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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

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 Insture of
Medicine. The members of the commimee responsible for the report were chosen for their special competences and with regard for appropri-
ate balance

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Copyright 2001 by the Nadonal Academy of Sciences. All rights reserved.

THE NATIONAL ACADEMIES

National Academy of Sciences
National Academy of Engineering
Institute of Medicine
National Research Council

The National Academy of Sciences is a privalcponprofil, self-perpctuating society of distinguished scholars engaged in scientific and engineering tosearch, 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 mandule that requires it to advise the federal governinent on scientific and technical matters. Dr. Bruce M. Alberts is presidcat 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 ibe National Academy of Sciences the responsibility for advising the federal government. The National Academy of Enginering also sponsors engineer ing programans aimed at meeting national needs, cacourages education and research, and recognizes the superior achievements of engineers. Dr. Williaro A. Wulf is president of the National Academy of Engineering

The Lastitute 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 hcalth of the public. The Institute ucts 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 Identity issues of medical care, research, and education. Dr. Kenneth L Shinc is president of the lastute of Medicine.

The National Rescarct Connell 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 agcacy of both the National Academy of Sciences and the National Academy of Engineering is providing servios to the governmeny 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. Wwf are chairman and vice chairman, respectively, of the National Research Council.

COMMITTEE ON THE SCIENCE OF CLIMATE CHANGE

RALPH J. CICERONE (Chair), University of Califomia, Irvine
ERIC J. BARRON, Pennsylvania State University, University Park
ROBERT E. DICKINSON, Georgia Institute of Technology, Atlanta
INEZ 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, Scattle

Consultant

DANIEL L ALBRITTON, NOAA/Acronomy Laboratory, Boulder, Colorado

Staff

VAUGHAN C. TUREKIAN, Swdy Director
DIANE L. GUSTAFSON, Senior Project Assistant

Foreword

This study originated from a White House request to help iaform the Administration's ongoing review of U.S. climate change policy. In particular, the wrinen request (Appendix A) asked for the National Academies' "assistance in identifying the areas in the science of climate change wbere there are the greatest certainties and uncertainties," and "vicws on whether there are any substantive differences between the IPCC (Intergovernmental Panel on Climate Changc) Reports and the IPCC summarias.” In addidon, based on discussions with che Administration, the following specific questions were incorporated into the statemcat 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 acceler. ating rase, 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 contribue lo climate change?

What other emissions are contributing factors to climate change le.g., acrosols, CO, black carbon soor), 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, show packs)?

By how much will temperatures change over the next 100 years and where? • What will be the consequences (c.8., 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 carly Junc-less than one month after submitting its formal request

The National Academics 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 Appeadix B for biographical information on committee members). In carly May, the committee held a confereoce call to discuss the specific questions and to prepare for its 2-day mecting (May 21-22, 2001) in Irvine, California. The committec reviewed the 14 questions and deter

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