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UNDERSTANDING ~ 1 11V12~1= CHANGE FEEDBACKS Panel on Climate Change Feedbacks Climate Research Committees Boa rcl on Atmospheric Sciences and Climate Division on Earth and Life Studies NATIONAl RESEARCH COUNCIL OF THE NATIONAl ACADEMIES THE NATIONAL ACADEMIES PRESS Washington, D.C. www.nap.edu

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THE NATIONAL ACADEMIES PRESS 500 Fifth Street, N.W. Washington, D.C. 20001 NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance. This study was supported by Contract No. NASW-01008 between the National Academy of Sciences and the U.S. Global Change Research Program (through the National Aeronautic and Space Administration) and Grant No. ATM-0136349 between the National Academy of Sciences and the National Science Foundation. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the organizations or agencies that provided support for the project. International Standard Book Number 0-309-09072-5 (Book) International Standard Book Number 0-309-52744-9 (PDF) Library of Congress Control Number 2003115368 Additional copies of this report are available from the National Academies Press, 500 Fifth Street, N.W., Lockbox 285, Washington, DC 20055; (800) 624-6242 or (202) 334-3313 (in the Washington metropolitan area); Internet, http://www.nap.edu Cover: A vortex street near the Canary Islands. A vortex street often occurs when cloud formations over the ocean are disturbed by wind passing over land or another obstacle. In this Moderate Resolution Imaging Spectroradiometer (MODIS) image from July 5, 2002, marine stratocumulus clouds have arranged themselves in rows, or streets, which are usually parallel to the direction of wind flow. Downwind of obstacles, in this case, the Canary Islands off the west African coast, eddies create turbulent patterns called vortex streets. Marine boundary layer clouds have a large effect on Earth's energy balance and understanding them is important to predicting the response of climate to human activities. Copyright 2003 by the National Academy of Sciences. All rights reserved. Printed in the United States of America

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THE NATIONAL ACADEMIES Advisers to the Nation on Science, Engineering, and Medirine The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Bruce M. Alberts is president of the National Academy of Sciences. The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. Wm. A. Wulf is president of the National Academy of ~ . . . ~ nglneermg. The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Harvey V. Fineberg is president of the Institute of Medicine. The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy's purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Bruce M. Alberts and Dr. Wm. A. Wulf are chair and vice chair, respectively, of the National Research Council www.national-academ ies.org

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v PANEL ON CLIMATE CHANGE FEEDBACKS DENNIS L. HARTMANN (Chair), University of Washington, Seattle ALAN K. BETTS, Atmospheric Research, Pittsforct, Vermont GORDON B. BONAN, National Center for Atmospheric Research, Boulder, Colorado LEE E. BRANSCOME, Environmental Dynamics Research, Palm Beach Gardens, Florida ANTONIO ]. BUSALACCHI, JR., University of Maryland, College Park AMANDA LYNCH, University of Colorado, Boulder* SYUKURO MANABE, Princeton University, New Jersey DOUGLAS G. MARTINSON, Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York RAYMOND NAIJAR, Pennsylvania State University, University Park EUGENE M. RASMUSSON, University of Maryland, College Park A.R. RAVISHANKARA, NOAA Aeronomy Laboratory, Boulder, Colorado DANIEL R. SAREWITZ, Columbia University, Center for Science, Policy, and Outcomes, Washington, D.C.** DIAN ]. SElDEL, NOAA Air Resources Laboratory, Silver Spring, Maryland GRAEME L. STEPHENS, Colorado State University, Fort Collins LYNNE D. TALLEY, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, California JOHN M. WALLACE, University of Washington, Seattle ANDREW J. WEAVER, University of Victoria, British Columbia, Canada STEVEN C. WOFSY, Harvard University, Cambridge, Massachusetts ERIC F. WOOD, Princeton University, New Jersey .,, A, Consultant MICHAEL J. PRATHER, University of California, Irvine NRC Staff CHRIS ELFRING, BASC Director PETER A. SCHULTZ, Study Director ROB GREENWAY, Project Assistant (from February 2002) CARTER W. FORD, Project Assistant (until December 2001) *Through February 2002. **Through August 2003.

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BOARD ON ATMOSPHERIC SCIENCES AND CLIMATE ERIC J. BARRON (Chair), Pennsylvania State University, University Park FREDERICK R. ANDERSON, Cadwalader, Wickersham & Taft, Washington, D.C. RAYMOND J. BAN, The Weather Channel, Inc., Atlanta, Georgia ROBERT C. BEARDSLEY, Woods Hole Oceanographic Institution, Massachusetts ROSINA M. BIERBAUM, University of Michigan, Ann Arbor RAFAEL L. BRAS, Massachusetts Institute of Technology, Cambridge MARY ANNE CARROLL, University of Michigan, Ann Arbor KERRY A. EMANUEL, Massachusetts Institute of Technology, Cambridge CASSANDRA G. FESEN, Dartmouth College, Hanover, New Hampshire MARGARET A. LEMONE, National Center for Atmospheric Research, Boulder, Colorado JENNIFER A. LOGAN, Harvard University, Cambridge, Massachusetts MARIO J. MOLINA, Massachusetts Institute of Technology, Cambridge WILLIAM J. RANDEL, National Center for Atmospheric Research, Boulder, Colorado ROBERT J. SERAFIN, National Center for Atmospheric Research, Boulder, Colorado ROGER M. WAKIMOTO, University of California, Los Angeles JOHN C. WYNGAARD, Pennsylvania State University, University Park Ax Off cio Members ANTONIO J. BUSALACCHI, JR., University of Maryland, College Park ERIC F. WOOD, Princeton University, New Jersey NRC Staff CHRIS ELFRING, Director ELBERT W. (JOE) FRIDAY, JR., Senior Scholar PETER A. SCHULTZ, Senior Program Officer LAURIE S. GELLER, Senior Program Officer AMANDA STAUDT, Program Officer DIANE L. GUSTAFSON, Administrative Associate ROB GREENWAY, Project Assistant ELIZABETH A. GALINIS, Project Assistant ROBIN MORRIS, Financial Officer Vl

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PREFACE The National Research Council (NRC), the operating arm of the National Academies, has over the years produced an extensive body of work focused on issues relevant to observing and understanding changes in climate. Several NRC reports have provided advice to guide the evolution of U.S. research priorities and, most recently, the Climate Change Science report (NRC, 2001a) concluded that Maintaining a vigorous, ongoing program of basic research, funded and managed independently of the climate assessment activity, will be crucial for narrowing these uncertainties.... The ability of the United States to assess fixture climate change is severely limited by the lack of a climate observing system, by inadequate computational resources, and by the general inability of government to focus resources on climate problems. Efforts are needed to ensure that U.S. efforts in climate research are supported and managed to ensure innovation, effectiveness, and efficiency. Although these statements are addressed toward the entirety of the climate research enterprise, the report also specifically discussed the importance of reducing the uncertainties associated with climate change feedbacks such as water vapor, clouds, and snow cover. This report is an attempt to look in detail at that challenge and identify ways to improve our understanding of climate change feedback processes. 'Decade-to-Century-Scale Climate Variability and Change: A Science Strategy (NRC, 1998a); The Atmospheric Sciences Entering the Twenty-First Century (NRC, 1998b); Adequacy of Climate Observing Systems (NRC, 1999a); Global Environmental Change: Research Pathways for the Next Decade (NRC, l999b); Improving the Effectiveness of U.S. Climate Modeling (NRC, 2001b); The Science of Regional and Global Change: Putting Knowledge to Work (NRC, 2001 c). vi'

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. . . V111 UNDERSTANDING CLING TE CHANGE FEEDBA CKS Over the past decade we have learned much about the complex natural processes that influence climate variability and change, and our ability to model climate has increased significantly. In addition, we have gained a better appreciation for the important connections between physical, biological, and social sciences in the climate system. We have also begun to better identify those parts of the climate system that are particularly important and not well understood, and therefore limit our ability to project the future evolution of Earth's climate. One of these critical areas is our understanding of the role of feedbacks in the climate system and their role in determining climate sensitivity. The ultimate goal of climate feedbacks research is to enable accurate predictions of the response of Earth's climate to specified natural or human-induced influences on the climate system. Clearly, humans play a major role in some feedback loops, particularly those involving biological and land surface processes, and it can be difficult to distinguish between the effects of human activities and natural processes. To maintain a sharp focus for this study, however, we have not addressed those feedbacks that depend on a human response to either amplify or damp an initial perturbation. We recognize, however, that research into climate change must ultimately embrace natural and human-induced feedbacks holistically. Furthermore, we do not address uncertainties associated with natural or human-induced climate forcing. The question addressed here is, "If humans provide specified inputs to atmospheric composition or changes in land surface that force changes in the climate system, how do natural climate processes influence our ability to project the response of climate to that forcing?" Although this report focuses in particular on climate change feedbacks, this research cannot be separated from other efforts to understand the climate system. Many of the research approaches and techniques used to better understand climate feedbacks will also shed light on other aspects of the climate system. Moreover, climate feedbacks research will take place in the context of continuing efforts to detect climate change, attribute climate change to specific causes, and make practical projections of future climates. For these reasons we have taken a broad view of what constitutes a feedback process, including in our definition not only the processes that affect the equilibrium response of global mean surface temperature to a specified forcing but also natural processes whereby climate changes feed back on the rate of warming, the magnitude of the climate forcing, or the spatial pattern of climate change. The Panel on Climate Change Feedbacks interpreted its charge (see Executive Summary) as a request for broad guidance on the key avenues of research to be pursued to better understand climate feedbacks and their role

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PREFA CE IX in climate sensitivity. Given our resources, we did not attempt to formulate a detailed research strategy, which would take significantly more time and community input than our process allowed. Rather, we sought to review the science of climate feedback processes broadly and call attention to those areas where additional focus might bear fruit in the near term. Thus, this report should be viewed as a starting point for the formulation of a long-term research strategy for addressing the issue of climate feedback processes and their role in climate sensitivity. Comprehensive disciplinary plans have in some cases already been carried out by groups of scientists convened by the agencies. Where this work is relevant to the report's discussion we have referred to it. The Panel's deliberations included some critical and difficult issues that did not make it into the final report. These include the scientific definition of uncertainty, its quantitative evaluation, and its relation to the development of policy options regarding climate change. I, like many scientists, believe that directed, high-quality research will increase understanding, and thus reduce uncertainty that is associated with incomplete knowledge of the climate system. My belief is that this improved knowledge can then be used to support decision making for the benefit of society, although this view is not universally accepted. In my view the proper and useful role of scientists is to use the scientific method to seek after natural truth as best we can. This being said, it is difficult to argue in a quantitative way that increased understanding will lead quickly to narrowing of the range of estimates of how much global warming will result from a doubling of atmospheric carbon dioxide. Indeed, new discoveries may widen the apparent possibilities. In this report we focus our attention on the key research questions necessary to better understand the feedback processes that we believe to be the most important. Larger questions are left to other groups to address. The primary funding for this study was provided by the U.S. Global Change Research Program, with additional funds from the National Science Foundation. Representatives from NSF and other relevant agencies, including the National Oceanic and Atmospheric Administration, the National Aeronautics and Space Administration, and the Department of Energy provided encouragement and input to the study, in particular through a meeting with the Panel in February 2002. In closing, I wish to thank the members of the Panel on Climate Change Feedbacks for their efforts in preparing this report. I also thank all those who participated in the Panel's information-gathering workshop in Boulder, Colorado, during August 2001 and in the Panel's meeting with federal agencies during February 2002 in Washington, D.C. Peter A. Schultz did an

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x UNDERSTANDING CLIMA TE CHANGE FEEDBACKS excellent job as study director and contributed greatly to this report. Chris Elfring made major contributions during the review and revision process. Dennis Hartrnann Chair

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ACKNOWLEDGMENTS This report has been reviewed in draft form by individuals chosen for their diverse perspectives and technical expertise, in accordance with procedures approved by the National Research Council's Report Review Committee. The purpose of this independent review is to provide candid and critical comments that will assist the institution in making its published report as sound as possible and to ensure that the report meets institutional standards for objectivity, evidence, and responsiveness to the study charge. The review comments and draft manuscript remain confidential to protect the integrity of the deliberative process. We wish to thank the following individuals for their review of this report: D. James Baker, Academy of Natural Sciences, Philadelphia, Pennsylvania Peter Gent, National Center for Atmospheric Research, Boulder. Colorado Anthony DelGenio, Goddard Institute for Space Studies, New York, N.Y. Isaac Held, National Oceanic and Atmospheric Administration, Princeton, New Jersey George Hornberger, University of Virginia, Charlottesville Richard Lindzen, Massachusetts Institute of Technology, Cambridge Berrien Moore, University of New Hampshire, Durham Gerald North, Texas A&M University, College Station Edward Sarachik, University of Washington, Seattle Tom Wigley, National Center for Atmospheric Research, Boulder, Colorado James Anderson, Harvard University, Cambridge, Massachusetts Although the reviewers listed above have provided constructive comments and suggestions, they were not asked to endorse the report's Xl

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. . Xll UNDERSTANDING CLING TE CHANGE FEEDBACKS conclusions or recommendations, nor did they see the final draft of the report before its release. The review of this report was overseen by Richard Goody, Harvard University, and Kuo-Nan Liou, University of California, Los Angeles. Appointed by the National Research Council, they were responsible for making certain that an independent examination of this report was carried out in accordance with institutional procedures and that all review comments were carefully considered. Responsibility for the final content of this report rests entirely with the authoring panel and the institution.

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CONTENTS EXECUTIVE SUMMARY The Need for Climate Feedbacks Research, 2 Priorities in Climate Feedback Research, 7 1 INTRODUCTION 15 2 CLOUD, WATER VAPOR, AND LAPSE RATE FEEDBACKS 21 Water Vapor, 21 Lapse Rate Feedback, 24 Cloud Feedbacks, 26 Why Has Progress on Cloud, Water Vapor, and Lapse Rate Feedbacks Been so Elusive? 32 Developing a Scientific Strategy, 35 3 SEA-ICE FEEDBACKS Overview of Sea-Ice Feedbacks, 43 Developing a Scientific Strategy, 44 4 OCEAN HEAT UPTAKE AND OCEAN CIRCULATION FEEDBACKS Mixing, Ocean Heat Uptake, and Climate Feedbacks, 50 Thermohaline Circulation Feedbacks, 53 TERRESTRIAL HYDROLOGY AND VEGETATION FEEDBACKS Terrestrial Hydrology, 60 Vegetation Feedbacks, 65 Developing a Scientific Strategy, 69 . . . X111 41 48 59

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xlv UNDERSTANDING CLIMA TE CHANGE FEEDBACKS 6 ATMOSPHERIC CHEMICAL FEEDBACKS An Example of the Multiple Dimensions of Climate-Air Chemistry Feedback, 78 Overview of Factors Controlling Climate, Chemistry, and Aerosol Interactions, 8 1 Developing a Scientific Strategy, 84 76 7 BIOGEOCHEMICAL FEEDBACKS AND THE CARBON CYCLE 88 Terrestrial Carbon Feedbacks, 90 Marine Biogeochemical Feedbacks, 97 8 MODES OF VARIABILITY 9 RECOMMENDATIONS Key Observations Needed to Monitor and Understand Climate Change Feedbacks, 111 Evaluating Progress in Understanding Climate Feedbacks, 113 REFERENCES APPENDIXES A Climate Change Feedbacks Workshop B Biographical Sketches of Panel Members C Acronyms 107 110 118 137 144 151