Abrupt Climate Change

Inevitable Surprises

Committee on Abrupt Climate Change

Ocean Studies Board

Polar Research Board

Board on Atmospheric Sciences and Climate

Division on Earth and Life Studies

National Research Council

NATIONAL ACADEMY PRESS
Washington, D.C.



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Abrupt Climate Change: Inevitable Surprises Abrupt Climate Change Inevitable Surprises Committee on Abrupt Climate Change Ocean Studies Board Polar Research Board Board on Atmospheric Sciences and Climate Division on Earth and Life Studies National Research Council NATIONAL ACADEMY PRESS Washington, D.C.

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Abrupt Climate Change: Inevitable Surprises NATIONAL ACADEMY PRESS 2101 Constitution Ave., N.W. Washington, DC 20418 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/Grant 50-DKNA-7-90052 between the National Academy of Sciences, NOAA’s United States Global Change Research Program, and the National Aeronautics and Space Administration. Additional funds for the impacts workshop were provided by the Yale National Bureau of Economic Research Program on Environmental Economics. 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. Library of Congress Cataloging-in-Publication Data Abrupt climate change : inevitable surprises / Committee on Abrupt Climate Change, Ocean Studies Board, Polar Research Board, Board on Atmospheric Sciences and Climate, Division on Earth and Life Studies, National Research Council. p. cm. Includes bibliographical references and index. ISBN 0-309-07434-7 I. National Research Council (U.S.). Committee on Abrupt Climate Change. 2002002017 Cover: This Spaceborne Imaging Radar-C/X-Band Synthetic Aperture Radar (SIR-C/X-SAR) image provided by NASA JPL and acquired from the Space Shuttle Endeavour, STS-59 (April 11, 1994), shows part of the vast Namib Sand Sea on the west coast of southern Africa, just northeast of the city of Luderitz, Namibia. This region receives only a few centimeters of rain per year. The colors are assigned to different radar frequencies. The magenta areas in the image are fields of sand dunes, and the orange area along the bottom is the surface of the South Atlantic Ocean. The bright green features in the upper right are rocky hills protruding through the sand sea. Because this radar penetrates through the sand, it can reveal sub-surface features such as former lakes, rivers, and drainage channels that have long since dried up as the climate changed. SIR-C/X-SAR is a joint US-German-Italian project that captures sophisticated images of Earth that are useful to scientists from many disciplines. (NASA JPL image). Additional copies of this report are available from the National Academy Press, 2101 Constitution Avenue, N.W., Lockbox 285, Washington, DC 20055; 800-624-6242 or 202-334-3313 (in the Washington metropolitan area); Internet: http://www.nap.edu. Copyright 2002 by the National Academy of Sciences. All rights reserved. Printed in the United States of America

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Abrupt Climate Change: Inevitable Surprises THE NATIONAL ACADEMIES National Academy of Sciences National Academy of Engineering Institute of Medicine National Research Council 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 Engineering. 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. Kenneth I. Shine 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 chairman and vice chairman, respectively, of the National Research Council.

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Abrupt Climate Change: Inevitable Surprises COMMITTEE ON ABRUPT CLIMATE CHANGE RICHARD B. ALLEY (Chair), Pennsylvania State University, University Park JOCHEM MAROTZKE, Southampton Oceanography Centre, United Kingdom WILLIAM NORDHAUS, Yale University, New Haven, Connecticut JONATHAN OVERPECK, University of Arizona, Tucson DOROTHY PETEET, National Aeronautics and Space Administration, New York, New York ROGER PIELKE, JR., Center for Science and Technology Policy Research, University of Colorado, Boulder RAYMOND PIERREHUMBERT, University of Chicago, Illinois PETER RHINES, University of Washington, Seattle THOMAS STOCKER, University of Bern, Switzerland LYNNE TALLEY, Scripps Institution of Oceanography, La Jolla, California J. MICHAEL WALLACE, University of Washington, Seattle Staff ALEXANDRA ISERN, Study Director (until July 2001) JOHN DANDELSKI, Project Manager (as of August 2001) CHRIS ELFRING, Director, Polar Research Board MORGAN GOPNIK, Director, Ocean Studies Board MEGAN KELLY, Senior Project Assistant (until April 2001) JODI BACHIM, Senior Project Assistant ANN CARLISLE, Senior Project Assistant

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Abrupt Climate Change: Inevitable Surprises Preface Large, abrupt climate changes have repeatedly affected much or all of the earth, locally reaching as much as 10°C change in 10 years. Available evidence suggests that abrupt climate changes are not only possible but likely in the future, potentially with large impacts on ecosystems and societies. This report is an attempt to describe what is known about abrupt climate changes and their impacts, based on paleoclimate proxies, historical observations, and modeling. The report does not focus on large, abrupt causes—nuclear wars or giant meteorite impacts—but rather on the surprising new findings that abrupt climate change can occur when gradual causes push the earth system across a threshold. Just as the slowly increasing pressure of a finger eventually flips a switch and turns on a light, the slow effects of drifting continents or wobbling orbits or changing atmospheric composition may “switch” the climate to a new state. And, just as a moving hand is more likely than a stationary one to encounter and flip a switch, faster earth-system changes—whether natural or human-caused—are likely to increase the probability of encountering a threshold that triggers a still faster climate shift. We do not yet understand abrupt climate changes well enough to predict them. The models used to project future climate changes and their impacts are not especially good at simulating the size, speed, and extent of

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Abrupt Climate Change: Inevitable Surprises the past changes, casting uncertainties on assessments of potential future changes. Thus, it is likely that climate surprises await us. When orbital wiggles and rising greenhouse gases warmed the earth from the last ice age, proxy records show that smooth changes were interspersed with abrupt coolings and warmings, wettings and dryings. By analogy, the expected future warming may come smoothly, but may come with jumps, short-lived or local coolings, floods or droughts, and other unexpected changes. Societies and ecosystems have an easier time dealing with slower or better-anticipated changes, so the abruptness and unpredictability of the possible changes may be disquieting. This report considers patterns, magnitudes, mechanisms, and impacts of abrupt climate changes, possible implications for the future, and critical knowledge gaps. The potentially large impacts and prediction difficulties focus special attention on increasing the adaptability and resiliency of societies and ecosystems. The committee notes that there is no need to be fatalistic; human and natural systems have survived many abrupt changes in the past, and will continue to do so. Nonetheless, future dislocations can be minimized by taking steps to face the potential for abrupt climate change. The committee believes that increased knowledge is the best way to improve the effectiveness of response, and thus that research on abrupt climate change can help reduce vulnerabilities and increase adaptive capabilities. I would like to thank the US Global Change Research Program and staff at the many agencies who are a part of USGCRP, for funding and participating in this study process. Thanks also to the Yale/NBER Program on International Environmental Economics for additional funding, and to committee members Bill Nordhaus and Dorothy Peteet for organizing and conducting the Impacts Workshop. The numerous participants at our workshops, the reviewers, and many other colleagues contributed valuable insights and encouragement. It has been my privilege to work with Study Director Alexandra Isern (now with the National Science Foundation), Polar Research Board Director Chris Elfring, Research Associate John Dandelski, and with Senior Project Assistants Megan Kelly, Jodi Bachim, and Ann Carlisle, of the National Research Council. I thank the Ocean Studies Board, the Polar Research Board, and the Board on Atmospheric Science and Climate for providing the impetus to do this study and oversight throughout the process. I would especially like to extend my deep appreciation to the committee’s members for their efforts in creating this report. By exploring

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Abrupt Climate Change: Inevitable Surprises new territory and working across disciplines, the committee has opened my eyes to exciting new frontiers. I hope that the readers of this report join us in seeing not peril but opportunity for improved knowledge leading toward a happier and more secure future. Richard B. Alley, Chair Committee On Abrupt Climate Change

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Abrupt Climate Change: Inevitable Surprises Acknowledgments The committee would like to express its appreciation to the many people who contributed to this report. In particular, we would like to thank the participants of the two workshops held as part of this study, especially those who gave keynote presentations at the Workshop on Abrupt Climate Change: William Ascher, David Bradford, Grant Branstator, Tony Broccoli, Wallace Broecker, Mark Cane, Bob Dickson, Isaac Held, Sylvie Joussaume, John Kutzbach, Jean Lynch-Stieglitz, Peter Schlosser, Jeff Severinghaus, Karen Smoyer, and Gary Yohe. These talks helped set the stage for fruitful discussions in the breakout sessions that followed. The steering committee is also grateful to Doug Martinson for stepping in at the last minute to lead one of the breakout discussions. In addition, the committee is grateful to the participants of the Workshop on the Economic and Ecological Impacts of Abrupt Climate Change for their insights and written reports, parts of which have been incorporated into this report: Craig Allen, Edward Cook, Peter Daszak, Mark Dyurgerov, David Inouye, Klaus Keller, George Kling, Walter Koenig, Carl Leopold, Thomas Lowell, Robert Mendelsohn, John Reilly, Joel Smith, Thomas Swetnam, Richard Tol, Ferenc Toth, Harvey Weiss, John Weyant, and Gary Yohe. The committee is also grateful to a number of people who provided important discussion and/or material for this report, including Vic Baker, Katherine Hirschboeck, and James Knox. The committee would like to acknowledge the generous financial sup-

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Abrupt Climate Change: Inevitable Surprises port provided by the U.S. Global Change Research Program, which requested this study, and the Yale National Bureau of Economic Research on International Environmental Economics, which provided funds for the impacts workshop. 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 participation in the review of this report: James Kennett, University of California, Santa Barbara Mahlon C. Kennicutt, Texas A&M University, College Station Roger Lukas, University of Hawaii, Manoa Robie Macdonald, Fisheries and Oceans Canada, British Columbia Vera Markgraf, University of Colorado, Boulder Stephen Rayner, Columbia University, New York Jeffrey Severinghaus, Scripps Institution of Oceanography, La Jolla Andrew Weaver, University of Victoria, British Columbia, Canada Gary Yohe, Wesleyan University, Middletown, Connecticut Although the reviewers listed above have provided many constructive comments and suggestions, they were not asked to endorse the conclusions or recommendations nor did they see the final draft of the report before its release. The review of this report was overseen by Robert Knox (Scripps Institution of Oceanography) and Stephen Berry (University of Chicago), appointed by the National Research Council, who 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 committee and the institution.

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Abrupt Climate Change: Inevitable Surprises Contents     EXECUTIVE SUMMARY   1     Improve the Fundamental Knowledge Base Related to Abrupt Climate Change,   2     Improve Modeling Focused on Abrupt Climate Change,   4     Improve Paleoclimatic Data Related to Abrupt Climate Change,   5     Improve Statistical Approaches,   7     Investigate “No-Regrets” Strategies to Reduce Vulnerability,   8 1   INTRODUCTION TO ABRUPT CHANGES IN THE EARTH’S CLIMATE   10     Definition of Abrupt Climate Change,   14     Scientific Evidence, Processes, and Consequences for Society and Ecosystems,   15     The Charge to the Committee,   17 2   EVIDENCE OF ABRUPT CLIMATE CHANGE   19     Interpretation of Past Climatic Conditions from Proxy Records,   19     The Younger Dryas as an Example of Abrupt Climate Change,   24     Abrupt Climate Changes Before the Younger Dryas Event,   36     Eemian Rapid Climate Change,   38     Holocene Rapid Climate Change,   40     Rapid Climate Changes in the Instrumental Period,   45     Patterns of Climate Variability,   48     Trends Recorded Instrumentally,   56     Synopsis of Observations,   69

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Abrupt Climate Change: Inevitable Surprises 3   PROCESSES THAT CAUSE ABRUPT CLIMATE CHANGE   73     An Overview of Mechanisms,   74     Atmosphere,   80     Abrupt Climate Change and Thermohaline Circulation,   89 4   GLOBAL WARMING AS A POSSIBLE TRIGGER FOR ABRUPT CLIMATE CHANGE   107     Changes in North Atlantic Thermohaline Circulation,   108     Limited Predictability Close to an Instability,   111     Changes in Natural Modes of the Atmosphere-Ocean System,   112     Possible Future Abrupt Changes in the Hydrological Cycle,   113     Ice Sheet Changes,   114     Outlook,   115 5   ECONOMIC AND ECOLOGICAL IMPACTS OF ABRUPT CLIMATE CHANGE   118     Recent Scientific Studies in the Ecological and Social Sciences,   120     Sectoral Approaches,   141     Modeling the Impacts of Climate Change,   149 6   FINDINGS AND RECOMMENDATIONS   153     Improve the Fundamental Knowledge Base Related to Abrupt Climate Change,   154     Improve Modeling Focused on Abrupt Climate Change,   159     Improve Paleoclimatic Data Related to Abrupt Climate Change,   161     Improve Statistical Approaches,   163     Investigate “No-Regrets” Strategies to Reduce Vulnerability,   164     Final Thoughts,   167     REFERENCES   168     APPENDIXES   203     ACommittee and Staff Biographies,   203     BBoard Rosters,   208     CWorkshop Agenda,   212     DWorkshop Participants,   214     EImpacts Workshop Program,   216     FImpacts Workshop Participants,   218     GAcronym List,   220     INDEX   223

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Abrupt Climate Change: Inevitable Surprises Abrupt Climate Change

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