - Nimbus 7 Total irradiance from the Sun Solar Irradiance Total solar irradiance is the energy source for Earth. The ultraviolet ERB 2003 - SORCE radiance influences stratospheric Total and spectral irradiance from the Sun ozone. The Earth’s radiation budget measures Absolutely Calibrated Broadband solar and 1979 - Nimbus 7 Earth Radiation Budget ERB changes associated with clouds, terrestrial radiance at the top of the atmosphere. temperature trends or volcanic eruptions. 1979 - SAGE I, Reactive chemicals affect UV Ozone and Trace Solar backscatter, solar occultation, thermal IR Gases radiation, air quality, and climate N imbus 7, TOMS, and microwave sounding SBUV 1995 - GOME Solar backscatter Aerosol Optical Depth Aerosols affect air quality, weather, 1999—MODIS, and climate MISR UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-28 Copyright National Academy of Sciences. All rights reserved.

208 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space 1981 - AVHRR on Sea Surface SST is needed for weather forecasting Thermal infrared and microwave imaging Temperature and monitoring ocean change. N OAA-7 Multi-wavelength reflected solar imager 1981 - AVHRR Estimation of the photosynthetic Vegetation Greenness N DVI activity indicates health and Index productivity of land vegetation Sea Surface height Global measurements of sea surface 1992 - TOPEX and Radar altimetry. quantifying sea level height are useful JASON rise, diagnosis and forecasting of El iño, and determining ocean heat N storage. anomalies of gravity field Spatial and temporal 2002 - GRACE Gravity measurements can be used to Mass Change monitor ocean mass, land surface total water storage and land ice mass changes. Laser Altimetry 2003 - ICESat Changes in land ice volume are an Ice Elevation important potential source of large sea level changes. Cloud/Aerosol vertical The vertical structures and properties 2006 - CloudSat, Radar and lidar profiling of cloud and aerosol structure profiles of cloud and aerosol layers are 2006 - Calipso important for weather and climate. Reflected solar spectrometer 2009 - GOSAT Important in understanding the factors Greenhouse gases , methane) controlling carbon fluxes and 2014 - OCO-2 (CO 2 2002 - SCIAMACHY atmospheric concentrations ********************************************************************************* Scope Within Non-Flight Program As noted throughout this section, NASA’s non -flight programs are essential to its overall all in approximately correct balance at the mission. These programs are performing well and are current time. Two small scope adjustments are recommended: NASA should make the following scope changes to its program elements: Recommendation 4.7:  Technology Program. Establish a mechanism for maturation of key technologies that reduce the cost of continuity measurements.  Applications Program. Redirect a small portion to new funding opportunities that focus specifically on taking early-stage ideas and expl oring how to move them into applications, including co-sponsorship with NOAA and USGS. Balance and Scope Within the Venture Program The Earth Venture Program was established to “create space-based observing opportunities aimed at fostering new science leaders and revolutiona ry ideas.” (NAS, 2007). To achieve this, NASA implemented three strands of Earth Venture elements . The first is the Earth Venture Mission opportunity (EV-M), which solicits stand-alone space missions with a cap of $150M. The second is the Earth Venture Instrument opportunity (EV-I), which solicits instrumentation for which NASA assumes the responsibility of identifying a launch opportunity. EV-I is solicited at approximately 18 month intervals. Finally, the Earth Venture Suborbital opportunity (EV-S), solicits suborbital studies with an approximately four-year cadence, selecting approximately five investigations per cycle, cost-capped at $30M each, lasting five years each. Through the implementation of this program, NASA has provided UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-29 Copyright National Academy of Sciences. All rights reserved.

209 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space six for instrumentation and two opportunities for two opportunities in the past decade for missions, The result has been that the program has succeeded suborbital proposals (for a total of eleven selected)). in fostering innovation and stimulating a vibrant Earth science community through the provision of multiple opportunities for large-scale observation capabilities. Even though the Earth Venture program was initia ted nearly a decade ago, only one EV-M has been flown yet, and one cycle of the EV-S has been launched (CYGNSS), none of the EV-I missions has of these programs are still not fully understood. The been completed. As a result, the relative benefits rth Venture program in its present form, but after committee fully supports the continuation of the Ea several of the EV-I missions and the contributions from CYGNSS are better understood, a cost/benefit analysis of the EV investments would help inform the amount and distribution of future investments in the program. Finding 4F: The Earth Venture program has provided increased opportunities for innovation in scientific Earth observations. However, it is t oo early in the program, with too little history, to assess the benefits of modifying the present 3-strand Venture structure or adjusting cost caps beyond the recommended addition of a Venture-Continuity strand. Recommendation 4.8: The Midterm Assessment, with a longer program history than is available to ESAS2017, should examine the value of each Venture strand and determine if the cadence or number of selections of any stra nd should be modified. In par ticular, the Venture-Suborbital strand should be compared to the approach of executing comparable campaigns through the research and analysis Program to assess whic h approach serves the community better. Budget Guidance and Decision Rules for Maintaining Balance The committee’s suggested decision rules have two components. First are guidelines for how to allocate funding that becomes available as current flight missions are completed (referred to as the “funding wedge”). Second are guidelines for ensuring vari ous aspects of balance in the program’s overall is that future budgets correspond to the FY2016 budget. The assumption, used throughout this report, budget adjusted for inflation. Computation of the fund ing wedge is described in detail in Chapter 3. The conclusion was that the cost to complete the Program of Record (the NASA-baseline missions already “in implementation”) was estimated to be $3.6B and the ‘funding wedge’ for new missions advocated in this report was estimated to be $3.4B. Overall Program Balance . To maintain program balance, the committee recommends (Recommendation 4.6) that ESD budget components should be approximately consistent with historical budgets. For the entire ESD budget, the following guidelines are recommended.  Earth Science research should be maintained at approximately 24% of the budget (within the range 22-26%). (This value of 24% includes 18% for openly competed research and analysis, and approximately 3% each for computing and administration).  The Applications program should be maintained at 2-3% of the budget. The Technology program should be increased from its current 3% to about 5%.   Flight programs , including Venture, should be 50-60% of the budget.  Mission Operations should be 8-12% of the budget. Allocation of the Funding Wedge Within Flight . As general guidance for allocating the funding wedge within flight programs, an appropriate dist ribution of the ESD investment is 35-45% in large UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-30 Copyright National Academy of Sciences. All rights reserved.

210 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space 10-15% in technology-related aspects of flight missions, 40-50% in medium and small missions, and development. No single mission should cons ume more than 25% of the funding wedge. Decision rules are most effective wh en budgets are managed carefully across Managing Budgets. uncertainties, traditionally results in significant ESD. Mission development, with its large costs and budget management challenges. Recommendation 3.3 provides specific guidance concerning the cost- aware management of missions in development. Decision Rules for Budget Changes . The committee expects that budgets will be different from e committee’s statement of task. A critical purpose the nominal assumptions made in accordance with th of decision rules is to maintain the scientific and technical capacity for a robust space-based Earth Science program when budgets change. Maintaining capacity is important, since that capacity takes a long time to build (in some cases, longer than the mission deve lopment time scale) and is easily disrupted. The committee places the highest priority on co ntinuity of critical missions, followed by competitive opportunities in the Earth System Explorer and Earth Venture lines, followed by the large is a balanced portfolio, it is important that no missions. However, because the highest overarching priority one aspect of the portfolio be reduced excessively, to keep others intact. budget reductions that impact the scope and/or cadence of the As a result, in managing potential new measurements of this decadal survey:  Reductions should first be accommodated by delaying the large missions.  If additional reductions are required, the medium -sized Designated missions should be delayed, unless these delays threaten the continuity of da ta sets that require continuous measurement.  Should continuity be threatened, the cadence of medium-sized competitive missions should be in the decade. The budgets for Venture and reduced but not to fewer than two competitions research and applications should not be reduced by more than 5% from their historical averages. These decision rules are intended to apply to the new missions recommended in this decadal survey. Because of the fraction of the budget consumed by the program of record in the first half of the decade, there is very little flexib ility to absorb budget reductions with the missions recommended in this survey until the second half of the decade. Should cuts to the program of record be required to address then the science priorities identified in the Science budgetary challenges in the first half of the decade, conjunction with the above decision rules - which and Applications Priorities table (Table 3.3), in by reducing large missions, then medium missions, then prioritize continuity and seek to absorb cuts first competitive missions - should be used as gui dance to inform such reductions. exceed the capacity of the above decision rules Large changes to the Decadal program (those that to address, in particular decisions which must bala nce continuity and the cadence of competition) should only be made subsequent to add itional review by the National Ac ademies Committee on Earth Science and Applications from Space (CESAS). In particular , NASA ESD should consult its standing scientific e Program of Record grows to consume more than advisory committees if: 1) the projected cost of th $3.6B in the coming decade, 2) more than one mission in this Decadal Survey is delayed more than 3 years, or 3) a mission in the program of record or required to meet the new measurements of this Decadal survey is lost prematurely. In such cases the above decision rules provide guidance to CESAS, but need not be strictly adhered to, as more flexibilit y may be needed to manage unforeseen events. In managing situations where additional budget becomes available, they should be used to increase the cadence of the new measurements and ass ociated missions recommended in this survey. In particular, expanding the breadth of observationa l objectives should be prioritized, potentially by increasing the number of Earth System Explorer competition opportunities. The ESAS 2017 SATM should be consulted for guidance on the scientific priorities when augmentations are possible. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-31 Copyright National Academy of Sciences. All rights reserved.

211 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space NOAA PROGRAMMATIC CONTEXT ng system priorities, in accordance with the This section provides guidance for NOAA’s observi committee’s statement of task whic h specified primary tasks to include: “1) how new technology may enhance current operations, and 2) what new science is needed to expand current operations, either to enable new opportunities or to in clude new areas of interest.” NOAA Role in Civil Observing System NOAA’s role with regard to space-based observations is specified in the 2014 National Plan for Civil Earth Observations (National Plan, 2014). NOAA’s primary responsibilities fall within “Sustained Satellite Observations for Public Services,” in c ontrast to NASA’s responsibilities which fall largely within the categories “Sustained Satellite Observati ons for Earth System Research” and “Experimental Satellite Observations.” Within the category, specific NOAA responsibilities are called out. rough additional policy directives, such as the These distinctions have been further clarified th OMB guidance accompanying the 2016 Federal B udget, as well as the Appropriations Committee ral Budgets, which direct NOAA to prioritize satellite programs Reports for the 2016 and 2017 Fede directly related to weather forecasting (as described in the Toward a National Strategy section of Chapter 2). This focus reflects a new budget reality for NOAA. Issues regarding the role of observations in support of NOAA’s non-NWS mission are discussed separately in this section. NOAA’s observing system role is thus dis tinct and different from NASA or USGS. Key distinctions, for the purpose of this discussion, include:  Operational responsibility for high-reliability observations. As noted in the statement of task, observations and delivery of services and “a critical requirement for continuity of NOAA has information to the public and commercial sector s.” This strong requirement has corresponding ogrammatic implementation methodologies, and new implications for observing system design, pr capabilities development.  Multi-decade development cycle . NOAA’s stringent availability requirements have led to observing system architectures with very long life-cycles. The current development paradigm lopment approaches or partnerships. It has involves multi-decadal cycles and impacts any deve for introducing new capabilities. also constrained NOAA’s agility  International operational obligations, and mandated international data sharing. NOAA has formal obligations for data sharing through agreements such as WMO-40 and the Joint Polar System (JPS) with EUMETSAT. This has implica tions for its use of commercial and alternate data sources, since their acquisition may imply obligations for sharing beyond NOAA. S 2017 is not intended as a primary planning activity for For these reasons and others, ESA NOAA space-based observations. Instead, ESAS 2017 was requested to provide guidance largely NOAA’s system beyond its baseline plan. associated with opportunities for improving Needs and Challenges The report of the NOAA NESDIS Independent Revi ew Team (IRT, 2017) provides an important perspective on the needs and challenges of NOAA’s space-based observations. The report’s objective is “independent assessment of NESDIS path forward and the capability of the enterp rise to embark on that path”. Key conclusions relevant to ESAS 2017 include:  The IRT concluded that NESDIS has a positive pa th forward and is “capable of embarking on that path.” UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-32 Copyright National Academy of Sciences. All rights reserved.

212 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space and property, with high-reliability space-based  NESDIS has a critical mission to protect lives observations an essential element.  NASA plays an important role in weather science, with relevance to NOAA. NASA (in particular, the NASA Goddard Space Flight Center) also plays a complex and evolving role in NOAA system development, recently including both JPSS and GOES-R which face ongoing challenges such as potential coverage gaps. Be tter definition and strengthening of the NOAA- NASA relationship is needed. The NESDIS stra tegic plan (NOAA, 2015) provides a framework for improving the partnership.  NESDIS has developed a strong strategic plan, wh ich recognizes the value of use-inspired science for advancing the NESDIS mission. However, the strategic plan suffers from being an, “internally-focused document, which limits its utility.” Science and Applications . NOAA’s primary planning activity at this time is an internal study called the NOAA Satellite Observing System Architect ure (NSOSA) performed within the Office of Systems Architecture and Advanced Planning (O SAAP), supported by a NOAA-chartered community study performed by the Space Platform Requirements Working Group (SPRWG). Both studies were ongoing at the time of ESAS 2017 and briefed to th e committee on several occasions. Several ESAS 2017 members were also SPRWG members. Although NOAA’s mission includes space weather, it was not a part of the ESAS 2017 study. NSOSA, in consultation with SPRWG, deve loped a formalized quantitative evaluation methodology to assess the cost and benefit of individual observations within the overall NOAA architecture (including the benefits/costs of rela tive improvements among observations), driven primarily by operational (rather than scientific) needs. Th e process is intended to inform NOAA management, which will make final decisions on observing system requirements. The current NOAA satellite system is expected to be replenished through the late 2020’s or early 2030’s without substantial changes. This program of record system is referred to as POR2025. The charter of NSOSA and SPRWG is to plan for changes to that system that could be implemented during the 2030’s and persist into the 2050’s. To accomplis h that, NSOSA/SPRWG prioritized possible changes to the system and identified those achieving the best cost-benefit performance as candidates for inclusion in the post-2030 system. In doing this, NSOSA/SPRWG also identified a set of “unsatisfied priorities” that reflect high priority NOAA requirements involving observations not selected for inclusion due to cost or technology readiness issues. Assessing these unmet n eeds is somewhat subjective, as it depends highly on unknown budget availability 10-30 years in the future. Recognizing this reservation, NOAA provided the committee with a preliminary summary of expected unsatisfied priorities, as identified by the NSOSA/SPRWG advisory process (which was ongoing at the time this report is being written). Th ese are listed in Table 4.6, along with corresponding priorities identified by ESAS 2017. From this table, it is clear that there are well-defined unmet needs which correspond closely to ESAS 2017 priorities. Th ere are notable exceptions, as well, such as the low- medium priority for regional IR and microwave sound ing; the Weather and Air Quality Panel felt that diurnal sounding capability is a high priority, and GEO-based sounding is one way to accomplish that. The table thus presents opportunities for NASA deve lopment activities that match NOAA’s “unsatisfied” priorities. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-33 Copyright National Academy of Sciences. All rights reserved.

213 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space TABLE 4.6 Opportunities for Improving the NOAA Operational Observing System Related ESAS 2017 Expected NOAA Programs or Targeted ed NOAA Priority and Rationale Observables Expect “Unsatisfied Priorities” Instrument Cost Reduction HIGH—Reduc ing cost of any system element Incubation program  enables greater system capability. NOAA has element limited capacity to invest in development  NASA ESTO activities that eventually reduce production cost. 3D Winds in Troposphere HIGH—High cost and low technology  Atmospheric Winds and Lower Stratosphere readiness impede inclusion in NOAA operational system. HIGH—High cost and low technology Global Precipitation Rate  Clouds, Convection, and readiness impede inclusion in NOAA Precipitation operational system. MEDIUM—Multiple new and often difficult Seasonal Forecasting  Many ESAS 2017 observations needed, notably upper ocean and Targeted Observables ocean-atmosphere coupling, along with assurance of continuity and ongoing cost reduction for existing observations. Ocean Surface Vector MEDIUM—Coverage is likely to be less than Ocean Surface Winds and  desired, with high-volume coverage presently Winds Currents costly. Global Atmospheric MEDIUM—Expect future systems to have Planetary Boundary  more soundings of at least moderate Soundings Layer precision/accuracy levels as compared to today, but high precision/accuracy IR and microwave soundings may be lacking. LOW to MEDIUM—Useful for forecaster GEO-based Regional IR Planetary Boundary  and Microwave Sounding nowcasting, but generally considered less Layer valuable than global sounding. NOTE: Based on a preliminary assessment of unsatisfied observing system priorities identified within the NSOSA/SPRWG process (intended to inform NOAA management which will determine final observing system requirements). Related ESAS 2017 priorities are included for comparison. The NOAA observations system plan for 2035-2050 currently is anticipated to Finding 4G: have unsatisfied priorities for global 3D fiel ds of winds, global precipitation, and other observables, along with a general need for observing system cost reduction. With some exceptions, these unsatisfied NOAA priorities generally align well with the ESAS 2017 recommendations to NASA. Programmatics . Providing guidance for advancing NOAA’s observing system requires starting with an understanding of how this has been accomplished historically. Appendix D of Earth Science and Applications from Space: A Midterm Assessment of NASA’s Implementation of the Decadal Survey (NRC, 2012) has an abbreviated history. Its general features include an initial strong interaction with NASA in terms of instrument and satellite development th at has become less strongly linked over time. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-34 Copyright National Academy of Sciences. All rights reserved.

214 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Today, NOAA faces challenges to an effective process for advancing their observing systems. These include: 1. Balancing Reliability Against Advancement. How should the need to advance be weighed against NOAA’s requirements for “continuity of observations and delivery of services”, given that advance is necessary to provide expected new observations and services in the future? 2. Determining Acceptable Risk. How much risk is acceptable to accommodate needed advances across observing system generation (block) ch anges? How much risk is acceptable to accommodate advance within observing system ge nerations? How can onramps be integrated to accomplish this? 3. Selecting Prioritization Methodologies. How should development advances be selected and prioritized? Some options include community-b ased input (e.g., SPRWG and NAS studies), and NASA/NOAA-provided Observing System Simulation Experiments (OSSEs) and Observing System Experiments (OSEs). 4. Accelerating Adoption of New Capabilities. How can NOAA make more rapid use of observing 9 system advances, avoiding internal bottleneck s in adoption, assimilation, and algorithms? 5. Leveraging External Sources. How can external advances in observing systems and data sources be integrated more rapidly? Deciding Between Make or Buy. Should observing system advances be accomplished within 6. NOAA, implemented by commercial partners through the procurement process, or pursued through partnerships? NOAA has internal policies that comprehensively address these issues, but they are in many cases insufficient to address the growing challenges NOAA faces. The result is an ineffective strategy for how s. Most fundamental is to rapidly advance its observing systems so as to meet the nation’s evolving need the first of these, by which NOAA’s appropriate co mmitment to reliability ev entually becomes an could be directly addressed by a NOAA policy that impediment to needed advances. All of these issues formally defines a coherent strategy and prioritiza tion for advancing the observing system, in addition to the critical mission assurance requirements. Observations for NOAA’s non-NWS Capabilities NOAA NESDIS has a broad range of real and potential users that extends well beyond its traditional (and most important) customer, the NWS a nd its provision of atmospheric weather forecasts, warnings and services. These users include th e remaining NOAA line offices, other agencies, international partners, commercial users (Box 4.8), academia and private citizens. As examples, their needs include detecting and forecasting harmful al gal blooms, understanding fish stock variability, forecasting high-seas wind and wave conditions, estima ting rainfall for the Pacific Islands, planning responses to coastal inundation, detecting coral bleach ing, and sea ice forecasting, along with performing reanalyses of various physical phenomena. At the same time, many (both real and pote ntial) users across the agency have expressed frustration with using data from satellites. Many see la rge weather satellite costs coming at the expense of is that a user has to be a satellite expert to their modest-by-comparison budgets. A common complaint understand and use the data. In gene ral, users need help accessing sate llite data and turning them into useful information, combining them with in-situ observations, and applying a combined product to meet a 9 NAO 216-105B: POLICY ON RESEARCH AND NOAA has recently introduced a policy referred to as DEVELOPMENT TRANSITIONS to better address this issue. It more clearly defines NOAA’s research-to- operations transition, including appli cations and commercialization. This text of this policy is available at http://www.corporateservices.noaa.gov/ames/admin istrative_orders/cha pter_216/216-105B.html. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-35 Copyright National Academy of Sciences. All rights reserved.

215 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space nt research question or feeding into an operational particular need—whether answering a societally releva environmental forecast. institutional framework within NOAA that systematically acquires, The origin of this issue is an processes and distributes satellite data from its ow n and foreign-partner satellites in support of NWS operational weather forecasting needs, which provides a degree of vertical integration. A corresponding institutional framework is lacking for the most pa rt in support of other (non-NWS) NOAA needs; data must be acquired from various satellites (some NOAA, but mostly NASA and international), appropriate products derived, and then distributed to a diverse user community spread across the agency. Meeting diverse NOAA needs—beyond the NWS— for satellite data involves acquiring satellite data from various NASA and foreign sources, generating suitable products, distributing them to users, and then working with those users to meet their needs and demonstrate benefits. Some user needs require timely access to near-real-time products, others require higher-level retrospective products at later times. There are costs associated with this process. And if funds are not available to demonstrate the utility of increase to provide a corresponding new operational the satellite data, it is difficult to justify a budget service to society. Since much of the satellite da ta come from outside NOAA, this results in lost opportunities, being unable to take advantage of othe r agencies’ and nations’ substantial investments in satellites in order to exploit their resulting data. Forecasting harmful algal blooms (HABs) is an exam ple of one of those needs. HAB forecasts in the western end of Lake Erie—which serves as th e water supply for Toledo and a dozen surrounding communities in Ohio and Mi chigan—determine those times when the water must either have a significant level of additional treatment or, in extreme situations, not used at all for drinking. Similar forecasts in the eastern (around Tampa, FL) and western (north of Brownsville, TX) Gulf of Mexico determine those times when shellfish beds must be closed. Both of these HAB events typically are annual events and may last for one to several months in duration. cations and Research (STAR) is responsible for Within NESDIS, the Center for Satellite Appli accessing various satellite sources a nd generating and distributing n ear-real-time products, while the the former national data centers) are responsible National Centers for Environmental Information (NCEI, pective products. These two organizati ons can be part of the solution for generating and distributing retros by recognizing, engaging and partnering with the br oad user base across NOAA. This could take the form of an internal Users Working Group, such as is em ployed at the NASA DAACs. Such a group would help satellite data products pot entially available—both interested users understand the variety of sources of those users to provide feedback to STAR and NCEI near-real-time and retrospective, and then empower to help prioritize which sources to access, what pr oducts to generate, and how to access those products. This would show users that STAR and NCEI are co mmitted to helping those users meet their needs, thereby demonstrating a responsive service attitude. This is a step that can be taken today. Finding 4H: NOAA has diverse communities of real and potential users with needs for satellite data that extend well beyond those asso ciated with the provision of weather forecasts ving more timely and easier access to user- and warnings. These users would benefit by ha friendly derived products that incorporate data from multiple sources. Such access would oducts of interest and combine them with data enable each user to work with those satellite pr from selected in-situ sources to meet specific operational and use-inspired research needs. Recommendation 4.9: NESDIS, working through its Cent er for Satellite Applications and Research (STAR) and National Centers fo r Environmental Information (NCEI) should establish an internal User’s Working Group (including Cooperative institutes and other NOAA partners) to (a) recognize the breadth of the potential user base beyond the NWS that would benefit from improved access to satellite data products; and (b) work in partnership with those users to prioritize requireme nts and how they might best be met. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-36 Copyright National Academy of Sciences. All rights reserved.

216 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space ********************************************************************************* BOX 4.8 Ocean surface wind modeling for maritime operations. The MetOp 50 km ASCAT scatterometer ocean su rface winds, along with in situ observations, presently serve as one of the primary drivers for th e GFS model that provides atmospheric forcing for NOAA’s WaveWatch III wave model (Bi et al., 2011) . This wave model has a global domain of approximately 1.25°x1.0° resolution, with nested re gional domains for the Northern Hemisphere oceanic 10 basins at approximately 0.5°x0.25° and approximately 0.25° resolution. This wave model serves seaport 11 cargo activity which accounts for 26% of the U.S. economy. One of its most critical applications is for operations at the Port of Long Beach, CA. The Port of Long Beach, combined with the Port of Los Angeles is the busiest port in the United States. 50% of California’s oil comes in through this Port, with only a 5-day storage capacity. With the longer 1,100- 1,300 ft vessel, entrance to the Port is constrained by the draft of the ship. The vessels must have 10% clearance under their keel. If the waves approach on the stern, the vessels will start to pitch, losing 9.6 ft of draft for each 1 degree of pitch. Presently there are five oil companies which lighten their tankers can enter the Port. The cost is $100,000 - $200,000 offshore California, such that the smaller oil tankers per day to hold a tanker offshore. In 2014, the Port, in partnership with Teso ro, California Oil Spill Prevention and Response, o, NOAA’s National Ocean Service, National Weather Jacobsen Pilots, the Marine Exchange of San Pedr ta Information Program (CDIP), and the California Service, the Army Corps of Engineers’ Coastal Da Parks and Recreation, contracted a company in Rotterd am, Protide, to calculate and provide the “go, no- based upon many parameters such as wave models, go” status to the Long Beach Pilots. This status is key role in providing init ializations and boundary tides and bathymetry. The WaveWatch III model has a conditions for higher-resolution nested models that produce the swell information used by the Protide application. At this time, the WaveWatch III model could benefit by higher te mporal (presently updates every 3 hours) and spatial resolution of the wind for ecast to help improve forecast accuracy. At the Long Beach location, the WaveWatch III model is known fo r either under predicting or over predicting the swell (Figure 4.8). Improving the existing satellite winds, which would result in higher wave model accuracy, will have societal economic, environmental and safety benefits. 10 http://polar.ncep.noaa.gov/wa ves/implementations.shtml 11 http://www.aapa-ports.org/advocating/PRDetail.aspx?ItemNumber=21678 UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-37 Copyright National Academy of Sciences. All rights reserved.

217 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space FIGURE 4.8 Nearshore Wave Prediction System (NWPS) model illustrating island sheltering and corresponding bi-modality of wave approach. Black ci rcle at the Port of Long Beach represents the critical area of vessel transit during an energetic south swell. (Figure courtesy of NOAA/National Weather Service). ********************************************************************************* Leveraging non-NOAA Observations its observations with international partners NOAA has a long and successful history of sharing and likewise benefiting from access to their data. Yet more is possible. Other Governmental and International Sources Data sources from other governmental and inte rnational sources offer an opportunity for increased information and value for a minimal investmen t, however realizing those benefits requires that such data actually be incorporated into the operational and research framework, which for a variety of the long and tedious process of understanding reasons, often does not happen. One impediment is alternate data sources, ensuring their quality, and est ablishing that using them to improve forecasts or meet other needs. NOAA has established processes for doi ng this with new systems of its own. For example, both JPSS and GOES-R each in clude a budget line (that are labeled Proving Ground for Risk Reduction ) to demonstrate product utility in a user setting; this enables the development of new and refinement of existing products for these two ope rational NOAA satellite programs. The associated funding for each is in the range of ~$10M+ in the ear ly years of a program to a few $M in later years. These budget lines are critical for the NWS to be able to successfully exploit the data coming from their new systems. But there is no analogous fundi ng opportunity for similar work on data from non- NOAA sources such as NASA and foreign satellites (esp ecially Copernicus), particularly to serve non- NWS needs. For example, while acc ess to observations of ocean surf ace vector winds from the Indian UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-38 Copyright National Academy of Sciences. All rights reserved.

218 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space available for the Ocean Prediction Center to actually ScatSat scatterometer is not an issue, no funds are Oceansat-3 - when it is launched in 2018. utilize its winds, or winds from its successor - Making the transition from NASA research (or foreign) missions to NOAA operations also requires management support. Both sides must recogni ze the importance of transitioning. It is important and appreciation of what NASA has to offer, as that NOAA leadership has a fundamental understanding well as the potential offered by non-NOAA satellite sources. NOAA should further leverage use of NASA, USGS, and Recommendation 4.10: international satellite observations to meet di verse needs of its line organizations, including those unrelated to weather—and thus not lose the opportunity to capitalize on substantial investments made by other organizations. As one step to accomplish this, NOAA should establish a budget line [similar to what is done for JPSS and GOES-R] in order to: a) facilitate access to and use of data fro m these non-NOAA sources, and b) demonstrate resulting benefits through broade ned collaboration with the NASA Applications and similar programs. Commercial and Other Non-Governmental Sources NOAA, to its credit, has recognized the potential benefit of commercial satellite data and is proceeding with projects to explore the opportun ities. Indeed, NOAA/NESDIS’s 2016 Strategic Plan suggests that, as an integral part of providing a comp rehensive and trusted set of products to serve users’ needs, NOAA will “Continue to diversify our portfolio by ingesting, validatin g and certifying data and information from within NOAA, our interagency and international partners and potential commercial 12 sources based on established priorities and requirement needs.” The recent NOAA Commercial Weather Data Pilot awards are demonstration projects to evaluate and demonstrate the quality of commerc r forecast models. The awards to ial data and its impact to weathe Spire Global and GeoOptics suggest the potential for small satellite launches to provide radio occultation ls. MISTiC Winds (Maschoff, et al., 2016) is a data into NOAA’s operational weather prediction mode proposed 27U CubeSat mission designed to improve shor t-term weather forecasting based on a miniature high resolution, wide field, thermal emission sp ectrometry instrument that will provide global tropospheric vertical profiles of atmospheric temper ature and humidity at high (3-4 km) horizontal and vertical (1 km) spatial resolution. Formations of th ree sequential spacecraft in one or multiple orbit planes could provide global 3D horizontal vector wind re trievals. Key remaining technical risks are being reduced through laboratory and airborne tes ting under NASA’s Instrument Incubator Program. With time, both the potential benefits and the risks are coming to be better understood. To be viable within NOAA’s operational system, NOAA insight is generally needed into data generation processes, calibration, validation, and other data quality characteristics. In some cases, this need conflicts with the needs of commercial providers to keep in formation proprietary for competitive purposes. This need also requires the commercial providers to have tion and validation on their done substantial calibra own, sometimes beyond what is needed for othe r customers. Commercial providers are similarly challenged by some of NOAA’s use-rights expectations , including sharing of data with all international partners. All of these issues, and more like them, reflect impediments to NOAA use of commercial data, something not surprising given that the availability of commercial space-based data sources is still in its infancy. With efforts on both sides, impediments can often be overcome. Given the critical operational role of NOAA, r obustness of data sources is essential. To be viable within NOAA’s operational system, commercia l and alternate data sources must be robust against loss of any single source/provider, if essential to NOAA core functions. To ensure 12 https://www.nesdis.noaa.gov/sites/default/files/a sset/document/the_nesdis_strategic_plan_2016.pdf UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-39 Copyright National Academy of Sciences. All rights reserved.

219 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space rs, NOAA can either engage multiple providers or recoverability in the event of lost sources/provide develop protocols for managing such losses. A full review of the potential benefits and ri sks of commercial data sources was beyond the scope of this committee, but we recognized the potential op portunity presented by this emerging data source for NOAA. NOAA should establish itself among the leading government Recommendation 4.11: rcial data sources, assessing both their benefits agencies that exploit potential value of comme and risks in its observational data portfolio. It should innovate new government-commercial partnerships as needed to accomplish that goal, pioneer new business models when required, and seek acceptable solutions to present barrier s such as international partner use rights. NOAA’s commercial data partnerships should en sure access to needed information on data characteristics and quality as necessary and appropr iate, and be robust against loss of any single source/provider if the data are essential to NOAA core functions. NASA Development Partnership NOAA’s partnership with NASA has been long and often productive. Several decades ago, NASA provided extensive development and flight prototyping support for advancing NOAA’s operational satellite systems (see Appendix D of NRC, 2012 for a more detailed description). Today, the NOAA’s needs and NAS A’s capabilities are well-matched. A str ong partnership can be very productive for both organizations (Box 4.9). This holds fo r both technology and scientific advancement. Technological Opportunities. Technology is advancing at a rapid pace in the space community, driven in large part by commercial and academic innovation. NOAA can benefit from these advances. l growth in microSat, nanoSat, and CubeSat The upcoming decade will likely enjoy an exponentia instrumentation, flights, and observations, with a co mmensurate explosive surge in collective contribution tions—from space weather to hydrology, spanning the to Earth System science, application, and opera atmosphere, ocean, and land surface. The mainstream emergence of “U-class” miniat urized satellites will significantly transform how we plan and conduct future Earth and space science research and operations, but only if the agencies are poised to take advantage of them. These spacecraft have masses no more than 1.33kg per unit (“U”) and are composed of multiple of 10×10×10 cm cubic units (e.g., 1U, 3U, 27U). They typically feature ployed on-orbit via previously planned missions commercial off-the-shelf (COTS) components and are de [e.g., through International Space Station resupply missions or accommodated as secondary (auxiliary) payloads on other launch vehicles such as NASA’s Educational Launch of Nanosatellites (ELaNa) and CubeSat Launch initiative (CSLI)]. One option is to exploit the proven capabilities offered by the NASA Earth Science Technology Office (ESTO), through a multi-agency funding and c oordination mechanism. The intent would be to resurrect an inter-agency technology maturation pr ocess to provide atmospheric observing technology the two agencies: NOAA’s lo w-risk and sustainable “on-ramps” that would account for the strengths of measurement set evolving as NASA matures new ob serving technologies to a high technology readiness limited technology matura tion. Through the addition level. Today, ESTO’s limited budget only allows for of NOAA’s future observing system needs, suitably supported, it would become possible for ESTO to oversee a technology maturation process that would deliver high Technology Readiness Level (TRL) instruments that would surpass a significant barri ency: beyond prototype er to an operational ag demonstration in a relevant environment (TRL6) to a system prototype demonstration in an operational environment (TRL7). The ESTO In-Space Validation of Earth Scie nce Technologies (InVEST) program element, intended to reduce the risk of new technologies in fu ture Earth science missions, is incubating many of UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-40 Copyright National Academy of Sciences. All rights reserved.

220 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space o Frequency Interference Technology Validation these satellites; e.g., CubeSat Radiometer Radi (CubeRRT); Compact Infrared Radiometer in Sp ace (CIRiS); CubeSat Infrared Atmospheric Sounder tives from academia and CubeSat (RainCube). Initia (CIRAS); and a Precipitation Profiling Radar in a Observations of Precipitation structure and storm industry are also breaking ground. The Time-Resolved Intensity with a Constellation of Smallsats (TROPICS) mission comprises 12 3U CubeSats in three low- ate the potential efficacy of NASA and other Earth orbital planes. These capabilities demonstr es to meet some of NOA organizations maturing new and innovative technologi A’s observational needs. ********************************************************************************* BOX 4.9 Opportunities and challenges inherent to a NASA-NOAA development partnership: Case study of geostationary IR sounding The high temporal sampling of geostationary hype rspectral sounders allows for rapidly evolving recasting of severe weather events, and will also weather to be observed from space to improve the fo istry observations. NOAA once had plans to include support air quality monitoring and atmospheric chem not to move forward with the plans. Part of a hyperspectral IR sounder on it GOES satellites, but chose t for such instruments. The history of this the reason cited by NOAA was lack of NASA developmen observational capability illustrates the opportun ities and challenges involved in a NOAA-NASA development partnership. In its evolution of environmental remote sen sing capabilities, NOAA (earlier the Environmental Science Services Administration-ESSA) has often re lied on new technology demonstrations by NASA that were subsequently transferred into NOAA miss ions. The polar-orbiting Nimbus satellite series had a number of scientific firsts, such as the High-resolu tion Infrared Sounder (HIRS) that flew on Nimbus 6 and led to the HIRS operational sounder on the TIRO S-N. NASA research and engineering also supported the development of the operational geostati onary satellite program. The SMS-1 (Synchronous Meteorological Satellite) was a NASA-developed, NOAA-operated spacecraft. SMS-1 and SMS-2 paved with way for the US Geostationary Operational E nvironmental Satellite (GOES) satellite program, which and Infrared Spin-Scan Radiometer (VISSR). included the multispectral imagery from the Visible In 1980, NASA added a sounding capability to the NOAA geostationary imager, the VISSR was successful in producing hourly soundings, but Atmospheric Sounder (VAS). The VAS demonstration eased spectral resolution to better resolve the vertical research and applications pointed to the need for incr changes in temperature and moisture. NOAA introduced an operational broad-ba nd infrared geostationary sounder in 1994 with GOES-8. This 19 spectral ba nd sounder successfully produced hourly observations over extended regions, including over soundings complemented the twice- the data sparse oceans. These daily international suite of radiosondes and helped de pict rapid changes in regional temperature, water vapor, and cloud cover for nowcasting severe weather. However, information content analyses have de monstrated that broad- band sounders still have limited vertical information and accuracy for atmos pheric profiling, when compared with hyperspectral IR sounders. To address this, NOAA planned to swap the broad-band GOES Sounder for a high spectral GOES (GOES-10), but the implementation of the resolution (hyperspectral) sounder on the third Geostationary High resolution Interferometer Sounder (GHIS) never materialized. During the systems Baseline Sounder (ABS) was studied to replace the design of the follow-on GOES-R series, an Advanced GOES sounder. The ABS measurements were to en able monitoring of the evolution of detailed temperature and moisture structures in clear skies with higher accuracy (better than 1 K temperature and 15% relative humidity root mean square) and impr oved vertical resolution (about 1 km) over the GOES broad-band Sounder. However, NOAA determined that a GEO advanced hyperspectral infrared sounder, without a prior technology demonstration, would offer unacceptable risk to an op erational agency, and the ABS was removed from the GOES-R series. In the 1990s, NASA developed a high spect ral resolution infrared sounder intended for geostationary testing; the Geosyn chronous Imaging Fourier Transfor m Spectrometer (GIFTS) was built, UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-41 Copyright National Academy of Sciences. All rights reserved.

221 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space NASA complete this program, but it was not but never launched. ESAS 2007 recommended that accomplished. NOAA has cited the lack of a NASA pr ototype as one reason there is currently no hyperspectral IR sounder on its GOES platforms. Other organizations have developed and are flyi ng IR sounders of the type interesting to NOAA. rometric Infrared Sounder (GIIRS) in December China successfully launched the Geosynchronous Interfe 2016. European Organisation for the Exploitati on of Meteorological Satellites (EUMETSAT) is developing GEO advanced hyperspectral infrared so unders (IRS) to fly operationally as a part of Meteosat Third Generation (MTG-3) in 2023. The GIIR tailed vertical layer-by- S and IRS will provide de ity that improves the capability to nowcast and to layer information on wind, temperature and humid initialize regional and global NWP models. Is development of a hyperspectral IR sounde r of sufficient importance to NOAA to support development? Does NASA retain an interest in doing so? Is a flight prototype needed, given other experience with such instruments? These are questions that a robus t NOAA-NASA partnership needs to be able to address. ********************************************************************************* Programmatic Opportunities. The programmatic history of NASA support for NOAA’s 2012, with a particular example illustrated in Box operational system is described in Appendix D of NRC, p is the Operational Satellite Improvement Program 4.7. An often-cited element of this partnershi 13 (OSIP) . While OSIP was a successful model for its time , it is not likely the right model for today. NASA and NOAA budgets are not matched to the OS IP roles, and the need for NOAA pathfinder development has been reduced. There is a need, howev er, to replicate much of the benefit that NOAA achieved through OSIP. The 2017 NESDIS Independent Review Team (IRT, 2017) noted that NOAA and NASA “could together define an R&D program specifically desi gned to develop and transfer technology to NOAA programs.” This Committee concurs, as long as resource contributions of each agency were matched the benefits derived by each. ogrammatic approach like OSIP is sufficient in However, the committee believes that no single pr today’s environment. The needs are more diverse, th e opportunities are broader, and the expectations are higher. Instead, NOAA can benefit from pursuing multiple programmatic approaches to both direct advances to its observing system and access advances that occur external to NOAA. NASA is clearly a central partner for pursuing system advances. As summarized in Recommendation 4.12, NOAA and NASA should establish a framework within which oppor tunities for advance are readily identified and pursued on an individual basis, as each opportunity has unique programmatic needs. This framework should enable implementation of specific project co llaborations, each of which may have its own unique requirements, ensuring: a. Clear roles, with both agencies contributing their expertise. b. Mutual interests , in which NOAA’s benefits are complemented by NASA benefits. c. , from the earliest program phases for identifying opportunities, to the Life-cycle interaction latest phases for ensuring successful tr ansition of lessons and knowledge. d. Multi-disciplinary methodologies , which may include contributions from requirements assessments, modeling, algorithm development, and even flight system alterations, etc. e. Multi-element expertise , which may involve several elements of NOAA (e.g., NESDIS and NWS) and NASA (e.g., ESTO and ASP) as well as established joint mechanisms such as JCSDA. 13 https://www.nap.edu/read/13042/chapter/4#24 UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-42 Copyright National Academy of Sciences. All rights reserved.

222 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space including transfers that provide full support for any share of f. Appropriate budget mechanisms collaborations, thus aligning resour ces with responsibility for execution. In order for NOAA to continue meeting user needs, advances in observing Finding 4I: system capability should receive priority comparable to the core objective of mission reliability. Recommendation 4.12: NOAA should establish, with NASA, a flexible framework for joint activities that advance the capability and cost -effectiveness of NOAA’s observation capabilities. specific project collaborations, each of which This framework should enable implementation of may have its own unique requirement s, and should ensure: a) clear roles, b) mutual interests, c) life-cycle interaction, d) multi-disciplinary methodologies, e) multi-element expertise, f) appropriate budget mechanisms. USGS PROGRAMMATIC CONTEXT This section provides guidance for USGS’s observi ng system priorities, in accordance with the committee’s statement of task whic h specified primary tasks to include: “1) how new technology may enhance current operations, and 2) what new science is needed to expand current operations, either to enable new opportunities or to in clude new areas of interest.” USGS Role in Civil Observing System The USGS is a research agency, embedded in a large and complex Interior Department. The USGS advances scientific understanding and provides b asic monitoring of natural hazards, water, energy ent, and the effects of climate and landuse change and minerals, status of ecosystems and the environm (Box 4.10). In addition to their scientific programs, therefor e, the USGS also has had extensive experience in archiving and managing remote sensing data, distributi ng data to a wide variety of research, management, cover and land-use change for many different policy and private users, and providing information on land- ong history with NASA and other US Government clients across the US Government. USGS has a l agencies in providing the main archive for Land Surface Imaging. Landsat, however, augmented by MODIS imagery in the Land Processes DAAC, h as been the main concern for decades. The USGS responsibility for managing the Landsat archive has taken many forms over the years of the Landsat missions. Early technological limitati ons on downloads from the satellites, augmented by a failure of a recorder on a later mission, meant that in practice, much of the global archive of Landsat data was held in foreign archives. USGS was responsible for managing the relationships among the global data archives, but the unavoidable outcome was that the US did not hold a complete global archive throughout most of the history of the measurements. With the retu rn of Landsat to the public sector with Landsat 7, and with the rapid development of information tec hnology, however, the USGS began the herculean task of coming up to date with both technological a dvancements and the changing goals of the Landsat mission. These included a complete revision of the pr ocessing system, reacquiring data for the US archive that previously existed only overseas, and revamping the cost of data to the users from both current acquisitions and the archive. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-43 Copyright National Academy of Sciences. All rights reserved.

223 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space ********************************************************************************* BOX 4.10 Landslide mapping can save lives and property. Volcanic activity, earthquakes, intense or long duration rainfall or snowmelt, fire, progressive hillslope steepening by canyon cutting rivers, and an thropogenic disturbance (e.g. road construction, deforestation) all may cause discrete areas of a la ndscapes to suddenly or progressively move as a landslide. These areas can be less than a meter and gr eater than a kilometer in width. Susceptibility to landsliding depends especially on steepness of the slope and the strength of the potential failure material (e.g. soil, bedrock, and artificial fill). The initia l failure material may mobilize as a rapidly flowing mixture of debris that can travel more than a k ilometer, entrain more material, and become highly destructive, especially if they head down canyons . Other landslides may move slowly, accelerating during rainy periods and slowing during droughts. Local topogr aphy tends to direct runoff, concentrating water that elevates pressure in the ground, reducing its stre ngth. Landslides also can leave topographic scars that can last 1000’s of years, and thus provide a visible record of potential unstable areas. Hence, the most important surface feature to document at high resolution is the surface topography. All landslides models rely on topographic data. od of heavy rainfall, the massive Oso landslide On March 22, 2014, after a three-week peri mobilized as a debris flow traveled across a river buried a small community of 35 single-family Washington. This 8 million cubic meter landslide is residences killing 43 people in Snohomish County, the deadliest landslide in the history of the continen tal United States. Figure 4.9 shows the landside and a series of maps based on lidar surveys of the area. Li dar light penetration enables both the mapping of the canopy structure of trees and the detection of the “bar e earth” wherever laser light can penetrate to the ground. The maps show the bare earth topography (a ll the vegetation digitally removed), revealing the numerous large ancient landslide scars that are otherwi se hidden by the dense forest cover. The shock of this deadly landslide and the clear evidence of past landslide activity in the area has lead the state of Washington to institute a program of lidar mapping across the state for hazard detection ellite-based lidar can be advanced such surveys (http://lidarportal.dnr.wa.gov/). If high-resolution sat could be done globally and repeat edly, providing not only a means to map landslide scars but also to use the topographic data to make forecast s about landslides. Such data woul d be a revolution in the field of hazard detection and prediction. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-44 Copyright National Academy of Sciences. All rights reserved.

224 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space FIGURE 4.9 March 22, 2014, the Oso landslide killed 43 people and caused more than $120 million in economic loss. A. aerial photograph of landslide. B. Ba re-earth shaded relief image derived from airborne lidar of river valley. C. outline of 2014 landslide and earlier landslides. D. Youngest (A) to oldest (D) landslides revealed by the high resolution lidar imagery (Figures (A) from M. Reid in Iverson et al., 2015, (B) and(C)) from Wartman et al. 2016; (D) is from Haugerud, 2014). ********************************************************************************* itment to a Sustainable Land Imaging capability, With the advent of the US Government’s comm USGS’ responsibilities for Landsat data evolved as we ll. USGS is now responsible for the entire operational and ground segments of the Landsat missi on; NASA is responsible for the planning, design, procurement, and launch of the satellite—which also gives it a primary responsibility for technological evolution of the measurements. In addition, USGS supports a Landsat Science Team, which considers both technological design issues and evolution, changes to algorithms for data processing and products to enable easier use of the data. distribution, and the design of standard data NASA, USGS, and ESA are now producing “harmonized” 30 m global multi-spectral imagery the union of Landsat-8, Sentinel-2a, and Sentinel-2b with an equatorial revisit frequency of 3.7 days, from multispectral data. This revisit freque ncy will drop to 3.0 days when Landsat-9 joins the team in 2020. These “harmonized” data will move land surface research and applications to 30-60 m from the current 500-1000 m spatial resolution of MODIS. ESA had a “block buy” of four Sentinel-2 imagers and has two imagers in reserve ready to be launched when needed. NASA may wish toconsider Landsat-10 and Landsat-11 to follow the example of Sentinel-2 for a block buy of two imagers with a wider-swath (300 km) and multispectral visible, near-infrared, short-wa ve infrared and thermal data, that would increase the equatorial revisit frequency me series 30 m data will be invaluable to 2.0 days for the harmonized data. Ti for many research and application purposes (Fisher et al. 2017, Lin and Roy 2017). In addition, the excellent Landsat-8 and Landsat-9 multispectral imagers are and will be the inter- calibration means for the commercial company Planet Labs to produce 5-m daily global data time series from constellations of cubesats. This “Landsat -based” inter-calibration service will be a major UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-45 Copyright National Academy of Sciences. All rights reserved.

225 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space ent of the commercial remote sensing sector at the contribution of NASA and the USGS to the developm few meter spatial scale. Through this combined capability, involving inte rnational and commercial partnerships, for the ility to follow individual ag ricultural fields through first time since the space age started we have the ab time and not deal with mixed pixels. This capability will greatly advance food security and famine early warning. Value to Users USGS has done a substantial amount of both in-house and extramural analysis of how the Landsat data are used in many different applicati ons areas. Their results are summarized in Figures 4.10 and 4.11. Both public sector and private sector inte rests are fully represented. Data access, which was historically a problem because of costs, was essentiall y solved in 2008 by making orders from the archive en, and is still increasing, as shown in Figure 4.12. free to users. Data usage has skyrocketed since th Government to be the second-most valuable The utility of Landsat was determined by the US satellite data source, behind only GPS. Much of this usage is in the public sector, so direct economic estimates of utility are difficult. More than 30 Federa l agencies and departments use the Landsat data, all 50 States, and a large number of companies. The USGS has estimated that the economic value to users exceeds $1.8 billion per year, and there are at least $400 million in savings in 16 government applications. Finding 4J: Extension of Landsat capability thr ough synergy with other space-based observations opens new opportunities for Landsat data usage, as has been proven with the European Space Agency (ESA) through cross-calib ration and data sharing for Sentinel 2. These successes serve as a model for future partnerships and further synergies with other space-based observations. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-46 Copyright National Academy of Sciences. All rights reserved.

226 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Image Pending FIGURE 4.10 UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-47 Copyright National Academy of Sciences. All rights reserved.

227 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Coastal science/monitoring/ma Other Use nagement 2.3% Education: K-12 ... Fish 2.4% 2.1% Emergency/disaster Range/grassland management science/management 2.4% 2.1% Land use/land cover Cryospheric science change 2.4% 13.5% Environmental regulation 2.5% Software development Geology 8.6% 2.7% Urbanization Ecological/ecosystem 2.9% science/monitoring 6.4% Technical training 2.9% Education: Biodiversity university/college conservation 5.5% 4.0% Urban planning and Climate development science/change 4.3% 5.1% Fire Agriculture forecasting science/management 4.9% Water resources 4.6% Forest 4.9% Agricultural science/management management/producti 4.7% on/conservation 4.8% FIGURE 4.11 Landsat usage (federal- and non-federa l government) usage by purpose and sector for a recent 1-year period (10/1/15-9/30/16).. mic benefits of the Landsat program, the USGS Because of the broad importance and direct econo ine the uses, potential uses, and both direct and has initiated a process by which it continues to determ indirect economic value of the program. This process surveys user communities and government and nd supports the Federal effort to evaluate how all private programs to determine how the data are used, a l for understanding how data should be processed, remote sensing products are used. It is essentia enhanced, archived, and distributed, and how the observing system should evolve to meet user needs. Recommendation 4.13: USGS should ensure that its process for understanding user needs is continued and enhanced throughout the life of the Sustainable Land Imaging (SLI) program. The studies and surveys that USGS has done to document the scientific and operational uses of Landsat should be repeated at appropriate inte rvals, so that progress can be tracked, and these studies should be broadened to incorporate the other components of the SLI program. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-48 Copyright National Academy of Sciences. All rights reserved.

228 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space EROS. Includes only downloads from the USGS delivers (Google Earth approximately 1 billion Landsat scenes month.) to users per Free and open data policy downloads from EROS Data Center. FIGURE 4.12 Cumulative Landsat image Partnerships of critical importance to the performance and The relationship of USGS and NASA is thus maintenance of the Sustainable Land Imaging program, of which Landsat is the primary set of measurements. Indications from both agencies are that th eir partnership is productive, and in fact, plans llenges that each agency will have to remain alert for the next mission are well along. There are four cha to, however, to ensure a successful long-term partnership. Challenge 1—Budgets. Although the operating and science team costs of the Landsat mission are ilding, and launching instruments and satellites, they small compared to the capital costs of developing, bu costs of the USGS budget, the vast majority of which are salaries. As are substantial compared to the base such, USGS must argue for their inclusion each year nt of Interior budget, in a much broader Departme to subsume too large a fraction of their overall and at the same time not allow the operational costs agency budget. Challenge 2— T echnological evolution of the main imager(s). NASA has typically been the agency that sponsors technology demonstrations as the measurement technologies evolve over time. een evaluated because there have been periods of overlap of different Consistency and continuity have b instruments orbiting at the same time, which allows inter-comparisons. Evolution of the measurement technologies will continue to be necessary, both to satis fy data continuity and to manage costs on the een met so far, continuing to meet it will require NASA side of the ledger. While this challenge has b in a way that is cautious enough to satisfy scientific specific steps to be taken to explore new technologies portant advances and potential efficiencies are not and applications users, but visionary enough that im overlooked. With the establishment of the long-term Sustainable Land Imaging (SLI) Finding 4K: ile maintaining standards. The major costs program, budgetary stability is now a priority wh UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-49 Copyright National Academy of Sciences. All rights reserved.

229 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space will remain the NASA development and launch costs. The USGS component (the ground operations, data archiving and distribution, and support of science team investigations) represents a proportionally large fraction of the total USGS budget. Recommendation 4.14: NASA should constrain cost grow th in the development portion of the Sustainable Land Imaging (SLI) partnership, and ideally reduce cost from one generation to the next. USGS should ensure budget growth is minimal, to avoid strain on the overall USGS budget. Challenge 3 - Technological evolution and relationships with the private sector. The advent of e or Amazon to ingest the entire Landsat archive cloud computing, and the ability of companies like Googl and make data quickly available for free, has creat ed valuable new opportunities for the use of large satellite data sets. But this evolution also means that USGS needs to evaluate its relationship with such companies, in analogous fashion to NOAA and NAS A in order to continue to provide essential governmental services in cost-effective ways. Similarly, the advent of new imagers with higher spatial resolution than Landsat, but that still retain the capability to do global surveys creates opportunities for USGS and NASA to consider how those capabilities might be incorporated in a broader Sustainable Land Imaging mission. There are many unanswered ques tions, especially with respect to calibration of instruments, reliability and cost of data access, and long-term access, but those need to be understood as soon as possible. Challenge 4 - International Interactions. NASA has done excellent work with ESA in comparing ta, but this is only the first step. USGS needs to continue to play and calibrating Landsat and Sentinel da an important role in this collaboration, as it provides critical guidance regarding the needs of both scientific and applications users. Partnerships and user communities associated with Sustainable Land Recommendation 4.15: and continue to expand. USGS should: Imaging (SLI) program should be protected  Ensure and continue to expand the benefits of SLI for its scientific and operational user communities. In partnership with NASA, further evaluate ways to more effectively cooperate with or use  emerging commercial capabilities for data ar chiving and dissemination and for imagery acquisition.  Work with NASA and international partners, continue to expand the use of international observation programs that complement and enhance SLI. REFERENCES Adams, Richard M., Kelly J. Bryant, Bruce A. McCarl , David M. Legler, James O’Brien, Andrew Solow, and Rodney Weiher. “Value of Improved Long ‐ Range Weather Information.” Contemporary Economic Policy 13, no. 3 (1995): 10-19. and water resources, Water Resource Research 53:2618–2626, doi:10.1002/ Babcock, Bruce A. “The value of weather informa tion in market equilibrium.” American Journal of Agricultural Economics 72, no. 1 (1990): 63-72. Balmaseda, M. A., Mogensen, K. and Weaver, A. T. (2013), Evaluation of the ECMWF ocean reanalysis system ORAS4. Q.J.R. Meteorol. Soc., 139: 1132–1161. doi: 10.1002/qj.2063 UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 4-50 Copyright National Academy of Sciences. All rights reserved.

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236 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space 5 Conclusion pplications from Space d ecadal survey, the space- At the time of the last Earth Science and A The Earth observing satellites were past their design based Earth observing system was in a critical state. lives (well past in many cases) with very few mission s in the queue. Given the importance of this space- based observations to our daily lives and our success as a society, the risk to our nation was great. Since that time, through careful management, strong international partners, the infusion of resources (though still significantly less funding than the period of the Earth Observing System in the 1990s), and innovation on the part of the technology, engineering, and scientific communities, NASA’s Earth science program has provided opportunity, resu lts, and impact for the nation and the world, in return for the investments made in understanding the planet on which we live. In addition, efforts and investments by NOAA and the USGS complement these capabilities and deliver value to the nation in terms of information that directly impacts our daily lives. Investments in the space-based Earth observation enterprise, which supports the quest for knowledge and the conversion of that knowledge value to citizens in this nation and throughout the worl d, have advanced science, served social interests, and mitigated environmental challenges and enhanced our nation’s prosperity. As we look to the coming decade, it is imperativ e that this momentum be built upon to realize the maximum value of investments in space-based Earth ob servation. Doing so effectively requires that we take an integrated approach that (a) fully capitaliz es on advancements and opportunities as they emerge, (b) stimulates innovation in the Earth system sci ence community, and (c) boldly seeks to meet the technical, fiscal, and programmatic challenges of the coming decade. nd was built on a foundation of input from across The ESAS2017 process sought to be inclusive a lop recommendations for the coming decade. The the science and engineering communities to deve stimulate innovation, serve the Earth science and priorities and recommendations are expected to applications community, and deliver value to the c itizens that provide the resources that support these pursuits. The program recommended is an implementable one, with cost estimates for the larger missions ed to keep costs of the medium-sized missions lower and promote validated, and with competition expect innovation. It achieves balance between flight and no n-flight elements of the NASA portfolio, paying specific attention to the balance between large and small missions, mission investment and science, continuity of observations and new observations, scien ce and applications, heritage technologies and new technologies. In addition, recognizing that unforeseen events , external budget pressures, and other various constraints can force difficult choices, the committee has developed a set of decision rules to inform NASA’s decision-making process on how to addr ess budgetary challenges. The committee also recognizes the potential for increased investments or additional funds being made available through partnerships and technological i nnovation, and offers guidance on how to use additional resources. NASA, NOAA, and USGS have faced a number of challenges in their ability to develop and maintain their portfolio in support of their missions . Given their constraints, they have managed these challenges well, and our Nation’s space-based observati on enterprise is able to provide information and value to its citizens. However, it is continually at risk: some needs go unmet and many opportunities are never realized, as limited resources constrain the prog rams. Without the infusion of additional resources, there will always be shortfalls in meeting national needs, but to partially mitigate against this, it is UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 5-1 Copyright National Academy of Sciences. All rights reserved.

237 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space imperative that the agencies find ways to implement their programs as cost effectively as possible through partnerships, programmatic innovation, exploitation of new technology, etc., as doing so will enable them to realize the full potential of their investments. Finally, a critical element of a successful civilian space-based Earth observation program is coordination among agencies that recognizes the roles and responsibilities of each, maps resources to the fulfilment of those responsibilities, and ensures a he althy interaction among those delivering the science, those developing technologies, and those developing and implementing applications. As each of these elements informs the other, and when executed in co ncert, with appropriate resource alignment, we will be in the best position possible to deliver an effec tive and successful Earth Science and Applications from Space Program. Earth science and applications from space have transformed the way we live. A better understanding of the Earth environment, and the re lationship humans have with it, will continue to produce scientific advances, drive economic opportuniti es, inform sound policy decisions, serve critical humanitarian needs, and much more. The coming decade provides new opportunities for making advances in each of these areas, building on yesterda y’s achievement and on today’s investment, to enable tomorrow’s success and ongoing prosperity. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 5-2 Copyright National Academy of Sciences. All rights reserved.

238 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Part II: Panel Inputs UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION Copyright National Academy of Sciences. All rights reserved.

239 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Copyright National Academy of Sciences. All rights reserved.

240 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space 6 Global Hydrological Cycles and Water Resources 6.1 Input Summary ... ... 2  ... 6 6.2 Introduction and Vision ...  6.2.1 Motivation and Context ... ... 6  6.2.2 Benefits of Prior Efforts ... ... 8  6.2.3 Challenges and Opportunities ... ... 10  6.3 Prioritized Science and Enabling Measurements ...  11 6.3.1 Science Goals, Questions, and Objectives ... 11  Science & Societal Goal H-1. Coupling the Water and Energy Cycles ... 11  Science & Societal Goal H-2. Prediction of Changes ... 21  Science & Societal Goal H-3. Availability of Freshwater and Coupling with Biogeochemical Cycles ... . 26  Science & Societal Goal H-4. Hazards, Extremes, and Sea Level Rise ... 28  6.3.2 Enabling Measurements ... ... 36  Energy and Water Fluxes in the Surface Layer ... 38   Precipitation and its Phase ... 42 45  Snow and Glaciers ... Groundwater Storage and Recharge ... 47  Water Quality ...  ... 49 Land Use and Land Cover ... 50  6.4 Resulting Societal Benefit ... ... 51  6.4.1 Use of Remotely Sensed Data to Manage Water ... 51  6.4.2 Ground Water at the Scale of Management Decisions ... 52  6.4.3 Remote Sensing of Snow Water Equivalent ... 53  6.4.4 Management of Agricultural Lands ... . 54  6.4.5 Floods and Droughts ... ... 56  6.4.6 Integration with Climate Models ... ... 57  6.5 References ... ... 57  UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-1 Copyright National Academy of Sciences. All rights reserved.

241 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space 6.1 INPUT SUMMARY Water—the medium for life—shapes Earth’s surface and controls where and how we live. Chemical, biological, and physical processes alter and are altered by water and its constituents. Water is the most widely used resource on Earth, its mass nearly 300 times that of the atmosphere. On this foundation, humans add engineered and social systems to control, manage, use, and alter our water environment for a variety of uses and through a variety of organizational and individual decisions. Figure 6.1. Earth’s freshwater landscape, including stores, transformations, and fluxes, from high mountain seasonal snow and glaciers, through ecosystems that may be managed, into our engineered agricultural, industrial, and urban landscapes. Courtesy of Jerald Schooner, from Dozier et al., 2014, https://www.researchgate.net/profile/Jeff_Dozier/publication/233531006_Living_in_the_water_environm ent_The_WATERS_Network_science_plan/links/09 12f5112a3dd736a8000000/Liv ing-in-the-water- environment-The-WATERS-Network-science-plan.pdf . Therefore, understanding the hydrologic cycle, monitoring, and predicting its vagaries, are of critical importance to our societies. Remotely sensed data play a key role in advancing our insight about Earth’s water resources. Missions such as the Tropical Rainfall Measurement Mission (TRMM), Global Precipitation Measurement (GPM) mission, Soil Moisture Active Passive (SMAP), and the Gravity Recovery and Climate Experiment (GRACE)—along with sensors of the Earth Observing System (EOS) including the Cloud-Earth Radiant Energy System (CERES), the Moderate-Resolution Imaging Spectroradiometer (MODIS), the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), the Atmospheric Infrared Sounder (AIRS), the Advanced Microwave Scanning Radiometers (AMSR-E and AMSR2), lidar altimetry (ICESat)—have provided important measurements of shortwave and longwave radiation, snow and glacier extent and change, soil moisture, atmospheric water vapor, clouds, precipitation, terrestrial vegetation and oceanic chlorophyll, and water storage in the subsurface, UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-2 Copyright National Academy of Sciences. All rights reserved.

242 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space among many others. Visual, infrared and lightning imagery from Geostationary Operational Environmental Satellites (GOES), especially GOES-16 and satellites in the GOES series through 2036, provide monitoring capabilities to improve now-casting and warning for extreme storms and associated responses to hazards. Together the Landsat 8 Operational Line Imager (OLI) and Thermal Infrared Sensor (TIRS), combined with the European Sentinel 2 satellites and the future launch of Landsat 9, will image Earth’s land area at 15-30 m spatial resolution every three days. Future planned missions like SWOT (Surface Water and Ocean Topography) will measure surface water elevations in lakes, reservoirs, and large rivers, and NISAR (NASA-ISRO Synthetic Aperture Radar) will enable detection of surface disturbance by identifying subtle changes in surface elevation. As a part of the Decadal Survey for Earth Science and Applications from Space, the Global panel was tasked with identifying the high-level integrative Hydrologic Cycle and Water Resources questions in understanding the movement, distribution and availability of water, how these are changing over time, and proposing the remote sensing measurements that will enhance and continue developments needed to address these questions and critical associated applications. Societal and Scientific The chapter identifies four Goals associated with the hydrologic cycle: (1) Coupling the Water and Energy Cycles; (2) Prediction of Changes; (3) Availability of Freshwater and Coupling with Biogeochemical Cycles; (4) Hazards, Extremes, and Sea Level Rise. Scientific advances toward these four goals will support the development of societal mitigation for risks the hydrologic to cycle (for example, contamination of drinking water supplies) or risks derived from the hydrologic cycle (for example, floods and droughts). Within each of the four Scientific and Societal Goals , the chapter identifies key scientific Quantifiable which, when addressed, will advance our scientific understanding toward the Objectives and Societal Goals Scientific Quantifiable Objectives serve as guideposts for identifying the . The scientific inquiries necessary to achieve progress toward each of the four goals, and as such they provide the basis for the suggested Enabling Measurements . Just as phases of the hydrologic cycle are linked, the are also linked. For example, simply Scientific and Societal Goals and Quantifiable Objectives quantifying the basic fluxes of the hydrologic cycle, precipitation, evaporation, streamflow, and groundwater flow will enable progress on all four Scientific and Societal Goals and many of the Quantifiable Objectives . The links between the Quantifiable Objectives are an important consideration for prioritizing the Scientific and the resulting Quantifiable Objectives the associated and Societal Goals, Enabling Measurements . suggestions for Scientific The priorities of the Panel are summarized in two tables. Table 6.1 below lists the and Societal Goals Measurement Objectives . The priorities listed in the with the associated highest priority Most Important table below are classified as Very Important (VI), and Important (I). The minimum (MI), ranking is Important owing to the criticality of water resources to water and food security, economic prosperity, and the health of the planet. Table 6.1. Summary of Science and Application Questions taken from the subsection on 6.3.1 Science Goals, Questions, and Objectives, along with their priorities organized as Most Important (MI), Very Important (VI), and Important (I). Science and Applications Science and Applications Objectives (MI = Most Questions Important, VI = Very Important, I = Important) Develop and evaluate an integrated Earth (MI) H-1a. Coupling the Water and Energy How is the water cycle System analysis with sufficient observational input to Cycles. changing? Are changes in accurately quantify the components of the water and energy cycles and their interactions, and to close the evapotranspiration and precipitation H1 accelerating, with greater rates of water balance from headwater catchments to evapotranspiration and thereby continental-scale river basins. precipitation, and how are these changes (MI) H-1b. Quantify rates of precipitation and its expressed in the space-time distribution phase (rain and snow/ice) worldwide at convective UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-3 Copyright National Academy of Sciences. All rights reserved.

243 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Science and Applications Objectives (MI = Most Science and Applications Questions Important, VI = Very Important, I = Important) of rainfall, snowfall, evapotranspiration, and orographic scales suitable to capture flash floods and the frequency and magnitude of and beyond. Quantify rates of snow accumulation, extremes such as droughts and floods? (MI) H-1c. snowmelt, ice melt, and sublimation from snow and ice worldwide at scales driven by topographic variability. Quantify how changes in land use, water (VI) H-2a. use, and water storage affect evapotranspiration rates, and how these in turn affect local and regional precipitation systems, groundwater recharge, temperature extremes, and carbon cycling. Prediction of Changes. How do anthropogenic changes in climate, land Quantify the magnitude of anthropogenic (I) H-2b. use, water use, and water storage interact processes that cause changes in radiative forcing, and modify the water and energy cycles H2 temperature, snowmelt, and ice melt, as they alter locally, regionally and globally and what downstream water quantity and quality. are the short- and long-term Quantify how changes in land use, land (MI) H-2c. consequences? cover, and water use related to agricultural activities, food production, and forest management affect water quality and especially groundwater recharge, threatening sustainability of future water supplies. Develop methods and systems for (I) H-3a. monitoring water quality for human health and Availability of Freshwater and ecosystem services. Coupling with Biogeochemical Cycles. (I) H-3b. Monitor and understand the coupled natural How do changes in the water cycle and anthropogenic processes that change water impact local and regional freshwater H3 quality, fluxes, and storages, in and between all availability, alter the biotic life of reservoirs, and response to extreme events. streams, and affect ecosystems and the (I) H-3c. Determine structure, productivity, and services these provide? health of plants to constrain estimates of evapotranspiration. Monitor and understand hazard response (VI) H-4a. in rugged terrain and land-margins to heavy rainfall, temperature and evaporation extremes, and strong winds at multiple temporal and spatial scales. Hazards, Extremes, and Sea Level (I) H-4b . Quantify key meteorological, glaciological, How does the water cycle interact Rise. with other Earth System processes to and solid Earth dynamical and state variables and processes controlling flash floods and rapid hazard- change the predictability and impacts of chains to improve detection, prediction, and hazardous events and hazard-chains (e.g. H4 floods, wildfires, landslides, coastal loss, preparedness. subsidence, droughts, human health, and (I) H-4c. Improve drought monitoring to forecast ecosystem health), and how do we short-term impacts more accurately and to assess improve preparedness and mitigation of potential mitigations. water-related extreme events? (I) H-4d. Understand linkages between anthropogenic modification of the land, including fire suppression, land use, and urbanization on frequency of and response to hazards. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-4 Copyright National Academy of Sciences. All rights reserved.

244 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Needs for monitoring and modeling of the water cycle, and its application to societal goals, cover the wide range of needs for a comprehensive understanding of the hydrologic cycle as it relates to freshwater availability, water quality for human health and ecosystem services, and prediction of extremes and hazards. It stretches from the accurate quantification of water and energy fluxes at the river basin scale, to accurate snow water equivalent measurements for water supply forecasting, to improved drought monitoring, to flash flooding hazard prediction, to changes in land use and water quality in highly coupled human-natural systems. And it stretches from recommendations to extend ongoing measurements, to new endeavors in detecting the phase (rain or snow) of precipitation, to measuring evapotranspiration, to new fields for application of remotely sensed data, such as water quality, groundwater recharge, effects of urbanization, water-modulated biogeochemical cycling, and prediction of hazard chains. Our science and applications rely heavily on data availability since the beginnings of remote sensing. Improvements sought mainly relate to water and energy fluxes at Earth’s surface— evapotranspiration, snow and ice melt, rainfall, snowfall, and recharge and withdrawal of groundwater. Table 6.2 presents the Priority Targeted Observables for the Science and Societal 1 Most Important or Very Important the Panel ranked as Targets/Objectives . The information is taken from Enabling Measurements . subsection titled Table 6.2. Priority Targeted Observables mapped to the Science and Societal Objectives, which were ranked as Most Important or Very Important. Summary text is included in the second column to illustrate the types of knowledge needed to achieve the Objectives. PRIORITY Targeted Observables Science and Applications Objectives Surface Characteristics H-1a and H-2a. Needed to measure rates of evapotranspiration Spectral albedo of snow, vegetation, - and quantify how land use affect them. H-1c. Needed to measure snowmelt, ice melt, and sublimation and soil Surface temperatures of snow, - from snow and ice. vegetation, and soil, covering diurnal cycle H-1c. Quantify rates of snow accumulation, and track Snow Depth and Snow Water snowmelt. SWE = depth × density, but depth is the main Equivalent (SWE) contributor to spatial variability. H-1a and H-2a. Needed to measure rates of evapotranspiration Soil Moisture Especially in the root zone - and quantify how land use affects them. H-1b. Improve identification of precipitation phase and rates Precipitation and Clouds of precipitation, especially when ice is present, and capture rainfall at orographic and convective scales. Terrestrial Ecosystem Structure H-2a. Improve the estimation of evapotranspiration. H-2a. Improve the estimation of evapotranspiration and Temperature, Water Vapor, PBL Height sensible heat exchange. H-3. Support emerging efforts to remotely sense water quality. Aquatic Biogeochemistry H-4a. Monitor hazards and response in rugged terrain and land-margins. H-1a and H-2. Monitor elastic and inelastic subsidence related Surface Deformation & Change to groundwater withdrawals. H-1c. Interferometric SAR might be able to measure snow density. 1 Not mapped here are cases where the targeted observables may provide a narrow or indirect benefit to the objective, though such connections may be cited elsewhere in the report. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-5 Copyright National Academy of Sciences. All rights reserved.

245 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space PRIORITY Targeted Observables Science and Applications Objectives H-1c. Help quantify rates of ice melt in basins where glaciers Ice Elevation contribute significantly to runoff. Implementing this program will enable the following scientific and applications advances: x Improved monitoring of precipitation and evapotranspiration, with the goal to measure and model each so that the accuracy of the estimation of their difference is less than rates of runoff or groundwater recharge: - . . . especially for rates of precipitation of mixed water and ice, so as to estimate snowfall as well as rainfall; . . . and in measurement and modeling of convective and orographic precipitation. - x Improved measurement and modeling of albedo of the components of Earth’s land surface— snow, ice, vegetation, and soil—to enable closing of the surface radiation balance to within 10% of the magnitude of the absorption: - . . . necessary to model evapotranspiration, snowmelt, and retrospective reconstruction of the snow water equivalent. x Understand how human modification of the land surface affects evapotranspiration, and the consequences for the hydrologic cycle. Understand how hazards in mountainous terrain and along coasts relate to weather extremes. With growing populations, the demands on our water resources are increasing. The study of our hydrologic cycle, and how it changes over time, is critical to understanding and quantifying freshwater availability, water quality and ecosystem health, and anticipating and managing risks due to extremes. Remotely sensed data have permitted the scientific community to develop broad new understandings of the water cycle at the scales of small basin to continents and the entire Earth, and advance socially important applications. This chapter’s priorities will, if implemented, support and enhance the continuation of that work for the benefit of society and for a safe and prosperous future. 6.2 INTRODUCTION AND VISION 6.2.1 Motivation and Context Water is the most widely used resource on Earth and, unlike other natural resources, water is a ubiquitous solvent and a medium for life itself. Fluxes of water connect the land to the atmosphere and the oceans. Water mediates Earth’s energy budget in the form of clouds, and it acts as a universal transport agent moving energy in the form of latent heat and all kinds of materials from sediments to bacteria across the planet (Evenson and Orndorff, 2013). The hydrologic cycle involves many processes (precipitation as rain or snow, evapotranspiration and evaporation, snowmelt, condensation, sublimation, surface runoff, infiltration, percolation, and groundwater flow) whereby water circulates between the atmosphere, land surface, and the oceans. To understand the physical structure, chemistry, biodiversity, and productivity of the biosphere, it is important to know how water moves and how water is stored in the Earth system (National Research Council, 2012). Further, the movement of water influences Earth’s biogeochemical cycles and the Earth’s climate (Vitousek et al., 1997). As Figure 6.2 shows, all components of the water cycle are linked at scales ranging from global to small basins, impacting and being impacted by human activities such as water withdrawals for agriculture and infrastructure development such as dams (Dalin et al., 2017). UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-6 Copyright National Academy of Sciences. All rights reserved.

246 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Figure 6.2. Interacting water and environmental processes, illustrating some couplings between natural and engineered systems and human processes in the water environment. Courtesy of Jeff Dozier, from Dozier et al., 2014, https://www.researchgate.net/profile/Jeff_Dozier/publication/233531006_Living_in_the_water_environm 12f5112a3dd736a8000000/Liv ent_The_WATERS_Network_science_plan/links/09 ing-in-the-water- environment-The-WATERS-Network-science-plan.pdf . The management of water resources is crucial for ensuring public health (Seid-Green, 2016) and securing the supply and allocation of water and food production to support human wellbeing, while st sustaining healthy ecosystems. This is a major challenge for the 21 century (Poff et al., 2016). Opportunities, however, exist to integrate ecological health and human water needs in a comprehensive UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-7 Copyright National Academy of Sciences. All rights reserved.

247 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space way (Gleick, 2000). In the Anthropocene, it is increasingly important to incorporate the human dimensions of freshwater use, to understand and predict aspects of freshwater resources (Konar et al., 2016). Beginning with the launches of Sputnik in 1957 and Explorer 1 in 1958, remote-sensing enabled an opportunity to observe the Earth system, especially its water cycle, and its changes in space and time from a global perspective (Vince, 2011; Lettenmaier et al., 2015). Measurements from spaceborne and airborne platforms advance understanding of the hydrologic cycle and water resource assessment, which can improve society’s ability to manage water in our ever-changing world. To understand the dynamics of the Earth’s terrestrial water cycle requires detailed in situ and remotely based measurements. Remote sensing has become a common tool in hydrology and water resources research, and because it enables a quantitative assessment of interconnections among multiple physical and biophysical and biogeochemical process across the world’s landscapes it has enabled and catalyzed the advancement of Earth System Science research and applications, and the fundamental role of the water cycle therein played (McCabe et al., 2017). The Earth-observation systems that produce these sustained observations constitute vital national infrastructure, providing well-established, direct benefits to society and the economy, such as protecting life and property and securing food and water during disasters (Seid-Green, 2016). However, in spite of the importance of water to humanity, ecology, and environment, a comprehensive global hydrological observing system for monitoring the storage and movement of Earth’s water does not exist (Rodell et al., 2015). The motivation for the Global Hydrologic Cycle and Water Resources panel’s work is to increase understanding of the hydrologic cycle from an integrated Earth System perspective. The intent is to provide a comprehensive perspective on the hydrologic cycle including impacts and feedbacks at key coupled human-natural interfaces (water resources, agriculture, urbanization and infrastructure, natural resource use, and stewardship). The panel addresses its important task according to four different organizing themes that capture both scientific and societal imperatives: (1) coupling of water and energy processes between land and the lower troposphere; (2) prediction with a focus on variability, biogeochemical cycling, and extreme events; (3) water use and availability of quality water; and (4) hazards such as floods, droughts and related fires, landslides and others that capture both scientific and societal imperatives. Section 0, 6.2 Introduction and Vision , provides a motivation and context of why the panel objectives are both scientifically compelling and societally important, and why now is a suitable time for investment into additional efforts to measure hydrologic parameters remotely using Earth observing satellites. We then review the improvements in understanding, monitoring, and predicting hydrologic processes and resource assessment by Earth-orbiting satellites since the last Decadal Survey. It includes a subsection on Challenges and Opportunities that identifies science and applications for which new or sustained measurements of hydrologic parameters are necessary to advance the hydrologic sciences and best serve society. Then Section 3 on 6.3 Prioritized Science and Enabling Measurements identifies and prioritizes 14 science and application objectives, which are categorized into broad societal questions based on coupled cycles, predicting change, water availability, and hazards. The subsection on Enabling Measurements describes how they can address the quantitative science objectives and questions. Section 4, 6.4 Resulting Societal Benefit, discusses priority measurements in the context of benefiting society, considering measurements that have broad application to water resource challenges such as availability of freshwater and hydrologic hazards. 6.2.2 Benefits of Prior Efforts The previous Decadal Survey (National Research Council, 2007a), emphasized the need for high- quality global estimates of precipitation, soil moisture, and snow-water equivalent. In addition to these variables, the previous Survey also noted that measures of surface-water storage and transport would UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-8 Copyright National Academy of Sciences. All rights reserved.

248 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space improve both modeling and an integrated understanding of the global water cycle. The four missions most relevant to the water cycle were: The already approved Global Precipitation Measurement (GPM) mission to provide estimates of x precipitation; A soil moisture mission to address this crucial part of the land-surface water balance; x A surface-water and ocean-topography mission to provide observations of water storage and x associated variability; and x A cold land processes mission to provide estimates of the water stored in snowpack. Since that time, substantial progress has been made. Space-based observations of the hydrologic cycle and water resources have improved both the scientific understanding and resulted in a variety of societal benefits. Some key achievements and transformational technologies include (Lettenmaier et al., 2015): x The Global Precipitation Measurement (GPM) mission, which contributed to developing a capability to forecast floods and droughts and understand how precipitation patterns change through time across local to regional and global scales. GPM provides improved measurements to help improve weather and climate models (Skofronick-Jackson et al., 2016); x The Gravity Recovery and Climate Experiment (GRACE), which contributed to the ability to measure the change in total water storage over large areas and information on global groundwater depletion (Famiglietti and Rodell, 2013; Alley and Konikow, 2015; Richey et al., 2015; Lakshmi, 2016); and x Soil Moisture Active Passive (SMAP) mission which included improvements to water and climate forecasting, flood and drought monitoring, and predictions of agricultural productivity (Entekhabi et al., 2010). SMAP has been providing soil moisture observations that have been calibrated and validated at various locations (Chan et al., 2016; Burgin et al., 2017; Colliander et al., 2017; Kim et al., 2017). In addition to pertinent missions that have already launched, scheduled launches also represent substantial achievement to help understand the hydrologic cycle and provide societal benefits. The Surface Water and Ocean Topography (SWOT) mission, scheduled for launch in 2021, will provide both water surface elevations and extent and thereby information about surface water storage and fluxes globally. The mission is expected to contribute to the understanding of individual lakes and reservoirs a few hundred meters in size and larger, and the information generated will aid the management of transboundary waters and ungauged basins (Biancamaria et al., 2016). The upcoming TROPICS mission—Time-Resolved Observations of Precipitation structure and storm Intensity with a Constellation of Smallsats (NASA EOS, 2017) to be launched in 2019—uses passive microwave spectrometry to provide for the first time high-revisit thermodynamic soundings and storm structure down to the boundary layer that can be integrated with high spatial resolution observations and rapid-refresh data assimilation systems to improve hydrological and hazard forecasts in remote regions generally and mid and large size 2 ungauged basins (> 300 km ). Other recommended missions (National Research Council, 2007a) included the Snow and Cold Land Processes (SCLP) mission and the HyspIRI mission. SCLP’s objective is to measure the snow-water equivalent (SWE), snow depth, and snow wetness over land and ice sheets. As a third phase mission still in formulation, its status is conceptual, with newer approaches to measuring SWE addressed in this report. The HyspIRI mission was recommended as a second phase mission for launch in the 2012 to 2016 period. Based on hyperspectral instruments designed to globally observe at high spatial and spectral resolution (Devred et al., 2013), such measurements provide an opportunity to assess ecosystem changes and functions, natural hazards such as volcanic eruptions and wildfires, and snow properties (Dozier et al., UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-9 Copyright National Academy of Sciences. All rights reserved.

249 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space 2009b; Hook, 2014). Since the last Decadal Survey, the mission concept has been refined to achieve needed measurements more economically, based partly on experience with hyperspectral observations of the lunar surface by the Moon Mineralogy Mapper (Green et al., 2011; Lee et al., 2015). Additionally, the ECOsystem Spaceborne Thermal Radiometer on space Station (ECOSTRESS) with a planned launch of May 2018 provides NASA the opportunity to collect very high spatial (35x75m) land surface temperature (LST) at a 4-day temporal resolution. This measurement addresses the H-1a measurement priority (“diurnal cycle of surface temperature (vegetation, soil, snow), at agricultural or topographic scales”) and can provide critical measurements that will help better design a spectrometry mission. To fully exploit the ECOSTRESS mission, its measurements plan needs to be expanded to cover all land areas rather than the current plan of selected regions and validation sites. 6.2.3 Challenges and Opportunities Over the last thirty years, NASA’s Earth Observing System (EOS) transformed Water Cycle science and applications by providing—for the first time—frequent multiscale observations over large spatial domains across the planet. From the privileged vantage point of space orbits, coordinated missions such as the Afternoon Constellation (A-Train) and a developing suite of precipitation sensors rely on measurements from multiple satellites and collaborations with international partners, mainly space agencies and centers in Europe, Japan, and India (CNES, ESA, JAXA, ISRO) to measure systematically key geophysical variables including shortwave radiation, atmospheric composition, clouds, precipitation, soil moisture, terrestrial vegetation and oceanic chlorophyll, water storage in the subsurface, and land subsidence, among many others, thus effectively establishing a de facto Earth Observing System. Such integrated observations show interrelatedness and feedbacks among seemingly removed processes and states, such as atmospheric composition and evapotranspiration, linking atmospheric pollution to clouds and surface temperature and water availability, and linking, in turn, public and environmental health to irrigation needs for food production and energy security. NASA’s initiation of the EOS idea was foundational to the advent and growth of Earth System Science and Applications. Where lead-time is paramount (e.g., seasonal climate for food production and water supply, 5-day weather forecasts for the construction industry, flashflood warnings for public safety, next-day snowfall for school closings), the integration of satellite-based observations and models through Data Assimilation Systems (DAS) significantly increased the predictability skill of existing forecast systems with implications for decision-making under uncertainty across weather and water socioeconomic sectors (Bolten et al., 2010; Magnusson and Källén, 2013; Pagano et al., 2014; Bauer et al., 2015). An entirely new service industry developed over the last two decades to provide specialized value-added information products and client-based modeling and observing systems (National Research Council, 2003; Benson, 2012; Mandel and Noyes, 2012; Acclimatise, 2014). In recent years, even as NASA’s original EOS missions surpassed expectations of longevity and utility, continuing to operate beyond their design life, the number of new satellite launches has declined and it is split between improved continuation missions (e.g., GPM and GRACE-Follow-On) and new missions (e.g., SWOT, NISAR). Benefitting from NASA’s early leadership in technological innovation, data access policy, and research and development, and more recently through international collaborations, the Program of Record of current and planned missions relies on mature (proven) technology to assure essential data continuity. Yet, prompted by developments in sensor technology, high-performance computing, and scientific advances over the last decade, the current Program of Record is inadequate for current and anticipated modeling capabilities, or to meet the data granularity and specific needs of data- driven decision-making in the near future. This report proposes a measurement plan that addresses these needs. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-10 Copyright National Academy of Sciences. All rights reserved.

250 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space 6.3 PRIORITIZED SCIENCE AND ENABLING MEASUREMENTS 6.3.1 Science Goals, Questions, and Objectives The linkages between the water cycle and freshwater availability, food and energy production, and environmental resilience highlighted in this Decadal Survey emerge from Grand Challenges of opportunity for a wide range of research programs (e.g., Trenberth and Asrar, 2014). Figure 6.3 and Table 6.1 summarize the goals, objectives, and assigned priorities from the details in Section 3. Quantifiable Objectives and the Measurements (in the next subsection) are intertwined, without a one-to- one mapping between them. Indeed, consistent with the ubiquitous role of the water fluxes connecting reservoirs and interfaces across the Earth System, many of objectives of lower priority would be achieved if specific higher priority objectives ( Most Important or Very Important ) are achieved. Figure 6.3. Schematic view of four science and societal themes and associated questions related to hydrology and water resources, and are relevant at all spatial and temporal scales. Source: E. Foufoula- Georgiou Science & Societal Goal H-1. Coupling the Water and Energy Cycles How is the water cycle changing? Are changes in evapotranspiration and precipitation accelerating, with greater rates of evapotranspiration and thereby precipitation, and how are these changes expressed in the space-time distribution of rainfall, snowfall, evapotranspiration, and the frequency and magnitude of extremes such as droughts and floods? Satellite-based observations available since 1979 have been used to generate multiple precipitation data sets (Huffman et al., 1997; Xie and Arkin, 1997; Adler et al., 2003; Xie et al., 2003; Ashouri et al., 2015; Xie et al., 2017) suitable for monitoring the water cycle at global scale. The frequency and space-time patterns of rainfall, snowfall, snowmelt, soil moisture and evapotranspiration control the water and energy cycles at basin, regional, and global scales. Changes in these patterns caused by climate change and human modification to the environment, coupled with increasing population and UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-11 Copyright National Academy of Sciences. All rights reserved.

251 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space per-capita demand for water, pose significant challenges in the management of water-resources systems, threaten water, food, and energy security, challenge the health of ecosystems, and increase susceptibility to hazards and their socio-economic consequences (e.g., Emori and Brown, 2005; Alexander et al., 2006; Wentz et al., 2007; Min et al., 2011; Trenberth, 2011). Changes in precipitation extremes, typically th th understood as the top 95 to 99.9 percentiles of daily accumulations, have been documented in many places (Kunkel et al., 2003; Emori and Brown, 2005; Groisman et al., 2005; Alexander et al., 2006; Berg et al., 2013), as well as in the duration of wet and dry spells (Zolina et al., 2013; Guilbert et al., 2015; Trepanier et al., 2015), and in the seasonality and phase (Barnett et al., 2005; Nayak et al., 2010). At the global scale, there is large spatial variability with both negative and positive trends over large regions at multiple spatial scales (Ashouri et al., 2015; CHRS Rainsphere, 2017) with high uncertainty depending on the length of the available precipitation data records, both rain gauge observations and satellite products. Accurately monitoring the timing, amount, phase (snowfall or rain), and vertical structure (hydrometeor composition) of precipitating systems globally and with sufficiently high spatial and temporal resolution to detect change and to quantify water availability at multiple scales from headwater catchments to continental river basins is an imperative challenge for the next decade. For basin-scale budget studies, estimating precipitation at spatiotemporal scales of 1 km and 1 hour are adequate, with temporal resolutions as fine as 5 minutes needed for urban flood warning and response (Berne et al., 2004; Emmanuel et al., 2012) and long standing engineering design standards (HEC22: Brown et al., 2009). Such observations will improve modeling of weather and climate, provide real-time warning for hazards such as floods and landslides, and increase the predictive understanding of teleconnections to attribute, anticipate, and manage environmental change. Accurate estimation of precipitation amounts and detection of changes is challenging over land, especially over complex terrain (Barros, 2013). For example, High Mountain Asia (HMA) contains the largest deposit of ice and snow outside the Polar Regions; here shrinking glaciers provide evidence of climate change in one of the world’s iconic regions, and the region plays a critical role in controlling the land surface energy balance, and downstream irrigation and freshwater availability in several densely populated river basins (Kehrwald et al., 2008). In the past few decades, a wide range of climatic changes, accelerated by economic developments and urbanization, has altered HMA’s radiation budget by increasing the temperature, depositing soot and dust in the snowpack that reduces its albedo, shifting the precipitation patterns, reducing snowfall, and amplifying the melting rate of glaciers and permafrost (Qui, 2008; Kaspari et al., 2014). HMA’s precipitation exhibits strong interannual variability (Barros and Lang, 2003; Barros et al., 2004; Lang and Barros, 2004), and its changes are still only poorly known because of the paucity of in situ observations and thereby the lack of validation of climate models. It is likely that future seasonal melting will shift the river peak flows towards the spring and decrease the water availability during the summer, posing risks to downstream water availability, impacting food and energy production and ecosystems (Immerzeel et al., 2010). This phenomenon and the lack of ground-based observations is not confined to HMA, but is evident in key mountain regions worldwide leading to their designation as the Third Pole that includes mountain ranges in North America, South America, and Europe (Stewart, 2009; Yao et al., 2012). Whereas the linkages between climate variability and hydrological drought at interannual and decadal scales are well established (e.g., Barros et al., 2017), there is large uncertainty in assessing the sensitivity of drought frequency to observed changes in global temperatures (Dai, 2012; Sheffield et al., 2012; Trenberth et al., 2014). However, just as warmer temperatures increase the water holding capacity of the atmosphere contributing to more extreme precipitation in some regions, higher temperatures— concurrent with regionally lower humidity that leads to greater potential evapotranspiration—can result in increased drought severity due to persistent decreases in soil moisture, increased plant water stress and degradation of plant productivity (Easterling et al., 2000; Weiss et al., 2009). Drought amplification by the interplay of concurrent and persistent high temperatures and low atmospheric moisture conditions is illustrated by Moran et al. (2014), who compared the dust bowl drought in the 1930s to the droughts in st the 1950s and the early part of the 21 century in the Western US. They found that the 1950s drought st was more severe than the 1930s and the early 21 -century droughts, even if the warm season temperature UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-12 Copyright National Academy of Sciences. All rights reserved.

252 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space was only 0.68°C above the historic mean. Changes in post-drought ecosystem composition due to invasive species during recovery from recent “hot” droughts (Willis and Bhagwat, 2009) pose further challenges in managing the interplay between water, food, energy, and ecosystem services. Interestingly, drought “busting” events that replenish regional soil moisture and aquifers, including atmospheric rivers in the western U.S. (Dettinger, 2013) and land-falling hurricanes and tropical cyclones in the South and Southeast (Brun and Barros, 2014; Lowman and Barros, 2016), are also associated with major hazards and destructive damage in complex terrain and in urban areas because of heavy precipitation, extreme winds, flooding and landslides (Guan et al., 2016a; Waliser and Guan, 2017). The complex and nonlinear interconnections between water availability and water use, extreme events, and hazards encompass spatial scales ranging from 100 m to 1000 km and temporal scales from minutes to years. They are iconic of the challenges presented to water cycle research, and a powerful motivation to monitor the Earth at high spatial and temporal resolution, which can only be accomplished systematically from space. Objective H-1a. Develop and evaluate an integrated Earth System analysis with sufficient observational input to accurately quantify the components of the water and energy cycles and their interactions, and to close the water balance from headwater catchments to continental-scale river basins. Error! Reference source not found. shows how the water and energy cycles are linked within the Earth climate system in many ways as well as how the various satellite missions have been used to observe these land and atmosphere variables. Objective H-1a underscores the need for a balanced research program combining observations and analysis systems. It also underscores the potential for scientific discovery that results from the integration of different observation types meeting requirements at distinct spatial and temporal resolution to probe interrelationships and feedbacks in the coupled Earth System. For example, surface evapotranspiration (and its equivalent latent heat) are common fluxes to both water and energy cycles. However, point-scale evapotranspiration is directly measured by lysimeters, which mainly are installed in agricultural research settings, or estimated from measurements of sap flow in individual trees. It cannot be measured remotely. Instead, sensible and latent heat flux modeled from in situ measurements are the components of the surface available energy that are the primary drivers of the surface boundary layer that influences the coupling of the land with the atmosphere (Ek and Mahrt, 1994; Betts et al., 1996; Betts, 2004) and heats the surface air. Thus, the key to estimating evapotranspiration lies in measuring, or modeling the variables and parameters that determine other terms of the energy balance equation —solar and longwave radiation, albedo, surface temperature, air temperature and atmospheric water vapor pressure in the boundary layer, and wind. Surface soil moisture influences the boundary-layer cloud development through the latent heat flux associated with evapotranspiration which in turn regulates surface temperature and thus the sensible heat flux and emitted longwave radiation, thereby affecting net surface radiation and available energy. (Findell and Eltahir, 2003; Betts, 2004; Ek and Holtslag, 2004). UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-13 Copyright National Academy of Sciences. All rights reserved.

253 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Figure 6.4. Closing the terrestrial water balance using remote sensing, showing needed measurements to measure the links between the energy and water cycles. Source: V. Lakshmi. Quantifying the components of the water and energy cycles at the Earth’s surface through observations and with sufficient accuracy to close water budgets over a wide range of river basin scales is a challenging problem that remains unresolved, but central to programs like the NASA Energy and Water System (NEWS) and the WCRP (World Climate Research Programme) Global Energy and Water Exchanges (GEWEX) (Rodell et al., 2015; Zhang et al., 2016). With evidence of increasing climate variability and change (Barnett et al., 2005), and increased utilization of water resources (Oki and Kanae, 2006), understanding the controls on these components from an Earth System Science perspective is imperative to assessing change, and to develop effective adaptation strategies. These processes are explicitly included in climate models, where the surface water and energy cycles are closed (fluxes in balance) by mathematical design at model-resolved scales that are unfortunately much coarser than the governing process scales, which are therefore not appropriately represented (i.e. parameterized). Proper characterization of states and fluxes is complex and requires many parameters, including landscape and vegetation characteristics (e.g. topographic variability, soil properties, land use and land cover, vegetation biophysical parameters, water and land management, to name a few), most of which are poorly measured across the globe and their effects are poorly understood. This results in high uncertainty and wide variability among predicted water and energy fluxes (Mueller et UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-14 Copyright National Academy of Sciences. All rights reserved.

254 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space al., 2011; Rodell et al., 2015; Wild et al., 2015; Zhang et al., 2016) that limits our understanding to changes in water availability, the proper partitioning of evaporation and transpiration and storage, and the effect of the vertical distribution of water vapor and cloud microphysics on precipitation among others that impact extremes like floods, droughts and heat waves. For example, Chaney et al. (2016) used global FluxNet data to improve the process parameterization in the Noah land surface model. But of the ~ 650 sites in 30 regional networks covering 5 continents, only 253 eddy covariance stations with a total of 960 site-years of data at the needed 30-minute time resolution have been harmonized, standardized, and gap- filled (ORNL, 2007) with only 154 sites open to the community. Further evaluation and quality control of the data reduced the useable number to 85. Many regions (South America, Africa, Asia, and Australia) have fewer than 3 or 4 sites. The existing observational vacuum handicaps the science and can only be addressed effectively through remote sensing observations in the context of a broader integrated Earth system analysis. What might comprise this context? Improved validation of remote sensing products through a significant increase in core sites (high x quality, multiple variable measurements sites over ~10×10 km grids that can resolve sub-grid flux heterogeneity at the 1 km scale or finer) and sparse validation sites measuring fewer locations or variables within a 10×10 km grid. This requires the coordination of space agencies and international bodies such as WMO and WCRP. x Development of high-resolution Earth-system models at spatial resolutions of 1 to 3 km, which can resolve watershed-scale water and energy states and fluxes with finer spatial heterogeneity and enable improved understanding and landscape management. Approaches could include elements such as the hyper-resolution land surface modeling based on tiling of complex hydrologic response units, which offers one approach for continental modeling at 30 m (Chaney et al., 2016). x Coordinated networks of in situ and remote sensing products that can improve the characterization of the land surface energy fluxes, and resolve surface solar and longwave -2 radiation balances within 10 W m accuracy at 1 km resolution globally, four times daily. Resolving the diurnal cycle is the desirable goal, but progress can be achieved through the integration of models and high-spatial resolution measurements at lower temporal resolution. x The upcoming ECOSTRESS mission (launch 2018) on the Space Station can provide useful landscape-scale (~70 m spatial and 4-day temporal resolutions) top of the canopy temperatures. Likewise, ongoing efforts to produce 30 m multi-spectral harmonized surface reflectance products through the fusion of Landsat-8 (and Landsat-9 in 2020) and Sentinel-2a and -2b with high-revisit frequencies (~3-4 day at the Equator, and 1-2 days at mid-latitudes) represent significant space-time resolution improvements over the highest resolution MODIS products currently available. Addition of a polar-orbiting imaging spectrometer to this constellation would enable spectroscopic interpretation and validation of the observations from multi-spectral sensors. x Development of assimilation techniques and data analytics that can provide the desired integration and synthesis from merging in situ, remote sensing, and hyper-resolution models. Given these advances, critical science and societal questions can be addressed, such as: 1. What are the impacts of increased atmospheric CO and other greenhouse gases on the coupled 2 water-energy-biogeochemical cycles, and do these modify water availability at basin to regional scales? 2. To what extent have the water cycle components and their variability changed, and have these resulted in changes to extreme events (floods and droughts)? UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-15 Copyright National Academy of Sciences. All rights reserved.

255 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space 3. How will monitoring and modeling of water and energy balance variables at the basin and field scales lead to improved management practices and resiliency? Objective H-1b. Quantify rates of precipitation and its phase (rain and snow/ice) worldwide at convective and orographic scales suitable to capture flash floods and beyond The Global Hydrology and Water Resource Panel assigned highest priorities to developing an Integrated Earth System analysis, which would integrate models and observations, and to measuring rainfall and snowfall and accumulated snow on the ground, that are key constraints and inputs into that analysis. Precipitation is the most important water flux in terrestrial hydrology, and thus precipitation measurements are key input variables in hydrologic and water resources models. It is equally important to a vast array of applications from agriculture, to ecosystem management, to climate monitoring and adaptation efforts, including risk-based engineering design of critical infrastructure from highways to water supply systems. For these reasons, precipitation has been at the forefront of NASA’s sustained mapping efforts at global scales along with NOAA ground-based radar networks for the Continental United States. Figure 6.5. Percentage changes of dry days over land relative to climatology based on NASA TRMM measurements. Shown are (a) time series (1998–2013) for annual, wet season, and dry season, respectively, and (b) trend pattern for dry season in units of percent change per decade. A significant global drying trend (3.2% per decade) over land is evident during the dry season. Source: Wu, H.-T. J., and W. K. M. Lau. 2016. Detecting climate signals in precipitation extremes from TRMM (1998–2013)— Increasing contrast between wet and dry extremes during the “global warming hiatus”. Geophysical Research Letters. 43:1340-1348. doi:10.1002/2015GL067371. Reprinted with permission; copyright 2016, American Geophysical Union.. NASA, in collaboration with the Japanese Space Exploration Agency, JAXA, launched the Tropical Rainfall Measurement Mission (TRMM) in November of 1997 to quantify tropical rainfall and the associated latent heating structure. The mission success went beyond quantifying mean rainfall over the global tropical oceans, and it spurred the development of innovative algorithms that used the TRMM radars as a way to calibrate existing passive microwave radiometers and sounders as well as infrared observations to increase the spatial and temporal resolution of precipitation (Huffman et al., 2007; Kummerow et al., 2015). Figure 6.5 shows how TRMM detected significant drying trends from 1998 to 2013, especially in the western and central U.S., Southern Africa, northeastern Asia, and Southern Europe and Mediterranean. These decreases are concurrent with positive trends elsewhere, resulting in spatial UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-16 Copyright National Academy of Sciences. All rights reserved.

256 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space variability at the global scale and large regions of statistically significant negative trends along the midlatitude storm track in the northern Atlantic and positive trends in the maritime subcontinent (Nguyen et al., 2017). The integration between a radar and sensors that provide wider spatial and temporal coverage was more fully developed in the second collaboration between NASA and JAXA precipitation efforts resulting in the Global Precipitation Measurement (GPM) mission launched in February of 2014 (Hou et al., 2014). GPM not only extends the time series of climate-quality precipitation radar observations from TRMM, but it also extends the core satellite observational domain to high latitudes, and it formalizes the calibration concept developed during TRMM to make a consistent precipitation product from a global constellation of passive microwave and infrared sensors that can include models and both research and operational satellites to produce 5 km, 30-minute precipitation estimates globally. One key improvement in GPM over TRMM is the ability to predict global extreme precipitation. Whereas there is a 50% match between TRMM Level 3 precipitation and ground-based rain gauges for determining the extreme precipitation, this number rises to 60% for GPM (Huffman et al., 2017) demonstrating increased spatial (0.1° versus 0.25°) and temporal repeat (3 hours versus half hour) monitoring capability. This improvement in estimating extreme rainfall in ungauged regions of the world has important implications for engineering (Olsen, 2015; Libertino et al., 2016). The Integrated Multi-satellitE Retrievals for GPM (IMERG) combines precipitation estimates from all available passive microwave observations with gaps filled using geosynchronous infrared precipitation estimates (Hsu et al., 1997; Hong et al., 2004; Joyce et al., 2004; Kummerow et al., 2015). These products are bei ng produced today and are expected to continue improving as the community learns to more fully exploit the dual frequency precipitation radars on GPM, as well as inter-calibration procedures to the diverse instruments in the constellation (Berg et al., 2016). Further, by leveraging dual-frequency dual-polarization radar measurements, improvements are expected in the detection and quantification of light and moderate rainfall that represent a significant fraction of the total precipitation, and in orbital mapping of 3D storm structures at the mesoscale. Precipitation is a multiscale process spanning a wide range of scales from the raindrop and raindrop cluster scales ( ȝ m to m) to the scale storm cells (100 m to 10 km) to the scales of organized systems (~100 km; e.g. tropical cyclones, fronts). Continuity of passive microwave instruments, which currently provide the longest records of any geophysical variables derived from space observations, is essential to monitor the variability of global precipitation from decadal to interannual to daily time-scales, especially over the world’s oceans, and to provide the large-scale context (regional to continental-scale) to ongoing measurements of precipitation, soil moisture, sea ice, and other variables sensitive to water fluxes at the land- and ocean-atmosphere interfaces including at short time-scales as high revisit passive microwave spectrometry from space becomes available (e.g. TROPICS mission). Numerous discussions within the precipitation community, reflected in multiple white paper submissions to this Decadal Survey, indicate the need and desire to continue to advance the quality of spaceborne instantaneous precipitation measurements not adequately covered by GPM, and to refine the spatial and temporal resolutions of precipitation estimates. For the latter, in particular, there is growing consensus that the key to success in this area is better process understanding coupled with assimilation into convection-resolving models that can provide continuous analyses and forecasts of precipitation at 1 km and 5 minutes to 1 hour scales, thus approaching the capabilities of ground based radars over developed regions of the world today. Advancing process understanding to properly model precipitation, particularly ice microphysics that can be gleaned from combined Doppler radar and radiometer information (Bryan and Morrison, 2012; Varble et al., 2014), or assimilate precipitation and its latent heating in convection resolving models to forecast small-scale intense precipitation could possibly revolutionize how we view Earth observing satellites from stand-alone measurement platforms to integral components of coupled observing and modeling systems (Stephens and Kummerow, 2007). In the case of models with parsimonious microphysics, a strong case can be made that observing hydrometeor vertical velocities within the context of large scale environmental conditions, as established from reanalyses such as MERRA (Rienecker et al., 2011), will provide useful constraints on microphysical parameterizations to capture the vertical structure UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-17 Copyright National Academy of Sciences. All rights reserved.

257 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space of precipitation in models which currently is lacking (Wilson and Barros, 2014, 2017), and thus to make high quality high-resolution model based analyses and forecasts of precipitation a reality. Advancing the quality of spaceborne instantaneous precipitation measurements does not require large leaps in technology, but rather innovation with regard to the development of multi-frequency instruments and measurement strategies to significantly refine their horizontal and vertical resolutions. This will address continuing issues with orographic precipitation as well as improve the detection and quantification of shallow precipitation in complex terrain, snowfall and light drizzle (Duan et al., 2015). A swath of 100 km is then needed for the calibration reference to operational microwave imagers, as well as operational sounders and visible and infrared imagers used in geostationary satellites. Albeit complementary, snowfall (precipitation rate) and snow accumulation (precipitation amount) pose distinct measurement challenges. Snowfall accuracy at short-time-scales (minutes to hours) is critical for winter weather forecasting with major implications for transportation and energy security applications, even leading to a snow impact scale for Northeast storms (Kocin and Uccellini, 2004). Currently, quantitative remote sensing of snowfall from satellites is largely limited to X- or W-band radar (Kulie and Bennartz, 2009; Heymsfield et al., 2016) for dry snow. Snow accumulation evolves from pulse contributions from a small number of individual storms with large interannual variability (Lang and Barros, 2004; Lundquist et al., 2013) to form snowpacks that grow in depth and spatial extent though the winter depending on environmental conditions and snowfall history. In the transition from the cold to the warm season, seasonal snow melts over a short period, weeks to several months, to produce runoff that is an essential source of freshwater resources in the foothills of high mountain regions and their adjacent plains such as the western U.S. (Bales et al., 2006). Not surprisingly, numerous reservoirs have been built to capture snowmelt runoff to further extend water availability. The global volume of glaciers outside the polar ice sheets is also not accurately known owing to their uncertain thicknesses, but this number does not change as rapidly as SWE. Worldwide, snowmelt and glaciers support about two billion people (Mankin et al., 2015); mountain snowmelt and glaciers support over a billion (Barnett et al., 2005). In the mountains themselves, snowmelt provides essential soil moisture late into the melt season (Harpold and Molotch, 2015), and melting glaciers supply water throughout the melt season, which in some regions is otherwise a dry season. The global inventory of glaciers outside the polar ice sheets comprises 0.35-0.41 m sea level rise equivalent (Radi ü and Hock, 17 2010; Grinsted, 2013), or 1.3-1.5 × 10 kg. Compared to the global annual river runoff to the seas of ~4.4 16 × 10 kg (Clark et al., 2015), the nonpolar glacier mass is equivalent to 3-4 years of global river runoff. Regionally and locally, annual glacier melt throughput, and the smaller magnitude net annual mass balance of glaciers is extremely important for water resources and economies, especially for some arid and semi-arid countries. Albedo, areal extent (snow covered area, SCA), and snow water equivalent (SWE) are important metrics of seasonal snow accumulation. The extent and albedo of seasonal snow govern the surface energy budget of large regions of the world at high latitudes and at high elevations, and therefore play a critical role in the water cycle of large regions of the world with implications for the interannual variability of climate at global scales (Fletcher et al., 2009). Passive microwave estimates of SWE can succeed when radiative transfer models are coupled with snow hydrology models via data assimilation, but uncertainty and retrieval error increase significantly in complex terrain, when snow is wet, generally when SWE exceeds ~200 mm (deep snowpacks), and when tree canopy cover exceeds ~20% (Lettenmaier et al., 2015; Dozier et al., 2016). Measurements of snowfall, or of accumulated snow on the ground, constitute an important unsolved problem in the hydrology and water resources in most of the world. The next section addresses some approaches to this issue. Objective H-1c. Quantify rates of snow accumulation, snowmelt, ice melt, and sublimation from snow and ice worldwide at scales driven by topographic variability UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-18 Copyright National Academy of Sciences. All rights reserved.

258 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space As noted in the discussion of Objective H-1b, remote sensing of snowfall rate is difficult because of the huge difference in the dielectric properties between water and ice in the microwave spectrum. Even measuring snowfall at a meteorological station is difficult, because catching falling snow in windy conditions generally misses much of it (Yang et al., 2005). Instead, our historical knowledge of the distribution of snow accumulation comes from measurements of the snow on the ground, either by manual surveys (Church, 1933; Armstrong, 2014) or snow pillows that sense the weight of the overlying snowpack (Cox et al., 1978). In mountain regions, where much of the snowpack in the middle and lower latitudes lies, topographic heterogeneity along with deep snow causes substantial uncertainty in our assessment of a major component of the hydrologic cycle. Even in regions like the Western U.S. with an extensive snow pillow network, the pillows are on nearly flat terrain and in most basins do not cover the highest elevations, so in some years they poorly represent the spatial distribution of the snow and the total volume of water stored in the basin’s snowpack (Bales et al., 2006). In the river basins of California’s Sierra Nevada, for example, the interquartile error in the forecast of the April–July runoff is 213% to 135%, but the error distribution has long tails, both positive and negative (Lettenmaier et al., 2015). In the world's great mountain ranges where surface data are sparse, precipitation estimates from lowland measurements, numerical weather models, or changes in sizes of glaciers span a large range of uncertainty (Kääb et al., 2012; Kapnick et al., 2014). This is further complicated by the fact that at high elevations snowfall input is highly intermittent and typically associated with specific types of storms and regional conditions (Lang and Barros, 2004). Therefore, seasonal snow accumulation and snowpack condition strongly depend on local inter-storm hydrometeorology and occasional rain-on-snow events (Guan et al., 2016a), that control snowpack evolution between snow storms. At mid and high latitudes, including the Arctic and sub-Arctic regions, complex topography (and landform) also plays a critical role in the spatial organization of snow accumulation as well as snowmelt dynamics at the transition between the cold and warm seasons impacting snow cover extent and duration, snow water equivalent, and albedo which feedback into regional and global climate (Derksen et al., 2012). Although significant progress has been achieved to estimate snow water equivalent from passive microwave observations (Kelly and Chang, 2003; Li et al., 2015; Shi et al., 2016) and also using coupled physical and passive microwave models (Kang and Barros, 2012; Langlois et al., 2012), the spatial resolution generally is too coarse for hydrological and eco-hydrological research and applications. A promising new development addresses the resolution limitation by taking advantage of overlapping footprints, to bring spatial resolution at 36 GHz down to ~3 km at the expense of some reduction in signal-to-noise performance (Long and Brodzik, 2016). Other shortcomings of passive microwave retrievals of snow water equivalent remain, particularly its low saturation threshold of about 200 mm SWE (Lettenmaier et al., 2015; Dozier et al., 2016). In regions like High Mountain Asia, the sparse measurement network supports neither seasonal runoff forecasts nor validation of precipitation models. In regions like the Western U.S. where in situ measurements and snow-depth measurements from airborne lidar are available, validation of snowpack resources computed with numerical weather models, such as SNODAS (NSIDC, 2016), show discouraging results with significant under- and over-estimates (Clow et al., 2012; Hedrick et al., 2015; Bair et al., 2016). Figure 6.6 shows the importance of snowmelt in the water supply of the western U.S. Three years of drought from 2013 through 2015 brought California’s reservoirs and groundwater to historically low levels. The storms in the winter of 2017 replenished the reservoirs, but the groundwater remained depleted because of the extensive pumping during the drought (Margulis et al., 2016). Forecasts of the snowmelt runoff are based on a network of surface measurements, but because the sites seldom cover the highest elevations as pointed out above, considerable snow amounts remain on the ground even after point-scale sensors indicate snow-free conditions (Rittger et al., 2016). The statistically based forecasts on average perform acceptably well, but occasionally generate errors of nearly a factor of two (Dozier, 2011). Therefore, estimating the spatial distribution of snow water equivalent (SWE) in mountainous terrain, characterized by high elevation, steep slopes, and spatially varying topography, is an important unsolved problem in mountain hydrology. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-19 Copyright National Academy of Sciences. All rights reserved.

259 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Figure 6.6. A five-year drought in California ended spectacularly in the winter of 2017, with the state emerging from one of its driest periods on record by enduring a wet winter. A steady stream of storms brought 175 percent of the long-term average of rain and snow to California between October 2016 and April 2017, The left image from the Landsat 8 Operational Land Imager (OLI) shows the tan bands around the shoreline of Trinity Lake, California’s third largest reservoir, in April 2015, when the reservoir’s level was at 59% of the historical average; the right image shows the reservoir in April 2017, with the level at 114% of the average. Source: NASA, https://earthobservatory.nasa.gov/IOTD/view.php?id=90062 Coupled with the problem of knowing the total quantity and spatial distribution of the snow accumulation are measuring and predicting its rate of melt, relating the rate of melt to environmental drivers, and the consequences of the rate and distribution of melt for water resources, glaciers, and ecosystems. The main drivers, absorption of solar and longwave radiation, vary with the solar geometry, atmospheric scattering and absorption, and illumination variability caused by topography (Marks and Dozier, 1992; Marks et al., 1992). Estimating these surface fluxes is also crucial to Objective H-1a, which requires addressing the components of the surface energy balance. As with any process driven partly by absorbed solar radiation, variability in snow albedo causes variability in the rate of melt. ሺ ሻ ߙ , an error in the measurement of albedo leads to a greater For surfaces with high albedo proportional error in absorption of the solar radiation (absorption ൌͳെߙ , so for greater values of ߙ closer to 1.0, a small error in ߙ causes a greater proportional error in ߙͳെ ). Changes in snow albedo are tied to changes in snow microstructure, specifically grain growth that reduces snow albedo at UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-20 Copyright National Academy of Sciences. All rights reserved.

260 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space wavelengths beyond about 1 m, and contamination by absorbing aerosols like dust and soot (Warren, μ 1982). These issues are discussed in the material about Objective H-2b. Science & Societal Goal H-2. Prediction of Changes How do anthropogenic changes in climate, land use, water use, and water storage interact and modify the water and energy cycles locally, regionally and globally and what are the short- and long-term consequences? Objective H-2a. Quantify how changes in land use, water use and water storage affect evapotranspiration rates, and how these in turn affect local and regional precipitation systems, groundwater recharge, temperature extremes, and carbon cycling Humans have altered the landscape by changing the vegetation cover over centuries but with increasing intensity over the latest decades. As a result, fluxes in the water cycle have already changed. Specifically, the evapotranspiration flux and the terrestrial surface water budget have changed dramatically over the historical record as a result of human alteration of the landscape. Because sensible and latent heat fluxes are fundamentally coupled by thermodynamics, these changes have already had significant impact on the terrestrial surface energy budget, including surface temperature and outgoing longwave radiation. 6 The large enthalpy of vaporization (2.5×10 J/kg) makes the latent heat flux due to evaporation a major term in the surface energy balance. Only one-fifth of the solar energy available to the Earth system is directly absorbed in the atmosphere. Half of the solar energy is first absorbed by the surface, and then latent heat flux and longwave radiation transfer it to the atmosphere. The latent heat flux is the most efficient dissipation mechanism available to return the surface to thermodynamic equilibrium upon solar forcing, and a major mechanism in zonally redistributing energy from the tropics, including the tropical oceans, to the higher latitudes. Latent heat flux and variations in it due to limiting factors over land such as availability of soil moisture are thus a major factor in the thermal forcing of the atmosphere at its base. It is also a source of moisture for the atmosphere that plays an important role in the formation of clouds, development of convection, and ultimately precipitation from local to regional scales (Aragão, 2012; Sun and Barros, 2014). By its control over buoyancy generation and moisture supply at the base of the atmosphere, evapotranspiration has a large influence on maintaining regional climate and affects the evolution of weather (Betts et al., 1996). In turn, small changes in the magnitude, seasonality and intermittency of precipitation and radiation can be magnified in the evapotranspiration signal. As a result, the future of evapotranspiration under a changing atmospheric composition may be even more uncertain. Because evapotranspiration is also a key conduit for biogeochemical substances, it is also critical to Earth’s biogeochemical cycle. Understanding how evapotranspiration has already changed and what consequences its changes have on ecosystems health, crop productivity, and climate are priority questions in Earth System Science (National Research Council, 1999; Canadell et al., 2000; Bondeau et al., 2007). Despite the importance of quantitative information on this flux and its historical change, only imperfect measurements—in situ or by remote sensing—provide estimation and mapping of evapotranspiration over regional or global areas. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-21 Copyright National Academy of Sciences. All rights reserved.

261 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Figure 6.7. Differences in ET (right panel) and GPP (mid-panel) for model simulations with and without the contribution of precipitation produced along the trajectories (black dashed lines) of land-fallen hurricanes (Tropical Cyclones, TCs) in the SE USA during September of 2004. Differences are calculated as monthly averages of (With-Without) simulation results. The right-hand panel shows the distribution of dominant soil texture over the region. Spatial resolution is 4 km × 4 km. Note the significant impact of TC precipitation on GPP and ET in the Piedmont region where shallow clay-rich soils predominate, and thus soil water storage in the root zone is limited. Source: Lowman, L. E. L., and A. P. Barros. 2016. Interplay of drought and tropical cyclone activity in SE U.S. gross primary productivity. Journal of Geophysical Research: Biogeosciences. 121:1540-1567. doi:10.1002/2015JG003279. Reprinted with permission; copyright 2016, American Geophysical Union. The water and carbon cycles are tightly linked via complex nonlinear feedbacks. Vegetation type and condition determine surface radiative properties (e.g. albedo) and root zone soil moisture uptake, and in turn root zone soil moisture availability modulates stomatal conductance, and consequently evapotranspiration (ET), and photosynthesis, and consequently Gross Primary Productivity (GPP). The photosynthesis process governs the metabolism of plants and it links the loss of vapor from the plant and the gain of carbon for biomass growth from the atmosphere. ET and GPP exhibit large spatial variability with topography, soil type, and land-use, as well as large seasonal and interannual variability with precipitation especially during the warm season (Yang et al., 2015; Lowman and Barros, 2016). Quantifying evapotranspiration and understanding its linkages is a grand challenge for Earth System Science in the coming decade, given the following principles and observations: x the central role of evapotranspiration in coupling the global water, energy and biogeochemical cycles; x the importance of the flux to the health and productivity of natural and agricultural ecosystems; x the already-realized several-fold changes in evapotranspiration through human alterations of the landscape; x the potential for amplification of these changes under climate change; and x the paucity, or total lack, of any direct estimates of evapotranspiration regionally and globally. The rates and spatial patterns continue to change as humans further modify the physical landscape and alter the vegetation cover. To understand their historical change, current state and future outlook, a more complete understanding of the processes that drive variations in these fluxes is essential. Basic questions include: UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-22 Copyright National Academy of Sciences. All rights reserved.

262 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space 1. How do the rates of evapotranspiration respond to human alterations to the physical landscape and These changes remain as gaps in our vegetation cover and to shifts in climate and its seasonality? knowledge of how the Earth System works. Understanding them is essential if we are to become better stewards of the terrestrial biosphere that we have appropriated so pervasively. Evapotranspiration flux can experience changes that amplify shifts in the precipitation and radiation forcing. Knowledge of changes in their magnitude and regional patterns in the future are critical to understanding the impacts of climate change. 2. How does the rate of evapotranspiration respond to changes in precipitation and radiative forcing? These rates are not understood and they are a source of uncertainty in assessment of climate change impacts (National Research Council, 2012). Evapotranspiration is a flux at the land-atmosphere interface. Its spatial variations are strongly related to soil type, topography, vegetation and climate. Their dynamics are affected by the variations in plant growth, weather and seasonal climate. To adequately characterize them, mapping at tens to hundreds of meters and temporal sampling at days to a week are needed at minimum (National Research Council, 2004). In situ monitoring is not a viable approach for collecting the required data. Observations are needed that span large areas, because installing and maintaining instrumentation at even a single site is costly and challenging (Baldocchi et al., 2001). In this regard, the upcoming ECOSTRESS space station mission provides a pathway to long term, space borne measurements needed for high resolution evapotranspiration estimates and to improve the remote sensing algorithms relying on the relationship between land- surface temperature (LST) and evapotranspiration. Also as a flux, evapotranspiration cannot be directly sensed as the rates do not uniquely correspond to the thermal or dielectric state of the soil and the vegetation at one level. Rather they are controlled by vertical and temporal gradients of state variables. Soil moisture is the fundamental state variable that directly controls evapotranspiration (Pollacco and Mohanty, 2012) . Vertical gradients in soil moisture drive evapotranspiration and water availability to plant roots. Vertical profiles of soil moisture need to be measured or estimated via integrated models and observations systems (e.g. H1a) in order to allow estimation of evapotranspiration. SMAP measures soil moisture in the top 5-10 cm and has enabled understanding of links between precipitation, surface soil moisture, and energy fluxes at very coarse spatial scales (10s km). Information about the top meter of the soil at spatial resolutions that capture the spatial variability in precipitation, energy fluxes and shallow subsurface flows would enable closing the water budget including water use by vegetation in the root zone. Objective H-2b. Quantify the magnitude of anthropogenic processes that cause changes in radiative forcing, temperature, snowmelt, and ice melt, as they alter downstream water quantity and quality. Snow is the brightest land cover in nature. In the solar spectrum, snow has a distinctive spectral signature—among the brightest natural substances in the visible wavelengths, reduced slightly in the near- infrared beyond 1 μ m, and dark beyond about 1.6 μ m in the shortwave-infrared—corresponding to the variability in the absorption properties of ice (Warren, 1982; Warren and Brandt, 2008). In the visible wavelengths, both ice and water are transparent to radiation, whereas in the shortwave-infrared both are strongly absorptive. Because snow is so distinctive, mapping of snow covered area was one of the first applications of remote sensing in the hydrologic sciences (Lettenmaier et al., 2015), and the combination of visible and shortwave-infrared bands enables discrimination between snow and clouds (Crane and Anderson, 1984). Characterization of snow and its rate of melt is critical for understanding the Earth system, and its role in regional hydrology for those river basins where people depend on snow- or glacier-melt for water resources. Snow’s high but variable albedo and low thermal conductivity together sustain stability of the boundary layer over vast regions (Levis et al., 2007). Our understanding of the strength of the simulated snow albedo feedback, however, varies by a factor of three in global climate models (Lemke et al., 2007), mainly attributed to uncertainties in snow extent and the albedo of snow-covered areas from imprecise UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-23 Copyright National Academy of Sciences. All rights reserved.

263 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space remote sensing retrievals (Flanner et al., 2009; Fletcher et al., 2009). Snow cover and its melt also ¿ dominate regional hydrology over much of the world. Not only does one- fth of Earth’s population depend on snow- or glacier-melt for water resources, people in these areas generate one-fourth of the global domestic product (Barnett et al., 2005). While long-term observations in many mountain ranges worldwide show a declining snowpack attributable to global warming (Mote et al., 2005; Shekhar et al., 2010), and thus declining glaciers caused by the overlying snow melting earlier in the spring, an equally important anthropogenic contribution lies in the increase in carbonaceous aerosols from combustion and dust from land degradation, darkening the snow and causing its warming due to greater absorption of solar radiation that accelerates melting (Painter et al., 2007; 2013; Kaspari et al., 2014). Earlier snowmelt also warms the climate indirectly by changing terrestrial radiative properties (albedo and emissivity) by earlier exposure of the underlying soil and vegetation. Locally, forest fires yield a source of charcoal that affects snow albedo for many years following the fire (Gleason et al., 2013). Earlier snowmelt affects the seasonal distribution of streamflows along with the quality of that water depending on the wet and dry atmospheric deposition of particles and chemicals into the snowpack (Williams and Melack, 1991a, 1991b). Moreover, management of forests implies management of water; for example in warmer climates, forest thinning retards the rate at which snow disappears (Lundquist et al., 2013). For these reasons, understanding and managing water from snow- and glacier-dominated basins requires tracking the energy sources that melt the snow and thereby the spatiotemporal distribution of snow and ice properties, especially its albedo as it varies with grain size and presence of absorbing aerosols (Warren, 1982), dust, and rock debris. The same processes govern the health of glaciers that comprise the iconic features of many areas of the world, as they incorporate the history of snowfall during the accumulation season and snow- and ice-melt during the ablation season. Objective H-2c. Quantify how changes in land use, land cover, and water use related to agricultural activities, food production, and forest management affect water quality and especially groundwater recharge, threatening sustainability of future water supplies. Agricultural activities involve the conversion of preexisting land uses into pasture or crops, and in many parts of the world, entail managed forest clearing. Most of the heavily irrigated regions were converted from grasslands or from other non-forested systems. Specifically, the impact of conversion of native land to agriculture alters the terrestrial water cycle in both quantity and quality. These changes in the land use and land cover affect infiltration, surface runoff, recharge to the groundwater, water quality, sediment loss, and surface albedo, as well as changes in the temporal dynamics of all of these processes. The water quality variables affected include erosion and sediment loss, total dissolved solids, nitrogen in the forms of nitrate or nitrite, and phosphorus. Impacts to the hydrologic cycle differ in non-irrigated and irrigated systems. In the non-irrigated, rain-fed case, the water input to the land does not change appreciably except due to weather and climate variability and change. Evapotranspiration, however, may increase or decrease due to changes in the vegetation water demand as well as rising temperatures and wind forcing due to climate change, affect groundwater levels, and in turn, streams and other groundwater-dependent ecosystems. The changes can be significant but difficult to determine, making unclear the cause-effect links between land use change and water quantity and quality. A particular challenge in rain-fed systems is the difficulty of resolving the precipitation and evapotranspiration sufficiently accurately to estimate groundwater recharge. As in natural ecosystems, the error in evapotranspiration measurements or estimates often exceeds the magnitude of groundwater recharge, a challenge to coupling surface and subsurface hydrologic processes. Much of the world’s food supply comes from irrigation in dry to moderate climates. Consequently, by far most of the water diverted, impounded or pumped by humans is for irrigated agriculture (Wada et al., 2013; 2014). Accordingly, irrigated agriculture has created massive dislocations in water stores and disruptions in the hydrologic cycle, as documented by GRACE (Richey et al., 2015), with the record expected to continue with GRACE Follow-On. In systems where substantial surface water UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-24 Copyright National Academy of Sciences. All rights reserved.

264 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space is available such as California, diversion of surface water for irrigation has caused massive increases in recharge and rising groundwater levels (Williamson et al., 1989; Faunt et al., 2009). In irrigated areas of California where groundwater pumping is not sufficiently high, elevated groundwater levels have caused soil salinity problems akin to those that caused the collapse of agriculture in Mesopotamia by circa 2300 BCE. Conversely, in many other areas of California excessive, uncontrolled pumping of groundwater has caused groundwater deficits and the attendant undesirable effects, including land subsidence, non- sustainable storage depletion, water quality degradation and increased energy costs (Cannon Leahy, 2016). The above themes where irrigated agriculture causes either water logged soils or non-sustainable groundwater depletion have been rather common occurrences throughout the world, including the Great Plains and southwestern U.S, North China Plain, India, North Africa, Australia and so on. Concomitant with the agricultural water quantity problems are ongoing degradation in groundwater quality owing to salt, nitrate and other contaminants inherent to agricultural practices. Worldwide, increasing difficulty in sustainably managing water quantity and quality, whether in the subsurface or on the surface, remains a major challenge to soil conservation, food production and the future of human civilization. This worsening situation calls for combined management of surface water and groundwater that is only possible through advanced, multiscale measurement of all the major water stores and the fluxes between them, with special emphasis on groundwater stores, evapotranspiration, precipitation, recharge, and surface water flows. While recharge in irrigated regions is reasonably well estimated based on knowledge of crop- water demands, amounts of applied water, and irrigation efficiencies, estimation of recharge in non- irrigated lands is much more challenging. There are several studies on recharge quantification. Scanlon et al. (2006) studied recharge in 140 sites in semi-arid and arid regions using the chloride mass balance technique. They found a longer scale variability of recharge rates that depends on El Niño/La Niña and other atmospheric and oceanic variability, and that in relatively warm arid and semiarid regions with thick vadose zones that little or no upland recharge occurs due to native plant adaptation and thermal gradients forcing water vapor upward. Furthermore, regional hydrogeology and the spatial organization of preferential recharge zones and pathways vis-à-vis precipitation patterns play an important role in redistributing groundwater regionally in space and time at multiple scales (Barros et al., 2017). Concentration of runoff in ephemeral channels or areas of high infiltration rates or karstic zones are necessary to overcome thermal gradients and low conductivities and high suction terms in unsaturated zone soils (Scott et al., 2000; Goodrich et al., 2004; Coes and Pool, 2007). Scanlon et al. (2007) carried out a comprehensive study of the impact of agriculture on water resources, both water quality and quantity. In some instances, the conversion of native vegetation to croplands could increase the recharge to the aquifers but degrade the water quality. However, the exact nature of this balance depends on the type of vegetation being replaced. In general conversion to agriculture increases the consumption of water and decreases streamflow and raises the water table and in some regions of the world causes waterlogging. However, groundwater fed irrigation generally lowers the water table in many parts of the world (Northern India, High Plains and Central Valley USA). As observations for estimation of changes in the terrestrial water cycle due to conversion of land cover from native to agriculture are limited, there has been a use of models such as the Soil Water Assessment Tool (SWAT) to study this issue (Arnold and Fohrer, 2005), and the Variable Infiltration Capacity (VIC) model has been used in the Great Lakes Region to study the impact of the conversion of forests and prairie grasslands into agriculture (Mao and Cherkauer, 2009). One of the earliest and most influential studies (Allan et al., 1997) emphasized the joint management of land and water and the fact that the two were deeply connected, and that the fractional area of agricultural land within a catchment was the best indicator of stream water quality (higher sediment and nutrient concentrations) based on a study of the River Raisin Basin in southeastern Michigan using a 20-year (1968-1998) study period. Similar land use impacts have been observed in water chemistry (phosphorus P, nitrogen N, and total dissolved solids TDS) in the Saginaw Bay catchment of Central Michigan (Johnson et al., 1997). Phosphate and nitrate in agricultural fertilizer and TDS are mobilized during storms due to the erosive nature of the rainfall impact UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-25 Copyright National Academy of Sciences. All rights reserved.

265 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space that dislodges the soil and end up in surface runoff (Hart et al., 2004; Ahearn et al., 2005) thus linking regional weather and climate, precipitation physics, and water quality. Science & Societal Goal H-3. Availability of Freshwater and Coupling with Biogeochemical Cycles How do changes in the water cycle impact local and regional freshwater availability, alter the biotic life of streams, and affect ecosystems and the services these provide? One of the biggest challenges facing human society is the availability of freshwater where it is needed, when it is needed. In fact, at times there is lack of water (droughts) and yet at other times there is too much water (floods). Global climate change, population and economic growth, land-use change, water quality degradation, and aging infrastructure are altering water availability and demand equilibrium at every scale (Sun et al., 2008; Padowski and Jawitz, 2009; Hering et al., 2013; Ajami et al., 2014). Coping with these changes and enhancing adaptive capacity of any region relies on informed and sustainable management of our limited water resources. Access to high resolution (both spatial and temporal) water quantity and quality data is key in enabling integrated water management and enhancing local human and ecosystem health. Objective H-3a. Develop methods and systems for monitoring water quality for human health and ecosystem services While water availability, environmental and ecosystem health, and social and economic wellbeing are directly affected by water quantity and quality, current Earth observation missions are not fully equipped to measure the quality of inland water bodies such as lakes, rivers, reservoirs, and estuaries, at an appropriate temporal and spatial scale, and enabling technology is still lacking. In addition, there are many water quality indicators that vary independently while having a combined effect on the remote sensing signals (Ampe et al., 2015). This phenomenon makes the sensing process much more complex and in need of methodologies that could invert gathered signals to effectively infer the accurate information (Chang et al., 2015). In recent years, there have been some attempts to infer water quality data and map related environmental and biogeophysical processes using the mosaic of available remotely sensed measurements (Usali and Ismail, 2010; Ampe et al., 2015; Chang et al., 2015; Fichot et al., 2015). However, these are limited in size and scope and they still need in situ data to establish and validate the inference methodology. To advance water quality sensing from space we need to understand: (1) Availability and potential utility of various sensing approaches (visible, infrared, and microwave) and how these technologies can be combined to monitor and measure water and ecosystem dynamics; (2) the temporal and spatial scales governing process dynamics and the measurement resolution needed to inform decision making process at the local level; (3) availability of inversion methodologies that have to be developed to leverage satellite data more effectively; and (4) the cost-benefit analysis of remote-sensing strategies (e.g. drones) alternative to space missions. Objective H-3b. Monitor and understand the coupled natural and anthropogenic processes that change water quality, fluxes, and storages, in and between all reservoirs, and response to extreme events In many parts of the world, complete transformation of land cover including deforestation, extensive row crop agriculture, and urbanization are affecting the eco-hydrology at local, regional, and continental scales. Change of fluxes and storages, and the transport and residence times of water in the terrestrial part of the landscape, in the streams, and in the subsurface affect bio-geochemical processes and impact water quality and stream biology. A prime example is the Midwestern U.S. where intensified UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-26 Copyright National Academy of Sciences. All rights reserved.

266 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space row crop agriculture—mainly corn and soybeans providing 40% of the global supply—along with drainage of wetlands and the extensive sub-surface tile drainage system needed to keep the plant roots dry during the growing season, has increased streamflow volumes and peaks (Guanter et al., 2014), which together with increased precipitation over the past decades (Groisman et al., 2012) have accelerated near- bank erosion, and increased nutrient loads contributing to the Gulf of Mexico hypoxia (Zhang and Schilling, 2006; Novotny and Stefan, 2007; Blann et al., 2009; Schilling et al., 2010; Belmont et al., 2011; Foufoula-Georgiou et al., 2015). Similar changes are observed in coastal areas where climatic and human impacts (e.g., sea level rise and local and upstream basin development that reduce water and sediment to the coast accelerating subsidence) contribute to increased salt water intrusion, coastal erosion, degradation of protective vegetation and marshes, and decline of unique biodiversity (Ericson et al., 2006; Syvitski et al., 2009; Tessler et al., 2015). Remote sensing observations of multiple coupled variables are needed therefore to achieve Objective H-3b (e.g., rainfall, soil moisture, photosynthesis, sediment/water interfaces, river migration rates, water stages, and vegetation composition) at spatial and temporal scales appropriate for detection of trends, local predictive modeling, and mitigation actions on the ground including urban expansion, deforestation, and agricultural practices. Collecting such comprehensive data sets which is prohibitive on the ground is necessary to understand and model the coupled human-natural system in an integrated Earth systems modeling perspective. Of special interest is the effect of extremes on the eco-hydrologic trajectories of landscapes at seasonal, annual and multi-decadal time scales and implications for global water cycling, landscape connectivity, and carbon storage. Objective H-3c. Determine structure, productivity, and health of plants to constrain estimates of evapotranspiration The components of evapotranspiration include: (1) transpiration from plants; (2) rain or snow intercepted by the plant canopy that subsequently evaporates or sublimates; and, (3) evaporation from the soil surface. Plant species, structure (individual and within a complex stand with understory), leaf area, and stomatal density all affect transpiration and interception. Improved knowledge of plant structure and its change over time (productivity and health) will therefore translate directly into improved estimates of evapotranspiration. Spectral remote sensing and associated retrieval methods have proven successful in identifying monoculture or spatially dominant plant species with automatic and supervised classification algorithms. Likewise, numerous vegetation indices have been developed to successfully identify plant chlorophyll, photosynthetically active vegetation, plant vigor, and the phenological cycles of annual and deciduous plant species (Huete et al., 2002). Spectral methods have had less success in complex multispecies stands where overstory shading, interwoven branches, and understory plants are present. In these instances, leaf area index (LAI) and surface roughness affecting canopy conductance are not well estimated (Dingman, 2014). Even within a monoculture stand some species such as cottonwood ( Populus fremontii ), LAI cannot be accurately estimated with spectral remote sensing and allometric relationships change with tree age (Farid et al., 2006). Multi-return and waveform lidar can address many of the shortcomings of spectral-based remote sensing noted above. Lidar employs a laser system on a platform that transmits laser pulses toward any object of interest at rates up to hundreds of thousands per second. The laser energy interacts with the object (e.g., vegetation, terrain, and structures) and some of the energy is reflected back toward the lidar receiver. If the position and orientation of the moving or stationary lidar instrument platform is known in time and space, the x, y, and z coordinates, and return intensity of the reflected portion of the laser pulse can be determined. Airborne, spaceborne, mobile, drone-based, and terrestrial laser scanning systems are advancing rapidly (Lefsky et al., 2002; Jensen, 2006). One National Academy study concluded (National Research Council, 2007b) that lidar should be acquired over the entire continental United States based solely on the benefits to improved flood plain mapping for the Federal Emergency Management Agencies Map Modernization program. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-27 Copyright National Academy of Sciences. All rights reserved.

267 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Lidar has been used extensively for forest inventory and structure (Lim et al., 2003), biomass and carbon stocks (Asner et al., 2012), estimation of leaf area index (Korhonen et al., 2011), and to constrain and improve evapotranspiration estimates (Farid et al., 2008; Mitchell et al., 2012). More recently, single- photon and Geiger lidar are being employed in non-defense applications. These new systems split a single laser pulse into numerous sub-pulses, which are then detected with segmented detectors. They offer higher effective pulse rates (~200 million samples per second) over linear mode lidar (~500,000 samples per second) with lower power needs (1-2W for single photo and 20-40W for Geiger) (Jasinski et al., 2016). A single photon lidar device is being deployed on the upcoming 2017 ICEsat-2 mission in the ATLAS instrument (Advanced Topographic Laser Altimeter System) with the primary goal of polar ice sheet thickness measurements (Abdalati et al., 2010). ATLAS will employ one laser split into six beams at roughly 10,000 pulses per second. If advances in lidar technology can continue apace so that within 10 to 15 years, spaceborne platforms can provide 20-30 return points per square meter that airborne platforms currently provide, a rich set of attributes on vegetation structure and productivity could be derived to constrain and improve evapotranspiration estimates. The additional benefits of these data to hydrology include improved topography for flow path and drainage derivations, surface water levels, snow depths, and near shore bathometry. Benefits to other disciplines would also be numerous, including landslide characterization, post-fire erosion, and post-fire recovery for hazards; carbon inventories for ecosystems, and ice volumes and depths for solid Earth. Science & Societal Goal H-4. Hazards, Extremes, and Sea Level Rise How does the water cycle interact with other Earth System processes to change the predictability and impacts of hazardous events and hazard-chains (e.g. floods, wildfires, landslides, coastal loss, subsidence, droughts, human health, and ecosystem health), and how do we improve preparedness and mitigation of water-related extreme events? Objective H-4a. Monitor and understand hazard response in rugged terrain and land-margins to heavy rainfall, temperature and evaporation extremes, and strong winds at multiple temporal and spatial scales This socio-economic priority depends on success of addressing H-1b and H-1c, H-2a, and H-2c. Natural disasters pose major threats to the livelihood and security of millions of people worldwide. Increasing trends of natural disaster impacts can be linked to amplification of the hydrologic cycle from climate-related events (Field et al., 2012), dramatic changes in land use and land degradation, and overpopulation in at-risk areas (e.g., coastlines, river margins, estuaries and deltas, mountainous regions) (Oki and Kanae, 2006). Recent estimates indicate that over 88% of natural disasters are water related (Adikari and Yoshitani, 2009) and are one of the greatest global threats to socio-economic development. Depending on the nature and location of the event, the number of people affected by natural hazards can range from hundreds to thousands (e.g., landslides), to several million (e.g., regional flooding and agricultural drought). Lasting impacts of natural hazards include significant damage to infrastructure, land degradation, loss of life, economic loss, and disease, and combinations thereof. Sustainable risk reduction of natural hazards consists of prevention and preparation as well as mitigation and response. Thus, it requires not only proper characterization and understanding of the events themselves, but also sufficient modeling and prediction of the processes that underlie and drive the hazards. Given the nature of water-related disasters, impacts from too much or too little water can result in a wide array of events including coastal and lake floods, flash floods, hurricanes, landslides and avalanches, subsidence, agricultural droughts, and water-borne epidemics. A key question to understanding these events is: How does the water cycle interact with other Earth System processes to change the probabilities, magnitudes, and frequency of these events? To this end, improved understanding of the quantification of their dynamics and impacts is needed. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-28 Copyright National Academy of Sciences. All rights reserved.

268 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Hazard process monitoring and modeling in at-risk areas such as mountainous terrain and land- margins such as coastlines, floodplains, deltas and estuaries, and land-water interfaces generally is particularly needed. Some key measurement variables common to these Earth System process and water- related hazards include precipitation, soil moisture, snowmelt, water depth, water flow, and atmospheric water vapor (for monitoring and predicting certain weather conditions). The ability to measure these variables at the temporal and spatial scales consistent with event dynamics is crucial for improved hazard response. For example, severe thunderstorms can be on a local (1-10 km) spatial scale and can evolve in a matter of minutes (Schmidt et al., 2009; Mathew et al., 2014). On the other hand, several slow-onset drought events have occurred in the past decade (Long et al., 2013; AghaKouchak et al., 2014) with magnitudes and intensities leading to regional and longstanding (months to years) impacts. Several applications of Earth Observations exist for improved hazard mitigation. In mountainous regions, frequent measurements of near surface soil moisture and of snow water equivalent (SWE) coupled with physically-based models can improve landslide susceptibility mapping by estimating antecedent soil moisture conditions and snowmelt and isolating the triggers for the onset of spring season landslides. Regular monitoring of streamflow can enable improved management of risk and water strategies in flood-prone areas. Evapotranspiration has been shown to be a critical hydrologic variable for capturing drought magnitude, intensity, and timing (Anderson et al., 2011; Fisher, 2014). At low elevations, along the land-margins of the world’s major rivers and coastlines, surface water elevation measurements from the Surface Water and Ocean Topography (SWOT) mission can potentially improve inundation mapping as well as serve as boundary conditions for high resolution models of rivers, storm surges, and coastal circulation. In addition to providing increased hazard security and forecasting, key questions to be addressed by the implementation of Earth observations needed for monitoring and modeling hazard response include the following: x How are changes in land use affecting evapotranspiration rates, and how do these in turn affect local and regional precipitation systems, temperature extremes, and carbon cycling? x How does the water cycle interact with other Earth System processes to change the probabilities and impacts of hazardous events and hazard-chains such as floods, wildfires, landslides, coastal loss, subsidence, droughts, and human and ecosystem health? x How do we improve preparedness and mitigation of water-related extreme events using measurements and integrated observing systems and models? x Can we improve our understanding of trigger mechanisms to improve predictions for all hazards and flooding and landslides in particular in headwater basins and along land- margins? x How do we improve our understanding of post-hazard landscape response using integrated systems and models toward more effective recovery and preparedness? Objective H-4b. Quantify key meteorological, glaciological, and solid Earth dynamical and state variables and processes controlling flash floods and rapid hazard-chains to improve detection, prediction, and preparedness This socio-economic priority depends on success of addressing H-1b, H-1c and H-4a. Floods and other hazards, like landslides, can devastate communities. In 1999, combined flash floods and landslides claimed the lives of 30,000 Venezuelans, displaced an additional 110,000 residents and destroyed 23,200 homes (IFRC, 2000). In July of 2012, a 7 m wall of water raced through the town of Krymsk in southern Russia killing 170 people and displacing 13,000 (Russia Today, 2012). Increased risk of persistent and intermittent inundation due to the interplay of More runoff in flat terrain is linked to increased storm activity, sea level rise, and storm surges, within the context of coastal landscapes hardened by built UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-29 Copyright National Academy of Sciences. All rights reserved.

269 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space infrastructure; the resulting increased risk of persistent and intermittent inundation poses a significant societal challenge (Tebaldi et al., 2012; Hallegatte et al., 2013). Figure 6.8. Predicted high tides at Boston near or exceeding the “nuisance flood” level of 68 ௗ cm above mean high water, and their relationship to sea level rise. Before 2010, the tides alone never exceeded flood levels; from 2011 onward, and likely into a future climate, sea level has risen and will rise sufficiently that tides alone can produce nuisance (i.e. frequent low-impact flooding). Catastrophic flooding that results in system failure can occur if a storm occurs on top of a high tide. Source: Ray, R. D., and G. Foster. 2016. Future nuisance flooding at Boston caused by astronomical tides alone. Earth's Future. 4:578-587, figure 5. doi:10.1002/2016EF0004 23. Reprinted with permission; copyright 2016, American Geophysical Union. Figure 6.8 shows how predicted sea-level rise interacts with storms to increase the probability of catastrophic coastal floods in Boston. In the U.S., the 2015 damages from flash floods reached $2.1B with 129 deaths (NOAA, 2015). While other flooding hazards have a broad impact on U.S. society, flash floods are the ones that consistently kill more people (Ashley and Ashley, 2008). Figure 6.9 provides a synopsis of reported flashflood occurrences over a five-year period in the continental U.S. Rainfall induced landslides are equally destructive. The USGS estimates the annual cost of landslides in the U.S. is between $2B to $4B and 25 to 50 people are killed by them (USGS, 2016). Worldwide, between the years 2004 and 2010, 2620 deadly landslides killed 32,000 people (Petley, 2012); though not all were rainfall induced, the great majority were. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-30 Copyright National Academy of Sciences. All rights reserved.

270 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Figure 6.9. National Weather Service reports of flash flooding from October 1, 2006 through December, 31 2011. Flash Flood points correspond to river gauge locations; polygons represent areas impacted by the flashflood, often coinciding with county boundaries. Note that flash-floods are largely underreported as many occur in ungauged basins, which frequently are not populated. Source: Gourley, J. J., Y. Hong, Z. L. Flamig, A. Arthur, R. Clark, M. Calianno, I. Ruin, T. Ortel, M. E. Wieczorek, P.-E. Kirstetter, E. Clark, and W. F. Krajewski. 2013. A unified flash flood database across the United States. Bulletin of the American Meteorological Society. 94:799-805, figure 3. doi:10.1175/BAMS-D-12-00198.1. Reprinted with permission; copyright 2013, Bulletin of the American Meteorological Society. Flash floods and shallow, rainfall-induced landslides are caused by high-intensity, high volume precipitation events coupled with the proper hydrological scenario which is defined by current soil moisture, the slope, shape and soil types of the basin, the impervious region in the basin, and the built drainage structures of the basin. As our climate warms, the hydrologic cycle is shifting toward an overall increase in heavy precipitation events across the U.S. (Groisman et al., 2004; 2005). The risk of and impact of flash floods and landslides increases with the frequency of intense or long-duration precipitation. Figure 6.10 shows the global distribution of landslide occurrences overlain on a global landslide susceptibility map (Kirschbaum et al., 2009). Because both hazards depend on precipitation and the physiographic characteristics of a region, both offer the possibility of mitigation through Early Warning Systems based on observed and forecast precipitation intensity coupled with a model of regional topography, geology, and land-use and land-cover (Hossain, 2006; Hong et al., 2007; Gourley et al., 2011; Sättele et al., 2015). Although uncertainty in estimating precipitation intensity in mountainous regions, as well as ambiguity with regard to the definition of susceptibility index classes, pose challenges to forecast skill, this methodology was used to increase situational awareness to the U.S. Army and its partners during the Peruvian floods and landslides in 2015-2016 linked to a strong El Niño. NASA global Precipitation Measurement (GPM) satellite precipitation estimates are already being used for flood and landslide now-casting purposes and to quantify risk (Wu et al., 2014; Kirschbaum et al., 2015; Stanley and Kirschbaum, 2017). Further, these models can be coupled to hyper-resolution physically-based models to pinpoint hotspots of slope instability and warnings (Tao and Barros, 2014b). UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-31 Copyright National Academy of Sciences. All rights reserved.

271 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Figure 6.10. Global landslide susceptibility index (Hong et al., 2007) plotted with the 2003 and 2007 landslide inventory data. The index integrates slope land-cover change, geology, road presence, and operational nowcasts of landslide potential are made by checking whether 1-day, 3-day and 7-day precipitation intensity thresholds and accumulations are met. Further details and up-to-date forecasts are available at https://pmm.nasa.gov/precip-apps . Source: NASA While an Early Warning System cannot stop a 7-m wall of water from descending upon a city, an Early Warning System can provide the lead-time necessary for people to move out of harm’s way. As noted by Di Baldassarre et al. (2010), Early Warning Systems were critical in reducing the impacts of floods in India and the Czech Republic. An effective Early Warning System depends upon informative input data and a model which uses that data to project into the future. Improving the predictability of the flashy events will depend upon improving the measurements of the input data as well as improving our understanding of the processes which control these phenomena and so improve our predictive models. Near real-time (i.e. latency depends on telecommunications or post-processing) monitoring—such as the WMO Global Telecommunications System (GTS, 2017), the Global Flood Monitoring System (GFMS, 2017), the Dartmouth Flood Observatory (DFO, 2017), and the Global Flood Detection System (GDACS, 2017)—can update initial conditions and identify “Early Watch Regions” to which modeling efforts can be directed aiming at improving forecast lead-times and situational awareness. Predictions and early warnings in mountainous regions remain the most challenging due to the challenges to precipitation retrieval in complex terrain and the very rapid rainfall-runoff response in steep terrain. Tao and Barros (2013) showed that by merging GPM-like observations with Quantitative Precipitation Forecasts (QPF) from the National Forecast Database (NDFD), significant improvements (up to 50%) could be attained in the skill of Quantitative Flood Forecasts (QFFs) using physically based models as long as revisit time is less than the response time of specific watersheds. Further, significant inroads toward longer warning lead-times can be expected through coupled prediction frameworks and data assimilation systems to integrate forecasts and observations (Tao et al., 2016a). Indeed, systems such as NOAA’s Rapid Refresh (Benjamin et al., 2015) with assimilation of ground-based radar data every 15 min at present can only be envisioned for operations outside CONUS by assimilating remote-sensing observations (e.g. satellite based radar as discussed in H1b). Big Data analytics provide unique opportunities for developing targeted UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-32 Copyright National Academy of Sciences. All rights reserved.

272 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space location-specific flood and debris flow forecasts with lead-times of hours to days (Kim and Barros, 2001; Campolo et al., 2003; Akhtar et al., 2009; Sättele et al., 2015). In many glacierized mountains, and Arctic lowland countries with glaciers, glacier thinning and retreat are causing the growth of glacial lakes. Glacier lake outburst floods are an important natural hazard in many of these places, such as Alaska, Greenland, Iceland, Peru, and Nepal (Bajracharya et al., 2007). Debris flows, ice and snow avalanches, landslides, and—on ice-capped volcanoes—lahars are frequent and sometimes deadly consequences of heavy precipitation and snow and ice accumulation on steep slopes. The interactions between the solid Earth and land cover and surface dynamics—for example, earthquake triggering of ice avalanches and landslides, landslide blocking of rivers, and consequent landslide dammed lake outburst floods—can be strongly influenced by the preceding history of the thermal state and retreat of glaciers, or by the seasonal state of shallow groundwater saturation of soils and snow melting (Kargel et al., 2016). Economically vital minerals and petroleum and gas extraction and transport from and through mountains and over lowland areas of permafrost and beside or across rivers, the routing pipelines, roads, and bridges, and the security of mountain villages and cities must consider the multitude of hazards due to rainfall runoff floods, snowmelt floods, glacier lake outburst floods, thawing permafrost, as well as glacier surges, landslides, snow and ice avalanches, and other physical elements and processes of the hydrogeological environment. The mountain hazard environment is changing around the world due to climate change as glaciers thin and retreat, snowpack and monsoonal precipitation patterns change, patterns and intensity of freeze-thaw processes shift, and vegetation communities are altered. Likewise, coastal hydrology in relation to rivers and lake and sea coasts is manifestly altered by the effects of climate change, floods, and land subsidence and uplift and general sea level rise. The tracking of hydrologic and hydrogeological natural hazards requires optical and radar methods of monitoring changes to glaciers and glacier lakes, snowpack, rivers, lakes, and vegetation; thermal monitoring of volcanoes; and high-resolution topographic mapping. The large datasets produced by satellite monitoring argue in favor of reliable semi-automated and autonomous hazard detection and monitoring, hazard susceptibility mapping, and hazard forecasting and real-time warnings (Kirschbaum et al., 2016), along with development of reliable human networks to support uniform, standardized data analysis. Because much of the world is not well instrumented, including many areas that are vulnerable to these high impact episodic hazards, spaceborne, remotely sensed data are needed to provide the precipitation and antecedent conditions. As with other Hydrologic priorities, the measurement of worldwide precipitation, Earth Science Objective H-1b, will be essential to enhancing our ability to predict Flash Floods and Landslides. The characterization of the antecedent conditions, both the fixed topographic variables and the dynamic variables like soil moisture, plant growth, and current river stage, will also be required, as is the case with Earth Science Objectives H-3a, H-3b, H-4a, and H-4b. Good measurements need to be integrated into models to forecast the likely future precipitation and then the resulting surface processes which may (or may not) result in a Flash Flood or Landslide. Objective H-4c. Improve drought monitoring to forecast short-term impacts more accurately and to assess potential mitigations This socio-economic priority depends on success of addressing H-1b, H-1c, and H-2c. Droughts have significant economic and societal impacts. Unlike other extremes, their onset tends to be slow, their persistence long, and their recovery poorly predicted. Wilhite (2000) estimated the average annual drought impact in the United States at $6 to $8 billion, while the estimated the damages from the 2015 severe drought in California was estimated to have $2.7 billion in damages in the agricultural sector alone (Howitt et al., 2015). Compared to other natural disasters, a greater proportion of the population can be affected by droughts. Although droughts can have larger impacts on gross national products in the more developed countries, the magnitude of impacts on people’s health and overall wellbeing is especially severe in less UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-33 Copyright National Academy of Sciences. All rights reserved.

273 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space developed regions. In Africa, droughts account for less than 20% of natural disasters but account for over 80% of the affected population (UNISDR, 2009). It is estimated that 300,000 people died in Ethiopia alone during the height of the Sahel drought in early 1980s (EM-DAT, 2017) due to crop failure, lack of drinking water, and disease. Half a million people are estimated to have died because of drought related impacts in Africa during the 1980s (Kallis, 2008). The impacts of drought are intimately linked to the vulnerability of a population to adverse conditions and how society responds within the constraints of changing economies. In general, loss of life is greater in developing regions, and the economic impacts are greater in the developed world. Timely determination from a drought early warning system and monitoring drought will aid the decision making process in order to reduce drought impacts (Wilhite et al., 2007). Drought is defined as a deficit (relative to an appropriately defined normal) of water in one or a combination of water stores (river, lake, reservoir, snowpack, soil water, or groundwater) or water fluxes (precipitation, evapotranspiration, or runoff). Depending on the water deficit, drought is usually classified as (1) meteorological (a negative departure from mean precipitation), (2) hydrological (a deficit in the supply of surface and subsurface water), or (3) agricultural drought (a deficit in soil moisture driven by a combination of meteorological and hydrological drought resulting in reduced supply of moisture for plants and crops). A variety of drought indices have been developed to reflect the various types of drought (Sheffield and Wood, 2011). Observations of hydrologic variables needed to estimate drought indices are scarce over large spatial regions that are of interest for drought monitoring and management, especially in the developing world. Precipitation is one of the best observed variables although near real-time gauge observations are limited in most regions outside the United States, and gauge networks are also sparse over much of the developing world (e.g. Africa). In situ soil moisture is one of the least observed aspects of the hydrologic cycle in terms of long term, large-scale measurements. Thus, to overcome the deficiencies in in situ observation systems, remote sensing of precipitation (both liquid and solid) as well as soil moisture are necessary for early warning drought monitoring. For meteorological drought, progress must be made on addressing the Earth science objective H- 1b, namely the improved monitoring of precipitation, and to gain knowledge on the predictability of rainfall at seasonal time-scales. For many regions of the globe, winter snow provides the needed water supplies in the summer seasons. Thus, advances in the monitoring of mountain snow packs, as quantified in the Earth science objective H-1c, is required to make progress on predicting drought in snow- dominated water supply systems. Better monitoring of snow packs and precipitation, when used with appropriate land surface models, can help improve river flow predictions, and therefore hydrological drought. Improved predictions of inflows into water supply reservoirs are a key requirement for early warning and planning of such hydrological droughts. For agricultural drought, soil moisture is the monitoring variable. There are currently four systems that provide soil moisture products at various spatial and temporal resolutions: MetOp with the advanced scatterometer (ASCAT) (Brocca et al., 2011; Wagner et al., 2013), JAXA’s Advanced Microwave Scanning Radiometer 2 AMSR2 with the C- and X Band passive radiometers on the GCOM- W1 satellite; ESA’s Soil Moisture Ocean Salinity (SMOS) L-band radiometer (Kerr et al., 2016) and NASA’s Soil Moisture Active Passive (SMAP) L-band radiometer and radar (Entekhabi et al., 2010). The preferred system is based on low frequency microwave active and passive remote sensing where the radiometer provides measurements with high sensitivity but at low resolution and the radar provides high resolution complementary capability. Airborne experiments have demonstrated that P -band measurements can provide potentially the basis for subsurface sensing of soil moisture in the rootzone. Objective H-4d. Understand linkages between anthropogenic modification of the land, including fire suppression, land use, and urbanization on frequency of and response to hazards UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-34 Copyright National Academy of Sciences. All rights reserved.

274 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space This objective is linked to H-2a, H-2b, and H-4a, H-4b, and H-4 c. Humans have altered landscapes for centuries through development of the built environment, conversion of woodlands to agricultural lands, and more recently, through extensive forest management policies. Conversion of the Earth’s surfaces to alternative or highly structured land cover significantly alters land-atmosphere processes, and ultimately, can increase the risk of hazards to human populations. Fire suppression has been a significant goal of forest management policies since the 1920s, facilitating an increase in large wildfires across the western United States (Westerling et al., 2006). Warmer spring and summer temperatures, coupled with lower than average precipitation, has also produced longer wildfire seasons with more frequent and larger fires (Morgan et al., 2008; Westerling et al., 2011). Climate change is also expected to increase fire risk and may lead to changes in vegetation types and increased fuel loads (Spracklen et al., 2009; McKenzie and Littell, 2017). Wildfires impact water resources that are in high demand in the arid West. The acute loss of vegetation reduces infiltration and enhances soil water repellency and decreases soil cohesion and organic matter (DeBano, 2000; Robichaud, 2000), ultimately increasing runoff and the risk of flooding, excessive erosion, and debris flows (Rulli and Rosso, 2007; Ebel et al., 2012). Kinoshita and Hogue (2011) documented elevated streamflow for seven years after fire in southern California (USA), while dry season flow increased for over a decade (Kinoshita and Hogue, 2015). Wildfires also impact water quality and threaten drinking water supplies (Smith et al., 2011; Stein et al., 2012; Burke et al., 2013). Nutrients associated with sediments (i.e. total phosphorous) increase in streams impacted by fire (Mast and Clow, 2008; Emelko et al., 2015). Studies on forest fires throughout the Western United States and Canadian Rockies also show increases in nitrate concentrations in receiving waters after forest fire (Riggan et al., 1994; Earl and Blinn, 2003; Bladon et al., 2008; Rhoades et al., 2011). Fire suppression policies, in conjunction with ongoing drought, have also caused extensive insect invasions in North America (Williams et al., 2010; Adams et al., 2012; Anderegg et al., 2013). The mountain pine beetle epidemic has affected conifer forests at historic levels (Raffa et al., 2008), with over 6 million forested ha in the U.S. and British Columbia impacted by bark beetles and more than 5 million ha affected by the mountain pine beetle (Meddens et al., 2012). This includes headwater catchments to the Colorado, Arkansas, Rio Grande, and Missouri Rivers. The progressive reduction in forest canopy due to bark beetle outbreaks does not completely remove the understory vegetation and canopy, making it challenging to predict the impact of bark beetle infestation on watersheds (Adams et al., 2012; Mikkelson et al., 2013). Recent work (Slinski et al., 2016) for 33 Western U.S. watersheds noted no significant change in peak flows or average daily streamflow following bark beetle infestations, and that climate is a stronger driver of streamflow patterns and snowmelt timing than insect forest disturbance for the studied systems. Biederman et al. (2015) also found a muted hydrologic response in eight infested catchments in the Colorado River headwater to beetle induced tree die off. Expectations of increased streamflow were not supported by observations. They attribute the findings to “increased transpiration by surviving vegetation and the growing body of literature documenting increased snow sublimation and evaporation from the sub-canopy following die-off in water-limited, snow-dominated forests.” Urbanization, or the addition of impervious land cover and related infrastructure, has some of the most significant impacts on land surface processes: increased runoff, decreased lag time between precipitation and runoff (Guan et al., 2016b) and larger peak flows (Sheng and Wilson, 2009), increasing the risk of flooding in highly urbanized areas and therefore nonstationarity in flood statistics (Cuo et al., 2009; Meierdiercks et al., 2010; Barros et al., 2014), increase in the heat island effect (Rizwan et al., 2008), decreased recharge of groundwater (Harbor, 1994; Rose and Peters, 2001), but increased groundwater recharge in the Southwestern United States due to the increases of flow and its concentration in ephemeral channels noted under objective H-2c (Kennedy et al., 2013), and development of extensive regional infrastructure to transport needed water to urban centers (Mitchell et al., 2001; 2003; White and Greer, 2006). Climate change is also altering regional precipitation and temperature patterns, increasing the risk of extreme events and urban flooding (Ekström et al., 2005; Berggren et al., 2012). UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-35 Copyright National Academy of Sciences. All rights reserved.

275 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Key needs in space-based estimates for land cover/land use change include: higher resolution soil moisture, including at scales for better estimates of plant water availability for evapotranspiration (i.e. less than 1 km and at a daily scale), estimates of plant biomass and density from microwave and visible/near- infrared sensors, including identification of plant type or species for which an imaging spectrometer would benefit, improved estimates of photosynthesis activity (infrared bands to determine plant activity and dynamics),and finer spatial resolution radiation terms, including skin and air temperature, both under clear sky and cloudy conditions. Other critical needs include precipitation at high spatial and temporal resolutions, and lidar for urban and plant structure and form. 6.3.2 Enabling Measurements We explain how those objectives described in the previous section (6.3.1) translate into measurements, the status of such measurements available from sensors in the Program of Record (Appendix A, already employed or those in the queue), and from new approaches to the interpretation of existing sensors. Finally, we explain how new measurements are needed to achieve the objective, and how those might be made. Generally, the objectives and the measurements are intertwined, without a one-to-one mapping between them. Indeed, many of the lower-priority objectives would be achieved because fulfilling them would result if specific higher-priority objectives are achieved. To illustrate the relationships, consider an objective designated Most Important in Table 6.1: Objective H-1a. Develop and evaluate an integrated Earth System analysis with sufficient observational input to accurately quantify the components of the water and energy cycles and their interactions, and to close the water balance from headwater catchments to continental-scale river basins . Fulfilling it requires success with Objectives H-1b and H-1c, and it also requires that we estimate evapotranspiration, and that requires that we estimate the surface energy balance, especially the surface radiative fluxes, so surface albedo and temperature are needed. To estimate water availability, we need to know about the soil moisture in the root zone, as well as the vapor pressure deficit between the leaves and the atmosphere. Finally, we gain additional information from knowing the species of the vegetation involved, and the vegetation structure. Table 6. maps the Most Important and Very Important objectives from Table 6.1 to measurements and potential technologies for achieving them, including use of sensors already in the Very Important Program of Record as well as potential new sensors. Notably, adding the objectives adds just one set of measurements, related to groundwater, beyond the set of measurements needed to achieve the Important objectives, in less detail Most Important . Table 6.4 provides the same information for the but illustrating that most of those objectives could be achieved if the most important and very important objectives were addressed. Table 6.3. Mapping Most Important and Very Important objectives in Table 6.1 to fluxes and state variables, and potential implementations. New measurements are indicated in italic font. Flux or state variable Objective Knowledge needed Potential technologies (priority order) improved algorithms with existing sensors in the Energy and water fluxes in Program of Record: radiative flux at surface, the surface layer, CERES, MODIS, VIIRS, downscaled to topography specifically GOES-16, Himawari, , which evapotranspiration H-1a MSG is not directly measured (MI) but inferred from the albedo (vegetation and soil, imaging spectrometer energy fluxes to get the separately) latent heat flux L- & P-band radiometer soil moisture in root zone and radar UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-36 Copyright National Academy of Sciences. All rights reserved.

276 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Flux or state variable Potential technologies Objective Knowledge needed (priority order) thermal infrared, in 4 μ m diurnal cycle of surface and 11 μ m spectral temperature (vegetation, soil, regions, multiple snow), at agricultural or platforms to get diurnal topographic scales cycle vapor pressure deficit in microwave & IR sounders boundary layer active sounders, radar or winds in boundary layer lidar vegetation species imaging spectrometer Landsat (e.g. LandFire vegetation structure extended globally), lidar passive and active microwave observations at finer spatial resolutions rain rate at both high and low Rainfall geostationary IR/VIS H-1b intensities observations to support (MI) operational applications (GOES series) Intensity of snowfall and higher frequency radar Snowfall mixed-phase precipitation and radiometer snow depth, weekly, at Ka-band or lidar altimeter topographic scale Snow water equivalent interferometric L-band or snow density (less heterogeneous than depth) S-band SAR or model H-1c Snowmelt rate imaging spectrometer radiative flux, albedo of snow (MI) thermal infrared, in 4 μ m separately from vegetation and m spectral and 11 μ soil, and surface temperature Sublimation from snow regions, multiple of the snow, at topographic platforms to get diurnal scale cycle Evapotranspiration same as for H-1a above H-2a land use and land cover Landsat and Sentinel-2, in Land use and land cover (VI) categories Program of Record Rainfall same as for H-1b above gravimetric measurements if can get to 50 km groundwater storage, at basin resolution scale (50 km or better) interferometric SAR or GPS to measure elastic H-2c Groundwater storage and changes rate of recharge (Precipitation recharge (MI) – Evapotranspiration to see H-1a and H-1b above accuracy finer than rate of recharge) rate of subsidence (to diagnose interferometric L-band or S-band SAR overdrafting) H-4a Hazard response to Same as H-1b, H-1c, H-2a, H-2c (VI) extremes UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-37 Copyright National Academy of Sciences. All rights reserved.

277 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space objectives in Table 6.1, in less detail than in Table 6., to fluxes and state Table 6.4. Mapping Important variables, and potential implementations. Flux or state variable Objective Potential technologies Knowledge needed (priority order) Snowmelt same as H-1c in Table 2 H-2b Water quality see H-3a below something like PACE at important biological, scale of rivers is too chemical, and physical H-3a imaging expensive, but variables (biggest connection Water quality H-3b spectrometer would between environment and address many problems human health) leaf and woody variables Landsat, imaging (need to address which Structure of vegetation H-3c properties are essential to spectrometer , lidar constrain evapotranspiration) H-4b Flash floods same as H-1b, H-1c, H-4a H-4c Drought monitoring same as H-1b, H-1c, H-2c Linkage between land same as H-2a, H-2b, H-4a, H-4b, H-4c, H-4d H-4d modification and hazards Details on the measurements, their role in achieving the objectives, and prospects for those measurements are given in the sections below. Some information about current capabilities is extracted from a recent review (Lettenmaier et al., 2015). Energy and Water Fluxes in the Surface Layer Radiative flux at surface, downscaled to topography Estimating the net solar and net longwave radiation at the surface, corrected for atmospheric attenuation, is crucial for calculating energy-driven water fluxes such as evapotranspiration or snowmelt. While the net radiative fluxes depend on the surface albedo and temperature, they are also driven by the incoming values of solar and longwave radiation, which are affected by the atmosphere and topography. A probing scientific question about Earth’s climate is the net radiation balance of the Earth system. For the hydrologic cycle, the concern mainly addresses the disposition of net radiation at the surface. At spatial scales of 100 km or so, and especially when averaged over time and space, the surface and top-of-atmosphere estimates of solar and longwave radiation from the CERES (Cloud-Earth Radiant Energy System) instruments on three different satellites—Terra, Aqua, TRMM—are generally viewed as satisfactory (Kato et al., 2011). For hydrologic analyses, however, the same values at temporal intervals for models and at spatial resolutions that reflect topographic variability are needed. In this case the estimates of solar radiation at the surface match station observations on average (Hinkelman et al., 2015), but in the mountains the average atmospheric properties of a CERES grid cell (~1°) are often different –2 than at specific locations, especially higher elevations where errors of several hundred W m can occur when clouds are present but not recognized by the sensor (Bair et al., 2016). Longwave radiation estimates are not as easily validated because in situ measurements are less available. Given estimates of incoming direct and diffuse solar radiation, longwave radiation, surface albedo, vegetation structure, and surface temperature, the net values at the surface can be estimated (Rittger et al., 2016). Probably, atmospheric properties can be downscaled from the CERES resolution using observations from the Program of Record, but this remains a problem to solve. Information about topography is available worldwide from the Shuttle Radar Topography Mission (Farr et al., 2007). UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-38 Copyright National Academy of Sciences. All rights reserved.

278 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space Information derived from Landsat imagery for the U.S. is available via the LANDFIRE product (Rollins, 2009), which is not processed worldwide, but could be. Albedo of vegetation, soil, and snow Estimation of energy fluxes such as evapotranspiration or snowmelt at the surface requires that we consider the radiative properties of the vegetation, soil, and snow including emissivity and albedo. Albedo is especially critical because it determines the magnitude of the net solar radiation term in the energy budget equations, and because it exhibits spatial, seasonal, and diurnal variability because of changes in local environmental conditions, and with illumination angle (Tao and Barros, 2014a; Bair et al., 2017). Broadband albedo of a surface is the convolution of the spectral albedo with the spectral distribution of solar radiation at the surface. The albedo of an arbitrary land cover cannot be directly measured with a multispectral sensor. Even if the atmospheric attenuation of the signal is corrected, estimating the albedo requires interpolating between the wavelengths where the reflected radiance is measured. Moreover, calculation of evapotranspiration or snowmelt might require the albedo of the surface of interest in a mixed pixel. To calculate the albedo of an arbitrary surface, or the albedo of individual constituents in a pixel, requires the spectral richness of an imaging spectrometer. Such a sensor can determine the broadband albedo without having to assume the shape of the spectral curve between separated bands, and it can provide enough information for a spectral mixing model to derive the fractional coverage of each surface in a mixed pixel, along with the reflective properties of each as well as viewing and illumination geometry. Such capability has been demonstrated with airborne sensors over both chaparral and snow environments (Roberts et al., 1998; Dozier et al., 2009b). For global coverage at biweekly or monthly intervals, a spaceborne imaging spectrometer would be needed. The HyspIRI mission was recommended as a second phase mission in the previous Decadal Survey (National Research Council, 2007a). That proposed sensor included spectrometer coverage in the solar reflected part of the spectrum (0.38 to 2.5 μ m) and multispectral coverage in the thermal infrared (3 to 12 μ m). HyspIRI remains in its design (“contemplation”) phase (Lee et al., 2015), possibly because its multitude of capabilities drive size and cost. The thermal infrared sensor can estimate atmospherically corrected surface temperature but also distinguish among the spectral emissivities of the silicate minerals (which lack features in the solar spectrum). However, the temporal resolution for spectral information about the surface—to determine various characteristics of vegetation, minerals in soil, and snow— requires less frequent observations than the temporal resolution for vegetation and soil temperatures to estimate evapotranspiration. Thus, there is an argument to separate these two capabilities, hence the recommendation of the National Academies’ Committee on the Future of Land Imaging to consider free flyers to acquire the thermal data (National Research Council, 2013). The ECOSTRESS space station mission, with 5 thermal bands, and a spatial-temporal resolution of 70 m and 4-day repeat with a diurnally progressive orbit offers a critical measurement platform to advance the science using thermal imaging. Experience with the Moon Mineralogy Mapper (Green et al., 2011), and prototype designs for an imaging spectrometer with Landsat-like coverage and spatial resolution (Mouroulis et al., 2016), show that an economical imaging spectrometer is feasible. Such an instrument would also address Most Important objectives described in Chapter 8, Marine and Terrestrial Ecosystems and Natural Resources Management . Soil moisture in root zone Spatial variations of groundwater recharge and evaporation are strongly related to soil type, topography, vegetation and climate. Their dynamics are affected by the variations in plant growth, weather, and seasonal climate. To adequately characterize them, mapping at spatial scales of tens to hundreds of meters and temporal sampling at days to a week are needed. The required spatial resolution is UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-39 Copyright National Academy of Sciences. All rights reserved.

279 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space dictated by the spatial scales of factors that affect the variations in soil moisture (topography, soil texture heterogeneity and vegetation distribution). The temporal requirement is dictated by the rate of change in soil moisture due to intermittent rainstorms and drying rates. Fluxes such as recharge and evaporation cannot be directly sensed as they do not uniquely correspond to any thermal or dielectric state. Rather they depend on spatial, vertical, and temporal gradients of state variables. The need to measure profiles of water vapor in the atmosphere and water in soils, to thereby allow estimation of these fluxes, sets the need for observations at multiple wavelengths to probe multiple depths. A second need is to sense the properties of the interface across which these fluxes are occurring. The atmosphere and the vegetation canopy that stand between the spaceborne sensor and the flux interface need to be as transparent as possible. This consideration leads to the selection of low- frequency microwave (less than 2 GHz) for the observations. Final requirements address sensitivity and resolution. Sensitivity is required if gradients in states and not the states themselves are the basis for estimation. Spatial resolution is required since variations in soil type, topography, solar illumination, and vegetation that drive recharge and evaporation vary across spatial scales. The sensitivity and resolution requirements lead to the selection of active and passive low-frequency microwave sensors. Put together, the space-based sensors should be: (1) multi-channel, (2) low-frequency microwave, and (3) active (radar) and passive (radiometer). A sensor package that combines all of these attributes is L- and P-band (around 1.5 and 0.5 GHz) active/passive. Capabilities for sensing the relevant surface and root zone states have been tested (Entekhabi and Moghaddam, 2007; Entekhabi et al., 2010; Tabatabaeenejad et al., 2015; Akbar et al., 2016; Chan et al., 2016). Several space programs, including JAXA’s PALSAR-2 (L-band active), NASA’s SMAP (L-band active/passive), NASA/ISRO’s NISAR (L-/S-band Active), ESA’s Biomass (P-band active) and CONAE’s SAOCOM (L-band active), are operational or in the development phase. They provide individual observation capabilities, but not the combined multi-frequency (L-/P-band) active/passive observation requirements. The ESA Biomass Mission P-band radar revisit rates are only seasonal. They are appropriate for biomass mapping but not for capturing surface water balance dynamics. The L-band SAR missions also have revisit rates that could be tens of days. Future combined L-/P-band system will require an operations concept that covers a wide swath for frequent revisit. A system study is required to determine the architecture of space-based sensing system for the multi-frequency active-passive observation requirement and to identify required technology development. An active-passive L-/P-band system would also support continuity of surface soil moisture estimation using the NASA SMAP and ESA SMOS missions. Soil moisture, a state variable of the land branch of the water cycle, has temporal variations ranging from daily to interannual. Long climate records are needed to characterize the variability of the water, energy and carbon cycles as affected by the soil moisture state. L-band radiometry of the Earth System has been continuous since 2009. The active- passive L-/P-band system will not only support the needed climate record but also enhance the capability for deeper sensing of the soil column. The system will also support finer spatial resolution to capture the heterogeneities induced by variations in topography, vegetation type, soil texture, and precipitation intermittency. Diurnal cycle of surface temperature Estimation of energy fluxes such as evapotranspiration or snowmelt requires that we consider the individual surface temperatures of the vegetation, soil, and snow. Remote-sensing methods to do this have been derived from thermal imagery in the 4 μ m and 11 μ m regions, for the purpose of detecting fires when they occupy only a small part of a pixel, as small as 0.01% (Dozier, 1981; Matson and Dozier, 1981; Giglio and Kendall, 2001). The same principle, based on the slope of the Planck equation versus temperature at different wavelengths, also enables the separation of temperatures of vegetation, soil, and snow as long as the differences exceed about 10K. Putting thermal sensors on a satellite constellation capable of spatial resolutions at the size of an agricultural field, as recommended by the National Academies’ report on the Future Of Land Imaging (National Research Council, 2013), would enable UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-40 Copyright National Academy of Sciences. All rights reserved.

280 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space measurement of the diurnal cycle of surface temperatures, and thereby improve the modeling of surface fluxes. One of the most important aspects of land surface dynamics is the continuous variability of land surface temperature. In the CONUS, spatially discontinuous networks of point-measurements of surface skin and boundary layer temperature—NOAA’s U.S. Climate Reference Network (NOAA USCRN, 2017) and others with the National Centers for Environmental Information (NOAA NCEI, 2017), USDA’s Soil Climate Analysis Network (NRCS, 2017), and the Ameriflux tower network (AmeriFlux, 2017)—can be used for validation and verification of remotely sensing estimates. Outside Europe and North American, however, in situ measurements of surface temperature are limited. From space, land surface temperature has been measured using various infrared and thermal sensors—HIRS2/MSU (Lakshmi et al., 1998), AIRS (Susskind et al., 2003), AVHRR (Price, 1984), MODIS (Wan and Li, 1997)—but all of these are at most twice a day (polar orbit) and only MODIS (on two platforms, Terra and Aqua) comes close to mapping a diurnal variability of surface at four different times of day. Recently launched geostationary satellites— U.S. GOES-16, Japan’s Himawari 8/9, China’s Fengyun-4, and Europe’s MTG —have the thermal band coverage needed to deconvolve skin temperatures from a variety of surfaces, and the value they provide of observing the diurnal variability of surface temperature can translate into better estimation of the outgoing longwave radiation and fluxes of sensible heat, latent heat, and conduction into or out from the soil. Clouds are another complicating factor, masking the land surface from visible, near infrared and thermal bands. Most land surface parameterization schemes in land simulation systems such as N/GLDAS (Mitchell et al., 2004; Rodell et al., 2004) and LIS, the Land Information System (Kumar et al., 2006) have a temporal resolution of half-hourly to hourly time steps and can readily use more frequent observations. While geostationary satellites provide fine temporal frequency, polar orbiting satellites provide spatial resolution at the scale of individual fields. Simulation studies will have to be undertaken to study the efficacy of surface temperature observations and the fusion of spatial and temporal resolutions. Surface temperature and vegetation observations are available at a finer spatial resolution than soil moisture and this along with the relation between soil moisture and diurnal change in surface temperature can be used to downscale the coarser spatial resolution soil moisture (Fang et al., 2013). Clouds obscure the surface from visible, near-infrared, and thermal infrared sensors, so the temperatures sensed during clear conditions must be used to model energy transfers within canopy and canopy-soil interactions (Jin and Dickinson, 2010). Vapor pressure deficit in boundary layer In understanding the energy and water fluxes in the surface layer, knowing the vapor pressure deficit (VPD) in the planetary boundary layer (PBL) is critical (Betts, 2004). No sensor in the Program of Record addresses temperature and humidity within the PBL with sufficient fidelity to directly estimate sensible and latent heat transfer from space. Currently the highest resolution operational instrument is the Infrared Atmospheric Sounding Interferometer (IASI) sensor on ESA’s Metop-A (Level 2 retrievals). IASI takes 8461 spectral samples í 1 m with a resolution of 0.5 cm between 3.62 and 15.5 ȝ after tapering, and has an instantaneous field of view that ranges from 12 to 39 km. In addition to the humidity data at meteorological stations and flux towers in the context of surface temperature, globally distributed weather balloon profiles (radiosondes) of quality-controlled air temperature, humidity and wind observations starting at 3 m above the surface launched twice or four-times daily are also available from NCEI among other repositories. Assessment of the IASI instrument (August et al., 2012) found that comparisons with ECMWF analysis fields for the temperature fields below 800 hPa had errors between 2.5 and 3.5 K on average. For humidity, the RMS differences are about 20% of the relative humidity, and with a smaller dynamic range than estimated by ECMWF. Similar results were found in comparisons with radiosonde data. August et al. (2012) caution against using IASI for the boundary layer at this time. It is believed that the relatively large errors can be reduced by improved retrieval algorithms (better channel selection, UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-41 Copyright National Academy of Sciences. All rights reserved.

281 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space improved land emissivity inputs) (Masiello et al., 2012) as well as joint use of information from AMSU and MHS/Metop. Whether sharpening the channel selection or advancing the algorithm development will lead to the needed improvements is yet undetermined, but is a potential path forward before investing in new hardware. Precipitation and its Phase Rainfall Precipitation is the longest measured parameter, with in situ observations dating back to 2000 BCE. Precipitation is critical both from a climate perspective, as a key marker of the speed of the water and energy cycle, and weather, affecting nearly all human activities. NASA and JAXA have led the global community in the global assessment of precipitation using passive and active microwave observations through their successful TRMM and GPM missions, while NOAA has led the way in ground based radar and geostationary IR/VIS measurements. Geostationary measurements at fine temporal and spatial resolution are useful for nowcasting and hazard warning applications, albeit with lower accuracy currently being exploited (Huffman et al., 2017). than microwave measurements. Synergies are Precipitation exhibits variability over a wide range of spatial and temporal scales (from a few meters to hundreds of kilometers in space, and from a few minutes to storm and seasonal scales in time. Ground radars can bridge these scales but are only available in industrialized countries and designed for civil defense purposes rather than routine monitoring or understanding. Rain gauges are more common but do not capture spatial variability and are sparse over much of Earth’s surface. Accurate estimation of precipitation from space is thus of paramount importance for improving weather and climate prediction models, closing water budgets at the catchment scale, providing global coverage of this most critical component of the water cycle, and for early prediction of severe storms. The Tropical Rainfall Measuring Mission (TRMM) satellite, launched in 1997, was the first of its kind to successfully carry a single polarization Ku-band (13.8 GHz) weather radar, along with a multichannel radiometer TMI (frequencies 10.65, 19.35, 21.3, 37.0, and 85.5 GHz, horizontal and vertical polarization except for the vertical-only water vapor channel of 21.3 GHz). TRMM considerably improved our understanding and estimation of rainfall over the tropics (Kummerow et al., 1998). The GPM mission followed in 2014 with a constellation of satellites covering the globe (Hou et al., 2014). The GPM core satellite includes a Dual-Frequency Doppler Polarization (DPR) radar (Ku and Ka band frequencies) and a passive microwave radiometer with 13 channels (10.7 to 183 GHz). GPM aspires to provide global estimates of precipitation at resolution of 5 km every 3 hours, and even finer resolutions (down to 2 km and 15 min) when combined with the constellation of geostationary VIS/IR imagers. Passive retrieval of rainfall from observed upwelling spectral radiance is challenging (e.g., Gopalan et al., 2010; Ebtehaj et al., 2016) mainly because of the background surface and atmospheric signal contamination. In microwave frequencies (6-to-200 GHz), the hydrometeor vertical profile is radiometrically active and alters the upwelling radiation largely through absorption-emission (over ocean) and scattering (over land). Whereas space-based radar measurements such as the DPR on GPM are the most promising with regard to spatial and temporal resolution of precipitation, retrieval quality is strongly tied to the algorithm’s ability to produce realistic descriptions of the vertical and spatial distribution of hydrometeors needed to quantify Path Integrated Attenuation (PIA) and scattering effects that modify the radar signal as it travels through a storm system (e.g., Iguchi et al., 2016). Radar-based quantitative estimates of precipitation (QPE) underestimate heavy rainfall due to strong attenuation effects when significant ice and large-sized water hydrometeors are present, and light rainfall is often missed due to the lack of significant scattering when hydrometeors are small. In complex terrain and mountainous regions generally, ground-clutter artifacts (excessive reflectivity when the radar signal is intercepted by the terrain) add ambiguity to the interpretation of reflectivity measurements from lower levels in the atmosphere resulting in QPE errors as large as 100% (Duan et al., 2015). Because orographic precipitation accounts for more than half of the world’s renewable freshwater resources and most UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-42 Copyright National Academy of Sciences. All rights reserved.

282 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space hydropower production, decreasing retrieval error and systematic uncertainty is critical to closing the basin water balance from headwaters to continental-scale basins. The challenge of providing accurate precipitation estimates everywhere and at spatial and temporal resolutions needed for accurate and timely prediction of extreme weather and floods remains a critical issue in global water and food security and in global health. From GPM this challenge will be met in the coming decade by combining products from several sensors, and learning from the coincidental active and passive sensors to improve physics-based and data-learning retrieval algorithms. Snowfall and mixed-phase precipitation A significant portion of precipitation at middle and high latitudes falls as snow (Liu, 2008). Snowfall and mixed precipitation retrievals from space, unfortunately, encounter all the difficulties discussed for rainfall, plus additional complications due to unknown surface characteristics as well as the details of the snow size, shape and density. Because ice is much more transparent than water in the microwave frequencies, mixed-phase precipitation comprising snowflakes and supercooled water droplets is particularly difficult to measure. Over snow-covered areas, where it is hard to disentangle the weak rain scattering signal at high frequencies from the snow-cover emission, satellite-based estimates of snowfall are unreliable. Yet, such retrievals are critically important for improving assessment of water storage and for constraining climate models. For example, outside Greenland and Antarctica, High Mountain Asia (HMA) contains the largest -1 deposit of ice and snow, roughly 8600-13600 GT, with a decreasing mass of 26±15 GT yr , largely due to reduced snow albedo caused by deposition of soot and dust (Kaspari et al., 2014), temperature increase, and decline of high-altitude snowfall (Pfeffer et al., 2014; Radi ü et al., 2014). Warmer temperatures over the past decade (IPCC, 2014) have not only increased evaporation and melting but have also shifted precipitation patterns to a more rain-dominant regime (Bhutiyani et al., 2010) and reduced the number of snowfall days (Shekhar et al., 2010). While there is a growing evidence of improved skills of climate models, studies confirm that their capability is severely limited in simulating precipitation and especially snowfall over HMA’s complex terrain (Turner and Annamalai, 2012). Reliable estimates from GPM might narrow this gap, but the problem of remotely sensing snowfall remains. Snow and mixed-phase precipitation measurements are challenging no matter the vantage point. Rain gauges that catch liquid precipitation do not effectively catch snowfall, especially in windy environments (Doesken and Judson, 1996; Yang et al., 2005). Manual measurements from snow boards are accurate, but too sparse for practical monitoring (Greene et al., 2016). Ground based radars suffer from the same issues related to snow size, shape and densities as the spaceborne sensors (Wen et al., 2017). The main shortcoming is that while we understand particle scattering from well-defined particle shapes such as needles, plates or rosettes, the more amorphous aggregates that often make up snow near the freezing level (where most snow falls), has been difficult to model particularly in the 0°C to -10°C range. New advances using triple frequency radars have made strides in this area (Kulie et al., 2014). Specifically, differences between Ku and Ka, as well as Ka and W band frequencies appear to contain much of the ice habit information, and in particular, offer a distinct signature of snowflakes and aggregates. Their results, confirmed by in-situ aircraft observations during the AMSR-E validation campaign in Wakasa Bay (Lobl et al., 2007) are encouraging. The GPM satellite flies Ku and Ka radars while CloudSat uses a W-band radar. In this respect, the needed technology is not new. It is flying today, albeit not on the same platform, and not with the needed sensitivity for snowfall detection (GPM minimum sensitivity at Ku and Ka bands limits detection to moderate and heavy rain and strongest snowfall events, thereby missing significant precipitation in middle and high latitudes). While the Wakasa Bay data and other work done with 3 frequency radars appears very promising, the current studies deal largely with stratiform precipitation that contains relatively little supercooled water and no mixed-phase precipitation. The phase of the precipitation is rather easily distinguished using –1 Doppler measurements to observe particle fall velocities. With terminal velocities around 1 m s for –1 –1 snow, 2-3 m s for graupel, and approximately 6 m s for liquid rain, the phase is easily discriminated UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-43 Copyright National Academy of Sciences. All rights reserved.

283 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space –1 resolution. This capability will be demonstrated by EarthCare, a joint ESA/JAXA mission with 0.5 m s scheduled to launch in 2019 (Illingworth et al., 2015), albeit not quite at the pixel level needed for many hydrologic applications. The supercooled water is more problematic as the water makes radar-only observations difficult to interpret. For this problem, the solution is seen in a combination of active and passive microwave sensors, also discussed in the rainfall section. The microwave signature is quite sensitive to the liquid water emission. Grecu and Olson (2008) were able to simultaneously retrieve cloud liquid water and ice contents for a lake-effect snowstorm using a W-band radar and a high frequency radiometer covering the GPM radiometer channels from 89 to 183 GHZ. A single frequency radar was sufficient in this study as the retrievals were for clouds over the water, so the background contribution to the signal was known. Specific improvements needed Falling snow can be detected by observing scattering signals in high frequency passive microwave sounders such as ATMS (Kongoli et al., 2015), and W-band radars like CloudSat (Stephens et al., 2002) also detect the backscatter of snow and ice hydrometeors. Nevertheless, quantitative snowfall retrievals remain elusive (Levizzani et al., 2011), but the work described in the previous section provides the basis for a three-frequency radar for snowfall detection and quantification. Frequencies currently flying on GPM (14 and 35 GHz) coupled with CloudSat (95 GHz) would constitute the basis of such a measurement. One of the radars should be equipped with Doppler capabilities to aid in the mixed-phase classification. Beyond that, it is postulated that the spatial resolution should be at least that of GPM and ideally better (perhaps 1-4 km) in order to minimize issues related to inhomogeneous radar field-of-views. The radar system should further be coupled with a high frequency radiometer, not unlike the current ATMS sensor, but with increased spatial resolutions to match the radar FOVs. As with GPM, this radiometer would also serve as a transfer standard to operational sounders that could achieve the necessary sampling to address the myriad of applications related to snowfall accumulations and water availability. Beyond better precipitation observations however, the next generation of improvements in weather and climate models critically relies on better understanding of precipitation physics. The rapid development and increasing use of global CRMs means that an understanding of atmospheric processes and feedbacks is rapidly becoming critical for the accurate prediction of not only weather, but global cloud regimes and climate as well. Improving our observations of dynamical and microphysical cloud processes with multi-frequency radars and Doppler velocities as proposed for new precipitation measurements would immediately contribute to addressing the following important unresolved problems through process evaluation or model constraints: Improvement in the manner in which the large ice species are parameterized – this will shrink x inaccuracies in surface precipitation rates, convective-stratiform precipitation and precipitation probability density functions (Bryan and Morrison, 2012; Adams-Selin et al., 2013; Tao et al., 2016b). x Improvement in the connection between vertical velocities and resulting ice hydrometeor species – properly representing vertical velocity reduces inaccuracies in the nucleation rates, numbers and sizes of cloud droplets and ice crystals and hence the hydrometeor size distributions (Saleeby and Cotton, 2005; Saleeby and van den Heever, 2013; Varble et al., 2014). x Improvement in the understanding of the partitioning between water and ice particles – accurately representing cloud microphysical processes reduces significant inaccuracies in the partitioning between the liquid and ice water species, the depth of the mixed-phase cloud region, the vertical redistribution and location of ice and liquid water, & upper-level detrainment of water vapor. x Improvement in the understanding and quantitative description of the vertical structure of microphysical properties of precipitation in regions of complex terrain – accurately representing UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-44 Copyright National Academy of Sciences. All rights reserved.

284 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space the vertical heterogeneity of hydrometeors at lower levels will significantly reduce systematic errors and uncertainty in orographic precipitation including layered stratiform systems with and without embedded convection. Together, these insights into cloud and hydrometeor behavior, coupled with corresponding aircraft campaigns that provide more in-depth views of the identified regimes, could revolutionize our understanding of clouds and our ability to predict their development. Snow and Glaciers The difficulties in measuring snowfall and mixed-phase precipitation are partly compensated by the fact that snow lies on the ground for a while before it melts or sublimates. Therefore a complementary strategy is to measure snow depth and density and thereby snow water equivalent (SWE = snow depth × snow density). In February 2017, NASA ran the first campaign of a multi-year plan (SnowEx, https://snow.nasa.gov/snowex) to test and validate a variety of airborne and ground-based instruments to measure snow properties. Some of the proposed technology developments for measurement of snow properties will be examined during SnowEx. Snow water equivalent (SWE) In many regions of the world where snowfall dominates precipitation, the snowpack comprises a larger seasonal cycle in water storage than the surface water reservoirs or groundwater (Zhou et al., 2016). Therefore, measurement of snow water equivalent on the ground has been a century-long component of hydrologic practice, especially for seasonal forecasts of streamflow (Church, 1914). The in situ data now comprise manual measurements at designated snow courses (Armstrong, 2014), typically monthly, and automatic measurements from snow pillows that continuously sense the weight of the overlying snowpack (Cox et al., 1978). For logistical reasons, snow pillows and other remote meteorological sites in the mountains all lie on nearly flat terrain, so they may poorly represent snow accumulation and melt rates on nearby slopes (Meromy et al., 2013). Even in mountain ranges with an extensive surface network like California’s Sierra Nevada, the in situ measurements do not accurately represent the spatial distribution or basin-wide volume of the snow water equivalent (Rice and Bales, 2010; Dozier et al., 2016). For example, Landsat images often show snow remaining even after all snow has melted from the surface stations (Rittger et al., 2016), so local reservoir managers lack necessary information to choose between maintaining storage or generating hydropower. For regions having more sparsely measured sites, or where data sharing is prohibited, such as much of High Mountain Asia, we rely almost entirely on remote sensing approaches to SWE determination. Passive microwave measurement of SWE is now a staple of remote sensing. Because ice is transparent in the microwave spectrum, whereas water is absorptive, the snowpack scatters and attenuates microwave radiation emitted from the soil. Moreover, the attenuation is greater at higher frequencies (shorter wavelengths) so the difference between brightness temperatures at different frequencies provides an index to the snow water equivalent (Chang et al., 1987). This approach works reasonably well in the prairies and tundra of North America and Eurasia, but the signal saturates when SWE values exceed about 20 cm (Kelly et al., 2003). Moreover, emission at microwave frequencies is on the tail of the Planck equation, so the tiny amount of radiation and implications of antenna design require that the pixels be large, 10 to 25 km. The heterogeneity of the surface in that large pixel (Vander Jagt et al., 2013) along with the deep snow often found in mountain ranges causes substantial uncertainty in our ability to assess a major component of the water cycle. Lettenmaier et al. (2015) argue that “Among all areas of hydrologic remote sensing, snow (SWE in particular) is the one that is most in need of new strategic thinking from the hydrologic community.” Potential avenues for improving the current inadequacies in assessing snow depth and water equivalent in the world’s mountains need further exploration with airborne missions and a resolve to implement a technology that shows acceptably accurate results in a variety of mountain snow settings. Because of the effect of topographic variability on snow accumulation and melt, spatial resolution needs UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-45 Copyright National Academy of Sciences. All rights reserved.

285 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space to be no coarser than ~100 m, and the SWE values that need to be assessed range from ~10 cm to several meters. Because snow changes more rapidly than other surface covers, temporal frequency should be about weekly. Figure 6.11 shows a promising approach; the NASA Airborne Snow Observatory (Painter et al., 2016) uses lidar to measure snow depth by comparing elevations measured weekly or biweekly throughout the snow season with the same topographic data acquired when free of snow in the summer. Snow water equivalent is estimated by multiplying the depths (at spatial resolution of ~3 m) by snow density derived from field measurements and a snowmelt model (Marks et al., 1999). The main source of uncertainty in SWE is thus reduced to only snow density, which varies 3× less than depth spatially (López-Moreno et al., 2013). Figure 6.11. Snow water equivalent estimated by NASA’s Airborne Snow Observatory in California’s Tuolumne River Basin in April 2014, by combining lidar measurements of depth with measured and modeled snow density. Source: T. H. Painter, NASA A scanning lidar’s narrow swath and the need for frequent altimetry over large areas perhaps makes the Airborne Snow Observatory’s approach difficult to implement from satellites Alternatively, a high-frequency (W- or Ka-band) radar altimeter or interferometer provides an alternative to measuring the snow depth. A Ka-band interferometer has been flown as an airborne sensor as GLISTIN, the Glacier and Land Ice Surface Topography Interferometer (Moller et al., 2011; 2017). The advantage of measuring snow depth and inferring SWE through density is not only a cheaper, more feasible implementation, but also that the snow depth measurement is insensitive to liquid water in the snowpack. Alternative methods to directly measure SWE at fine spatial resolution through backscattering from synthetic aperture radar (Shi and Dozier, 2000b, 2000a) showed promising results, but only in dry snow and only with a multi- frequency multi-polarization radar. But density can also be measured either by polarimetric (Li et al., 2001) or interferometric L-band (1.4 GHz) SAR, hence such measurements from the Program of Record (e.g. NISAR) could compliment the measurement of snow depth, because density does vary in systematic ways that may affect the calculation of SWE. Generally, densities are lower at higher elevations and on slopes that receive less radiation (Wetlaufer et al., 2016). Snow and ice melt Energy used to melt seasonal snow and glaciers depends on other enabling measurements described in the subsection on Energy and Water Fluxes in the Surface Layer . Their estimation from satellite data depends on downscaling the coarse satellite estimates from instruments like CERES, along with accurate measurements of snow and ice albedo. Because many regions where snow and ice are important in the hydrologic cycle are in the mountains, many of the pixels are mixed, i.e. containing some combination of snow, ice, vegetation, and soil. Spectrally unmixing these pixels and determining the albedo of the snow and ice component would be improved by measurements from a spaceborne UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-46 Copyright National Academy of Sciences. All rights reserved.

286 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space spectrometer, which would also support information needs identified by other panels, Decadal Survey particularly chapter 8 on Ecosystems. Sublimation from snow Snow disappears from the land and from glaciers by two mechanisms other than snowmelt—wind transport and sublimation (the direct transition from ice to water vapor). Wind transport can be identified by measurements of snow water equivalent. Estimation of sublimation depends on many of the measurements needed to estimate evapotranspiration, with the problem being somewhat easier because the surface vapor pressure can be reliably estimated from measurement of the surface temperature. Groundwater Storage and Recharge Groundwater storage and depth to water table Approximately 98% of all circulating freshwater (excluding glaciers and ice caps) on Earth is groundwater, i.e., below the water table (Feder al Council for Science and Technology, 1962). In the context of water resources, whether worldwide or in a local watershed, the groundwater stores of freshwater are vast, which is also one reason why much of the fresh groundwater is fairly old, typically a century to a millennium. That is, the larger the water volume, the longer the water residence time. In most groundwater systems, both the old and relatively young (a few decades to a century) the water is usable and replenishable, although the geology and confinement strongly affect the rate of replenishment of deeper groundwater. Aquifer replenishment occurs thru recharge at the top of the groundwater system, the water table. Importantly, the recharge benefits deeper groundwater by bolstering fluid pressure, the changes in which can propagate regionally through the groundwater system much faster than the water itself. Despite the vast volume of groundwater, only a limited quantity of it can be pumped without causing detrimental effects, which include chronic groundwater depletion, land subsidence, depletion of groundwater-dependent surface water and ecosystems, and groundwater quality degradation (e.g., seawater intrusion in coastal zones). Because of the vastness of groundwater reserves, the consequences of exploiting them, and the way that groundwater is replenished, the essential metric is the in change groundwater storage rather than the total quantity. In developed aquifer systems, changes in elevation of the water levels in wells can be measured and changes in storage calculated. Remotely sensing depth to water table with ground penetrating radar suffers from interference from water in the soil above the groundwater. In many parts of the world, however, measurements of depth to water table are seldom made, which is one reason why local and global awareness of major groundwater depletion in places like India, China and North Africa did not come to light until the emergence of GRACE data (Richey et al., 2015). Moreover, even in many monitored groundwater basins and especially in the sedimentary basins that contain most of the major aquifer systems, the groundwater occurs under semi-confined conditions in which the calculation of storage change based on groundwater level data is difficult and typically infeasible without the use of well-calibrated groundwater models. Accordingly, the capability of GRACE (Tapley et al., 2004) for detecting real-time changes in groundwater storage is in concept a highly relevant and positive development. The main limitation of GRACE is that the scale of its measurements is much larger than the scale of most groundwater systems or of typical water resources management regions. The current GRACE scale of measurement is approximately 400-500 km (Famiglietti and Rodell, 2013), while the scale of most water management basins or problems is on the order of 10-50 km (Alley and Konikow, 2015; Lakshmi, 2016). This disparity in scales of the GRACE measurements and the hydrologic system or problem means that measurements from GRACE in even large groundwater basins such as California’s Central Valley include not only the changes in groundwater storage in the major sedimentary aquifers, but also the changes in UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-47 Copyright National Academy of Sciences. All rights reserved.

287 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space snow in the adjacent mountain range, soil moisture, and fractured-rock groundwater, which all must be measured and modeled sufficiently to separate them from the important major aquifer storage changes. NASA announced the end of the GRACE mission in late October 2017; one of the pair of satellites has run out of fuel. The overall objectives of the GRACE Follow-On (to be launched in 2018) for measuring groundwater change in storage are highly relevant for both water management and understanding of global water balances. It is critically important, however, that the technology be advanced sufficiently to get the resolution down to scales relevant to water resources management (e.g., ~50 km). This scale of measurement is also most appropriate for better understanding of the almost entirely unmonitored changes in subsurface water storage in mountainous regions. Recharge In undeveloped groundwater systems, recharge is typically balanced by groundwater discharge (e.g., spring flow, stream baseflow, subsea discharge), driving regional and local groundwater flow system dynamics and to a large extent also keeping the groundwater systems fresh. Furthermore, in most undeveloped groundwater basins that do not discharge groundwater to oceans (e.g., Post et al., 2013), the recharge is balanced by discharge such that the net, regional recharge (i.e., recharge minus discharge) is essentially nil. Although local recharge can be substantial, net recharge does in fact depend on scale, often approaching zero at larger scales in undeveloped systems. In developed groundwater systems, groundwater pumping can be sustainable or unsustainable, depending on whether the pumping magnitudes exceed the recharge and the reductions in natural discharge that pumping commonly induces. Accordingly, the net recharge can increase as the pumping increases and strongly influences whether the groundwater pumping will lead to unsustainable overdraft of the groundwater system. A major unknown in both developed and undeveloped groundwater systems is the recharge. There are many ways to estimate recharge. At the landscape scale a water budget approach that accounts for precipitation, evapotranspiration, and runoff can in theory be used to calculate recharge as the residual. A major limitation of this approach is that the errors in measuring or estimating precipitation and evapotranspiration often exceed the magnitude of recharge. As the accuracy of satellite-based evapotranspiration measurements improves, direct estimation of associated recharge rates will become more feasible. One very important case where a water balance approach has worked well for estimating recharge is in irrigated croplands, which consume more groundwater worldwide than any other use (Döll et al., 2012; Scanlon et al., 2016). Irrigation has not only caused massive, often uncontrolled increases in groundwater pumping in many parts of the world, it has also resulted in significant increases in recharge because typically only about 50 to 80% of the water applied to the crop is consumed (evaporated and transpired) by the crop, with most of the remainder typically recharging the groundwater. In such agricultural systems, because the evaporative water demand of the crop is easier to estimate and because the recharge tends to be greater than in non-agricultural watersheds, water budget calculations of recharge can be fairly reliable. Nevertheless, as water scarcity increases and irrigation efficiency improves, the recharge from irrigation will decrease. In turn, the need for continual, more accurate monitoring of crop evapotranspiration will only increase. Besides irrigation water management, the other forcing that will affect recharge is climate change. Warming will tend to increase potential evapotranspiration, decreasing recharge; but on the other hand, climate induced changes in vegetation due to reduced soil moisture and deeper groundwater levels may result in less actual evapotranspiration. Here again, our ability to monitor spatial and temporal changes in evapotranspiration, and in turn groundwater recharge and runoff, will hinge on future improvements in satellite-based methods to measure evapotranspiration. Subsidence and elastic groundwater storage changes Earth’s surface fluctuates both up and down due to groundwater storage changes that are referred to as either elastic or inelastic. All aquifer systems undergo elastic changes in storage wherein decreases UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-48 Copyright National Academy of Sciences. All rights reserved.

288 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space in fluid pressure cause modest amounts of aquifer system compaction that release groundwater from storage, and increases in fluid pressure cause modest amounts of aquifer system expansion, taking groundwater into storage (Amelung et al., 1999; Galloway et al., 2000). In unconsolidated to semi- consolidated sedimentary basins that contain most of the world’s major aquifers, the aquifers are confined or semi-confined, and hence a key mechanism by which groundwater comes into, or out of storage on the daily to monthly time scales is through these elastic processes, rather than solely through fluctuations of the water table itself. In the confined and semi-confined aquifer systems, water levels in the wells are not the same as the water table, and fluctuate more than the water table by orders of magnitude, and on shorter time scales. Whether in fresh groundwater systems, oil and gas reservoirs, or geothermal fields, the elastic changes in storage are not easily monitored. Moreover, since in most of the world including parts of the U.S., groundwater levels are not sufficiently monitored, measurements of land surface deflections caused by elastic groundwater storage changes are valuable for discerning quantities and mechanisms of groundwater storage changes (e.g., Amelung et al., 1999). Elastic changes in storage can manifest in land - surface deflections on the order of 10 mm, which is within the capability of InSAR, which has resolution of 5-10 mm. Any future improvements in this resolution would obviously benefit real-time monitoring of groundwater storage changes, especially when complemented with information from GRACE and sparse groundwater level measurements. The finer spatial resolution of NISAR (Rosen et al., 2016) will further benefit monitoring of groundwater withdrawal consequences, as well as the subsurface geologic structures that affect it. Subsidence, also referred to as “inelastic compaction,” occurs when declines in fluid pressure are sufficient to increase effective stress (sediment grain-to-grain stresses) to an extent not previously experienced in the geologic burial history of the sedimentary package of coarse and fine sediments (Galloway et al., 2000). This important form of groundwater overdraft is increasingly symptomatic of increasing over-exploitation of groundwater resources. Prior to availability of InSAR, real-time knowledge of subsidence and its associated, permanent losses in groundwater storage capacity and damage to surface structures did not come to light until the damage was already done. InSAR revolutionized our ability to monitor subsidence in real-time and led to unanticipated discoveries about use of land surface data for determining previously unrecognized subsurface complexities. Again, NISAR and future missions will significantly enhance this capability. High resolution GPS monitoring of the land surface has also recently been used to detect cm- scale deflections in the Earth’s crust in response to crustal loading and unloading caused by total change in subsurface water content (Borsa et al., 2014). Future improvements in tracking not only subsidence but also groundwater storage will lie in the joint use of GRACE, InSAR, NISAR and GPS. Collectively, this approach will become increasingly essential for local and regional groundwater management. Water Quality One of the essential but overlooked elements of regional water availability and sustainable water resources management is water quality. Water quality issues are local and need to be observed at a finer temporal and spatial resolution in order to be used and incorporated into local decision-making. Some progress has been made utilizing satellite-based observations for monitoring and assessing eutrophication impacts in coastal waters (Schaeffer et al., 2012; 2013). However, the current technologies embedded in our Earth observing satellite systems are not adequate to gather the kind of signals that could be used to infer various biological, chemical or physical variables at a finer scale to map regional or local water quality. The pixel resolution of platforms such as MODIS and Landsat or the forthcoming Sentinel Ǧ 3 mission is too coarse for this purpose (Lee et al., 2014). In order to evaluate and manage water quality for inland waters, we need to measure turbidity, salinity, colored dissolved organic matter (CDOM), te mperature, sediment load and chlorophyll-a. Spatial resolution of 30-60 m is required in order to have observations that are interpretable and detectable to UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-49 Copyright National Academy of Sciences. All rights reserved.

289 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space evaluate the state of inland water bodies such as rivers and lakes at various geographical scales (Hestir et al., 2015; Turpie et al., 2015). The desired temporal resolution is daily to weekly observations. Advancement in spectrometer imaging technologies, at spatial scales of rivers, inland water bodies and coastal regions, can ultimately address some of these problems. More focus is also needed on developing algorithms to infer water quality variable based on receiving signals while addressing land adjacency and atmospheric effects (Lee et al., 2014). In addition, considering the advancement of drones and other imaging technologies, there are possibilities for regions with extensive water quality challenges to rely on these technologies to collect water quality data at a finer temporal and spatial scale. In January 2017, NASA Earth Sciences Division issued solicitation A.30 Remote Sensing of Water Quality , to identify investigations that could improve the measurement of water quality from spaceborne and airborne sensors. Selections for the funded investigations were announced in July 2017. Findings from them could affect future recommendations and prospects for remotely sensing water quality from space. Land Use and Land Cover Vegetation species There has been a long and successful history of obtaining vegetation classifications at the community level with existing sensors such as Landsat, SPOT, and MODIS (Xie et al., 2008). Classification and mapping at the species level requires finer spatial resolution, generally less than 5 m (IKONOS, Quickbird). Classification at the community or species level does not generally provide the condition of the vegetation, for example whether a grassland’s grass is 50 cm or 2 cm tall, owing to grazing or different stages in growth phenology. Finer spatial resolution multispectral sensors will be required for more reliable identification of plant species. The relative vigor or level of plant stress is another important attribute to constrain evapotranspiration estimates that is not measured by community or species level classifications. Thermal remotely sensed measurements have been used as a measure of plant moisture stress to improve evapotranspiration retrievals (Anderson et al., 2004; 2007) also see Diurnal cycle of surface temperature subsection). Another relatively recent technique to measure plant vigor is through solar-induced chlorophyll fluorescence (SIF) (Joiner et al., 2011). SIF is an indicator of photosynthetic activity and efficiency at the molecular scale (Meroni et al., 2009). When plants are water or temperature limited, photosynthesis is reduced but light absorption continues. To compensate, plants decrease the release of fluorescent photons at wavelengths of 690 to 800 nm. Imaging spectrometers with ultra-fine spectral resolution ( ׽ 0.02–0.05 nm full-width half-maximum) in the range centered around 760 nm now enable accurate and global fluorescence retrieval (Frankenberg et al., 2013). GOSAT (Greenhouse Gases Observing Satellite) and the Orbiting Carbon Observatory (OCO-2) have spectrometers that enable retrieval of chlorophyll fluorescence (Frankenberg et al., 2012). The ESA FLEX (FLuorescence Explorer) is designed specifically to monitor global terrestrial vegetation for steady-state chlorophyll fluorescence and is scheduled to be launched in 2022. Vegetation structure As noted above, spectral remote sensing methods have had less success in complex multispecies stands where overstory shading, interwoven branches, and understory plants are present. Lidar (Light Detection and Ranging) can address many of the shortcomings of spectrum-based remote sensing. With high pulse rates exceeding 500,000 pulses per second, airborne systems can achieve ~10 to 30 multi- return points per square meter. Even in dense foliage, lidar point returns (x, y, z, and returned intensity) typically sample not only the top of the canopy and ground but numerous points within the canopy. Resulting point clouds can be analyzed for a variety of vegetation structural attributes. If the plant species is also known, greater inferences can be made regarding plant structure using species specific allometric relationships such as LAI, biomass, and above ground carbon. Multi-sensor airborne lidar systems with UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-50 Copyright National Academy of Sciences. All rights reserved.

290 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space co-registered multi-spectral sensors are available to simultaneously tackle the vegetation species identification and vegetation structure estimation. Single photon counting lidar systems are advancing rapidly and a 10-kHz system is being deployed on the upcoming 2018 IceSAT-2 mission in the ATLAS instrument (Advanced Topographic Laser Altimeter System). This system will provide point returns along six tracks at a spacing of roughly 70 cm. Ideally, a future system could provide ground point densities of approximately 8 points per square meters on bi-weekly or monthly time scales to provide growth phenology. Growth phenology of major commodity crops, coupled with plant growth models would provide a powerful tool for improved crop market projections and early warning of famine. Synchronized lidar, traditional multi-spectral, and narrow band spectrometer measurements coupled with multi-data fusion techniques would further address multiple research objectives recommended by several other Decadal Survey panels. 6.4 RESULTING SOCIETAL BENEFIT The crucial question for our interactions with water is: How can we protect ecosystems and better manage and predict water availability and quality for future generations, given changes to the water cycle caused by human activities and climate trends? The problem of sustainable water resources management is itself a grand research challenge not only because prediction of future forcings is challenging, but also because real-time measurements of the state of the hydrologic systems, including the essential stores and fluxes of surface water and groundwater, are commonly lacking (National Research Council, 1991, 2012). The recommended enhancements to remote sensing of the water cycle focus on critical research questions and provide insights that enable us to address societal needs for understanding of water systems. Water will become even more critical and difficult to manage in a highly variable future that involves new and growing stressors on the global water resource, all of which contribute to changes in quality, quantity, and availability of water (Zimmerman et al., 2008). These stressors—changing climate, evolving land use and urbanization, shifting water demands for food, energy and fiber, growing and migrating populations, and individual and societal decisions—complicate efforts to ensure clean water to support humans and ecosystems. Balancing the needs of water for people and water for critical and sensitive ecosystems presents a significant challenge for natural, social, and engineering science. There is widespread agreement with regard to the need to develop methods and application-oriented frameworks for using satellite data in water and ecosystems resources management, to monitor and protect public health and agriculture, and to manage disaster preparedness and response (Hossain et al., 2016). 6.4.1 Use of Remotely Sensed Data to Manage Water NASA’s Applied Earth Sciences Program has developed over the last decade a portfolio of demonstration activities focused on transferring research to operations, toward bringing NASA’s observations and models to national and international partners through regional centers (e.g. SERVIR network), training programs, and capacity building. A track record of successful collaborations with FEMA and state agencies already exists including reconnaissance of the 2016 Mississippi and Louisiana flooding to support FEMA (Federal Emergency management Agency) disaster response. Increasing the use of satellite-based observations by the public in the future requires a comprehensive understanding of how observations and models are integrated in sector-specific decision-making toward providing products with information content tailored to meet stakeholder needs. For example, reservoir operations for flood control, water supply, and power generation benefit from river forecasts (Boucher et al., 2012), and more accurate forecasts permit more effective reservoir operations, either through improved seasonal forecasts via improved initial conditions or via more accurate interannual meteorological forecasts (Anghileri et al., 2016). Enhancements to both the initial conditions and the interannual forecasts will be enabled by the Goals, Objectives and Measurements proposed here. Estimates of snow water equivalent drive many of the water supply forecasts, especially in the Western U.S. The measurements and the modeling which will result are specifically designed to improve basin snow water equivalent estimates that can be used to UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-51 Copyright National Academy of Sciences. All rights reserved.

291 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space update forecast model states. The Objective H-1a includes measuring evaporation more accurately, thereby leading to improved formulations of the evaporative fluxes in both meteorological and hydrologic models which can lead to improvements in the interannual forecasts. Availability of reliable information about water at scales from small basins to Earth itself can improve decisions. Recognizing that decisions about managing water and adapting to its scarcities and abundance will be made regardless of the availability of information, we can examine instances where better information has led to decisions that benefit society, either by increasing revenues, decreasing costs, or providing humanitarian benefits that are harder to measure but at least as valuable. In addition, understanding how information is used by various decision-makers or planners is key toward developing more effective use of models and tools in water management (Meyer et al., 2013; Matte et al., 2017; Thiboult et al., 2017). 6.4.2 Ground Water at the Scale of Management Decisions A major challenge in hydrology and water resources management is the lack of integration of groundwater and surface water. This challenge is directly linked to the considerable difficulty of measuring groundwater recharge, which is largely the residual of precipitation and evapotranspiration. Unfortunately, the errors inherent to measurements of precipitation and evapotranspiration often exceed the magnitudes of recharge, rendering this key coupling between surface water and groundwater poorly defined in the past, present and future. The ongoing improvements in remote sensing of precipitation and evapotranspiration will significantly improve our ability to better couple the surface and subsurface parts of the hydrologic cycle. With current knowledge, directly remotely sensing groundwater storage is feasible only at very coarse scales using GRACE (Richey et al., 2015) or possibly by examining hysteresis in elastic subsidence and rebound, for which there is some intriguing evidence (Chaussard et al., 2014). Inelastic subsidence, whereby groundwater withdrawals have permanently lowered the land surface, has been measured with InSAR (Amelung et al., 1999), but at that point the non-sustainable withdrawal of groundwater has already occurred. Sustainably managing all the major stores of circulating freshwater, most of which are in the subsurface, would benefit from a technology to measure changes in groundwater storage at a scale at which basins are managed, ~50 km. As of the date of this publication of the Decadal Survey , such a technology with sufficient evidence of its usefulness does not exist. A GRACE-like instrument would be useful at the 50 km resolution, but the GRACE Follow-On does not achieve that. InSAR has applications in other areas of Earth Science, and if such a technology is selected for launch in the 2024-2034 time frame, then exploring its use in estimating groundwater balance through measurement of elastic subsidence is a fruitful research area for NASA and its partners. UNEDITED PREPUBLICATION–SUBJECT TO FURTHER EDITORIAL CORRECTION 6-52 Copyright National Academy of Sciences. All rights reserved.

292 Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space 6.4.3 Remote Sensing of Snow Water Equivalent Figure 6.12. Information about the snowpack in California’s Tuolumne River Basin from the traditional method based on a network of snow sensors (black lines in the figures) and from NASA’s Airborne Snow Observatory (bars in the figures) for the years 2013 through 2016. The left y-axes show the basin-wide snow water equivalent as a percentage of the median April 1 value, while the right y-axes show snow water equivalent measured from the Airborne Snow Observatory. In 2013, the surface network provided an adequate picture of the snow resource, but in 2014 and 2016, the surface network underestimated the snow. Source: C. Graham and T. H. Painter, NASA Figure 6.12 shows an example of how remotely sensed information increased confidence in water management. The Tuolumne River in California’s Sierra Nevada flows into Hetch Hetchy Reservoir, which supplies water for San Francisco, protects against