RADIATIVE FORCING OF CLIMATE CHANGE

EXPANDING THE CONCEPT AND ADDRESSING UNCERTAINTIES

Committee on Radiative Forcing Effects on Climate

Climate Research Committee

Board 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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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties RADIATIVE FORCING OF CLIMATE CHANGE EXPANDING THE CONCEPT AND ADDRESSING UNCERTAINTIES Committee on Radiative Forcing Effects on Climate Climate Research Committee Board 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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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties THE NATIONAL ACADEMIES PRESS 500 Fifth Street, NW Washington, DC 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. Support for this project was provided by the National Oceanic and Atmospheric Administration under Contract No. NASW-01008. Any opinions, findings, and 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 0-309-09506-9 (Book) International Standard Book Number 0-309-54688-5 (PDF) Library of Congress Control Number 2005921687 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: Images obtained from the Clouds and Earth’s Radiant Energy System (CERES) instrument on board NASA’s Aqua satellite on June 22, 2002. The image on the front cover shows the amount of infrared energy, or heat, emitted by the Earth and its atmosphere to space. Clear, warm land regions (shown in yellow) emit the most heat. High, cold clouds (shown in blue and white) emit less heat to space. The image on the back cover shows the amount of sunlight reflected back to space. Clear ocean areas (shown in dark blue) reflect the least amount of sunlight back to space, and clouds and snow-covered surfaces (shown in white and green) reflect the most sunlight back to space. SOURCE: National Aeronautics and Space Administration (NASA) Goddard Space Flight Center. Copyright 2005 by the National Academy of Sciences. All rights reserved. Printed in the United States of America

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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties THE NATIONAL ACADEMIES Advisers to the Nation on Science, Engineering, and Medicine 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. 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-academies.org

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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties COMMITTEE ON RADIATIVE FORCING EFFECTS ON CLIMATE DANIEL J. JACOB (Chair), Harvard University, Cambridge, Massachusetts RONI AVISSAR, Duke University, Durham, North Carolina GERARD C. BOND, Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York STUART GAFFIN, Columbia University, New York, New York JEFFREY T. KIEHL, National Center for Atmospheric Research, Boulder, Colorado JUDITH L. LEAN, Naval Research Laboratory, Washington, D.C. ULRIKE LOHMANN, ETH Zurich, Switzerland MICHAEL E. MANN, University of Virginia, Charlottesville ROGER A. PIELKE, SR., Colorado State University, Fort Collins VEERABHADRAN RAMANATHAN, Scripps Institution of Oceanography, University of California, San Diego LYNN M. RUSSELL, Scripps Institution of Oceanography, University of California, San Diego NRC Staff AMANDA STAUDT, Study Director PARIKHIT SINHA, Program Officer GERALDEAN H. LANTIER, Christine Mirzayan Science and Technology Policy Intern ELIZABETH A. GALINIS, Senior Program Assistant ROB GREENWAY, Senior Program Assistant

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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties CLIMATE RESEARCH COMMITTEE ANTONIO J. BUSALACCHI, JR. (Chair), University of Maryland, College Park LEE E. BRANSCOME, Environmental Dynamics Research, Inc., Palm Beach, Florida JAMES A. COAKLEY, JR., Oregon State University, Corvallis JULIA E. COLE, University of Arizona, Tucson JUDITH A. CURRY, Georgia Institute of Technology, Atlanta CLARA DESER, National Center for Atmospheric Research, Boulder, Colorado DAVID KAROLY, University of Oklahoma, Norman ROBERT J. LEMPERT, RAND Corporation, Santa Monica, California LINDA O. MEARNS, National Center for Atmospheric Research, Boulder, Colorado GERALD A. MEEHL, National Center for Atmospheric Research, Boulder, Colorado PETER B. RHINES, University of Washington, Seattle W. JAMES SHUTTLEWORTH, University of Arizona, Tucson LYNNE D. TALLEY, Scripps Institution of Oceanography, La Jolla, California HASSAN VIRJI, International START Secretariat, Washington, D.C. YUK YUNG, California Institute of Technology, Pasadena NRC Staff AMANDA STAUDT, Senior Program Officer ROB GREENWAY, Senior Program Assistant

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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties BOARD ON ATMOSPHERIC SCIENCES AND CLIMATE ROBERT J. SERAFIN (Chair), National Center for Atmospheric Research, Boulder, Colorado FREDERICK R. ANDERSON, McKenna Long & Aldridge LLP, Washington, D.C. ROBERT C. BEARDSLEY, Woods Hole Oceanographic Institution, Massachusetts MICHAEL L. BENDER, Princeton University, New Jersey ROSINA M. BIERBAUM, University of Michigan, Ann Arbor RAFAEL L. BRAS, Massachusetts Institute of Technology, Cambridge MARY ANNE CARROLL, University of Michigan, Ann Arbor WALTER DABBERDT, Vaisala Inc., Boulder, Colorado KERRY A. EMANUEL, Massachusetts Institute of Technology, Cambridge CASSANDRA G. FESEN, Dartmouth College, Hanover, New Hampshire JENNIFER A. LOGAN, Harvard University, Cambridge, Massachusetts VERNON R. MORRIS, Howard University, Washington, D.C. WILLIAM J. RANDEL, National Center for Atmospheric Research, Boulder, Colorado ROGER M. WAKIMOTO, University of California, Los Angeles JOHN C. WYNGAARD, Pennsylvania State University, University Park Ex Officio Members ANTONIO J. BUSALACCHI, JR., University of Maryland, College Park ERIC F. WOOD, Princeton University, New Jersey NRC Staff CHRIS ELFRING, Director PAUL CUTLER, Senior Program Officer AMANDA STAUDT, Senior Program Officer JULIE DEMUTH, Program Officer PARIKHIT SINHA, Program Officer ELIZABETH A. GALINIS, Senior Program Assistant ROB GREENWAY, Senior Program Assistant DIANE GUSTAFSON, Administrative Coordinator ANDREAS SOHRE, Financial Associate

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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties Preface Radiative forcing is a way to quantify an energy imbalance imposed on the climate system either externally (e.g., solar energy output or volcanic emissions) or by human activities (e.g., deliberate land modification or emissions of greenhouse gases, aerosols, and their precursors). The concept of radiative forcing has been central for guiding climate research and policy over the past two decades. There are several reasons for this. It provides a simple yet fundamental index that allows us to look at how climate change is driven by the energy imbalance of the Earth system. It is successful in predicting change in global mean surface temperature as computed from climate models and it, thus, allows quantitative comparison of the contributions of different agents to climate change. It is easy to compute and is reproducible across models and therefore offers a convenient common metric on which policy research and recommendations can be based. New studies on climate forcing agents not conventionally considered have, however, raised doubts as to the continued viability of the radiative forcing concept. For example, the climatic effects from light-absorbing aerosols or land-use changes do not lend themselves to quantification using the traditional radiative forcing concept. Aerosol effects on clouds are difficult to describe in terms of simple radiative forcing. These challenges have raised the question of whether the radiative forcing concept has outlived its usefulness and, if so, what new climate change metrics should be used. To address these issues, the U.S. Climate Change Science Program (CCSP) asked the National Academies to undertake a study to evaluate the current state of knowledge on radiative forcings and to identify relevant

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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties BOX P-1 Statement of Task for the Committee on Radiative Forcing Effects on Climate This study will examine the current state of knowledge regarding the direct and indirect radiative forcing effects of gases, aerosols, land use, and solar variability on the climate of the Earth’s surface and atmosphere and it will identify research needed to improve our understanding of these effects. Specifically, this study will: Summarize what is known about the direct and indirect radiative effects caused by individual forcing agents, including the spatial and temporal scales over which specific forcing agents may be important; Evaluate techniques (e.g., modeling, laboratory, observations, and field experiments) used to estimate direct and indirect radiative effects of specific forcing agents; Identify key gaps in the understanding of radiative forcing effects on climate; Identify key uncertainties in projections of future radiative forcing effects on climate; Recommend near- and longer-term research priorities for improving our understanding and projections of radiative forcing effects on climate. research needs. In response, the Committee on Radiative Forcing Effects on Climate was formed. The committee was charged to examine the current state of knowledge of how gases, aerosols, land use, and solar variability force the climate system, identify key gaps in understanding, and recommend research priorities (see Box P-1 for the full statement of task). This report presents the committee’s findings and recommendations. The committee began its discussions with a good dose of skepticism about the continued viability of the radiative forcing concept. In the end, however, one of our major findings is that the concept retains considerable value. It needs to be expanded to account for the vertical and regional structure of radiative forcing and also for nonradiative climate forcings. We propose several new research avenues that should be pursued to accomplish this expansion. We present an agenda for addressing uncertainties in forcings and climate effects from conventional and nonconventional agents. We make specific recommendations for using past climate records to improve our understanding of the relationship of radiative forcing to climate change and for developing an observational strategy aimed at continuous monitoring of climate forcing variables for the indefinite future. Finally, we examine ways to improve the application of radiative and nonradiative forcing metrics in policy analyses directed at climate change.

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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties The committee met four times over the course of a year to gather information and to deliberate over findings and recommendations. We thank the following speakers who shared their knowledge with the committee: James Anderson, Harvard University; Theodore L. Anderson, University of Washington; Gordon Bonan, National Center for Atmospheric Research; Thomas Crowley, Duke University; Kea Duckenfield, National Oceanic and Atmospheric Administration (NOAA); Jerry Elwood, Department of Energy; David Fahey, NOAA Aeronomy Laboratory; Jay Fein, National Science Foundation; Peter Gent, National Center for Atmospheric Research; James Hansen, National Aeronautics and Space Administration (NASA) Goddard Institute for Space Studies; Dennis Hartmann, University of Washington; Eugenia Kalnay, University of Maryland; Yoram Kaufman, NASA Goddard Space Flight Center; James Mahoney, U.S. Climate Change Science Program; Kenneth Mooney, NOAA; Richard Moss, U.S. Climate Change Science Program; V. Ramaswamy, NOAA Geophysical Fluid Dynamics Laboratory; Daniel Rosenfeld, Hebrew University; Susan Solomon, NOAA Aeronomy Laboratory; Graeme Stephens, Colorado State University; Lucia Tsaoussi, NASA; and Josh Willis, Scripps Institution of Oceanography. The committee hopes that this report will be useful to the U.S. Climate Change Science Program in mapping future research directions to improve our knowledge of radiative and other climate forcings, their variability, and their impacts on climate. Daniel J. Jacob Chair

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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties 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: Tami C. Bond, University of Illinois James A. Coakley, Jr., Oregon State University Robert E. Dickinson, Georgia Institute of Technology James A. Edmonds, Pacific Northwest National Laboratory Jonathan A. Foley, University of Wisconsin Peter R. Gent, National Center for Atmospheric Research Richard Goody, Harvard University Venkatachalam Ramaswamy, Geophysical Fluid Dynamics Laboratory William J. Randel, National Center for Atmospheric Research Stephen E. Schwartz, Brookhaven National Laboratory Although the reviewers listed above have provided constructive comments and suggestions, they were not asked to endorse the report’s conclusions or recommendations, nor did they see the final draft of the report

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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties before its release. The review of this report was overseen by John H. Seinfeld, California Institute of Technology. Appointed by the National Research Council, he was 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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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties Contents     EXECUTIVE SUMMARY   1 1   INTRODUCTION   11      Earth’s Climate System,   12      Radiative Forcing: Perturbation to Earth’s Energy Budget,   16      Theoretical Development of the Radiative Forcing Concept,   19      The Radiative Forcing Concept and Climate Policy,   23 2   STATE OF SCIENTIFIC UNDERSTANDING   28      Well-Mixed Greenhouse Gases,   30      Tropospheric and Stratospheric Ozone,   32      Direct Effect of Aerosols,   34      Indirect Effect of Aerosols,   39      Land-Cover and Land-Use Changes,   44      Solar Forcing,   49      Volcanic Eruptions,   57      Ocean Color,   59      Teleconnections and Radiative Forcing,   60 3   RADIATIVE FORCING OVER EARTH’S HISTORY   63      Geological Timescales,   63      Glacial-Interglacial Timescales,   70      Holocene,   71      Last 1000 Years,   75      Last 25 Years,   80

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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties 4   RETHINKING THE GLOBAL RADIATIVE FORCING CONCEPT   83      The Current Concept,   83      Global Mean Radiative Forcing with Adjusted Tropospheric Temperatures,   87      Global Mean Radiative Forcing at the Surface,   89      Regional Radiative Forcing,   92      Regional Nonradiative Forcing,   93      Ocean Heat Content,   98 5   UNCERTAINTIES ASSOCIATED WITH FUTURE CLIMATE FORCINGS   100      Future Emissions of Greenhouse Gases and Aerosols,   100      Solar Variability,   113      Land-Use and Land-Cover Change,   113      Abrupt Climate Change,   115 6   RESEARCH APPROACHES TO FURTHERING UNDERSTANDING   117      Observations of Radiative Forcing and Response,   117      Laboratory and Process Studies for Aerosols,   132      Atmospheric Reanalysis and Data Assimilation,   133      Relating Concentrations of Greenhouse Gases and Aerosols to Sources,   134      Climate Forcing and Response over Earth’s History,   138      Climate Models,   140 7   RECOMMENDATIONS   144      Expanding the Radiative Forcing Concept,   145      Addressing Key Uncertainties,   148      Improving the Observational Record,   152      Addressing Policy Needs,   155     REFERENCES   159     APPENDIXES     A   BIOGRAPHICAL SKETCHES OF COMMITTEE MEMBERS AND STAFF   191 B   STATEMENT OF TASK   197 C   GLOSSARY AND ACRONYMS   198