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CLIMATE CHANGE SCIENCE
AN ANALYSIS OF SOME KEY QUESTIONS

Committee on the Science of Climate Change
Division on Earth and Life Studies
National Research Council



NATIONAL ACADEMY PRESS
Washington, D.C.



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Page i CLIMATE CHANGE SCIENCE AN ANALYSIS OF SOME KEY QUESTIONS Committee on the Science of Climate Change Division on Earth and Life Studies National Research Council NATIONAL ACADEMY PRESS Washington, D.C.

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Page ii NATIONAL ACADEMY PRESS 2101 Constitution Avenue, N.W. Washington, D.C. 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. International Standard Book Number 0-309-07574-2 Additional copies of this report are available from: National Academy Press 2101 Constitution Avenue, N.W. Lockbox 285 Washington, D.C. 20055 (800) 624–6242 (202) 334–3313 (in the Washington metropolitan area) http://www.nap.edu Printed in the United States of America Copyright 2001 by the National Academy of Sciences . All rights reserved.

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Page iii 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. William 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. William A.Wulf are chairman and vice chairman, respectively, of the National Research Council.

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Page iv

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Page v COMMITTEE ON THE SCIENCE OF CLIMATE CHANGE RALPH J.CICERONE (Chair), University of California, Irvine ERIC J.BARRON, Pennsylvania State University, University Park ROBERT E.DICKINSON, Georgia Institute of Technology, Atlanta INEZ Y.FUNG, University of California, Berkeley JAMES E.HANSEN, NASA/Goddard Institute for Space Studies, New York, New York THOMAS R.KARL, NOAA/National Climatic Data Center, Asheville, North Carolina RICHARD S.LINDZEN, Massachusetts Institute of Technology, Cambridge JAMES C.McWILLIAMS, University of California, Los Angeles F.SHERWOOD ROWLAND, University of California, Irvine EDWARD S.SARACHIK, University of Washington, Seattle JOHN M.WALLACE, University of Washington, Seattle Consultant DANIEL L.ALBRITTON, NOAA/Aeronomy Laboratory, Boulder, Colorado Staff VAUGHAN C.TUREKIAN, Study Director DIANE L.GUSTAFSON, Senior Project Assistant

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Page vi

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Page vii Foreword This study originated from a White House request to help inform the Administration's ongoing review of U.S. climate change policy. In particular, the written request ( Appendix A) asked for the National Academies' “assistance in identifying the areas in the science of climate change where there are the greatest certainties and uncertainties,” and “views on whether there are any substantive differences between the IPCC [Intergovernmental Panel on Climate Change] Reports and the IPCC summaries.” In addition, based on discussions with the Administration, the following specific questions were incorporated into the statement of task for the study: What is the range of natural variability in climate? Are concentrations of greenhouse gases and other emissions that contribute to climate change increasing at an accelerating rate, and are different greenhouse gases and other emissions increasing at different rates? How long does it take to reduce the buildup of greenhouse gases and other emissions that contribute to climate change? What other emissions are contributing factors to climate change (e.g., aerosols, CO, black carbon soot), and what is their relative contribution to climate change? Do different greenhouse gases and other emissions have different draw down periods? Are greenhouse gases causing climate change? Is climate change occurring? If so, how? Is human activity the cause of increased concentrations of greenhouse gases and other emissions that contribute to climate change? How much of the expected climate change is the consequence of climate feedback processes (e.g., water vapor, clouds, snow packs)? By how much will temperatures change over the next 100 years and where? What will be the consequences (e.g., extreme weather, health effects) of increases of various magnitudes? Has science determined whether there is a “safe” level of concentration of greenhouse gases? What are the substantive differences between the IPCC Reports and the Summaries? What are the specific areas of science that need to be studied further, in order of priority, to advance our understanding of climate change? The White House asked for a response “as soon as possible” but no later than early June—less than one month after submitting its formal request. The National Academies has a mandate arising from its 1863 charter to respond to government requests when asked. In view of the critical nature of this issue, we agreed to undertake this study and to use our own funds to support it. A distinguished committee with broad expertise and diverse perspectives on the scientific issues of climate change was therefore appointed through the National Academies' National Research Council (see Appendix B for biographical information on committee members). In early May, the committee held a conference call to discuss the specific questions and to prepare for its 2-day meeting (May 21–22, 2001) in Irvine, California. The committee reviewed the 14 questions and deter

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Page viiimined that they represent important issues in climate change science and could serve as a useful framework for addressing the two general questions from the White House. For the task of comparing IPCC Reports and Summaries, the committee focused its review on the work of IPCC Working Group I, which dealt with many of the same detailed questions being asked above. The committee decided to address the questions in the context of a brief document that also could serve as a primer for policy makers on climate change science. To aid in the presentation, the questions have been organized into seven sections, with the questions addressed in each section listed in italics at the beginning of that section. While traditional procedures for an independent NRC study, including review of the report by independent experts, were followed, it is important to note that tradeoffs were made in order to accommodate the rapid schedule. For example, the report does not provide extensive references to the scientific literature or marshal detailed evidence to support its “answers” to the questions. Rather, the report largely presents the consensus scientific views and judgments of committee members, based on the accumulated knowledge that these individuals have gained—both through their own scholarly efforts and through formal and informal interactions with the world's climate change science community. The result is a report that, in my view, provides policy makers with a succinct and balanced overview of what science can currently say about the potential for future climate change, while outlining the uncertainties that remain in our scientific knowledge. The report does not make policy recommendations regarding what to do about the potential of global warming. Thus, it does not estimate the potential economic and environmental costs, benefits, and uncertainties regarding various policy responses and future human behaviors. While beyond the charge presented to this committee, scientists and social scientists have the ability to provide assessments of this type as well. Both types of assessments can be helpful to policy makers, who frequently have to weigh tradeoffs and make decisions on important issues, despite the inevitable uncertainties in our scientific understanding concerning particular aspects. Science never has all the answers. But science does provide us with the best available guide to the future, and it is critical that our nation and the world base important policies on the best judgments that science can provide concerning the future consequences of present actions. I would especially like to thank the members of this committee and its staff for an incredible effort in producing this important report in such a short period of time. They have sacrificed many personal commitments and worked long weekends to provide the nation with their considered judgments on this critical issue. Bruce Alberts President National Academy of Sciences

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Page ix 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 NRC'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: Betsy Ancker-Johnson, General Motors Corporation (retired), Austin, Texas Wallace S.Broecker, Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York David L.Dilcher, University of Florida, Gainesville Thomas M.Donahue, University of Michigan, Ann Arbor James R.Holton, University of Washington, Seattle Jerry D.Mahlman, NOAA/Geophysical Fluid Dynamics Laboratory (retired), Princeton, New Jersey Syukuro Manabe, Institute for Global Change Research, Yokohama City, Japan Pamela A.Matson, Stanford University, California William D.Nordhaus, Yale University, New Haven, Connecticut Eugene M.Rasmusson, University of Maryland, College Park Susan Solomon, NOAA/Aeronomy Laboratory, Boulder, Colorado Andrew R.Solow, Woods Hole Oceanographic Institution, Massachusetts Grace Wahba, University of Wisconsin, Madison Although the reviewers listed above have provided 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 Richard M.Goody (Harvard University) and Robert A.Frosch (Harvard University). 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 committee and the institution. We would also like to thank the following individuals for their input regarding the IPCC process: John Christy, Haroon Kheshgi, Michael Mann, Jerry Meehl, Berrien Moore, Michael Oppenheimer, Joyce Penner, Ray Pierrehumbert, Michael Prather, Venkatachalam Ramaswamy, Ben Santer, Piers Sellers, Susan Solomon, Ron Stouffer, Kevin Trenberth, and Robert Watson.

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Page xi Contents SUMMARY 1 1     CLIMATE, CLIMATE FORCINGS, CLIMATE SENSITIVITY, AND TRANSIENT CLIMATE CHANGE 6     Climate, 6     Climate Forcings, 6     Climate Sensitivity, 6     Transient Climate Change, 7 2     NATURAL CLIMATIC VARIATIONS 8 3     HUMAN CAUSED FORCINGS 9     Greenhouse Gases, 9     Aerosols, 12     Climate Forcings in the Industrial Era, 12 4     CLIMATE SYSTEM MODELS 15 5     OBSERVED CLIMATE CHANGE DURING THE INDUSTRIAL ERA 16     The Occurrence of Climate Change, 16     The Effect of Human Activities, 17 6     FUTURE CLIMATE CHANGE 18     Estimating Future Climate Change, 18 7     ASSESSING PROGRESS IN CLIMATE SCIENCE 22     Intergovernmental Panel on Climate Change, 22     Research Priorities, 23 APPENDIXES     A     Letter from the White House 27     B     Biographical Sketches of Committee Members and Staff 28

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