A PLAN FOR A RESEARCH PROGRAM ON Aerosol Radiative Forcing and Climate Change

Panel on Aerosol Radiative Forcing and Climate Change

Board on Atmospheric Sciences and Climate

Commission on Geosciences, Environment, and Resources

National Research Council

NATIONAL ACADEMY PRESS
Washington, D.C.
1996



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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change A PLAN FOR A RESEARCH PROGRAM ON Aerosol Radiative Forcing and Climate Change Panel on Aerosol Radiative Forcing and Climate Change Board on Atmospheric Sciences and Climate Commission on Geosciences, Environment, and Resources National Research Council NATIONAL ACADEMY PRESS Washington, D.C. 1996

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change AN EXAMPLE OF A REGIONAL AEROSOL DISTRIBUTION This NASA shuttle photograph (opposite page), taken on a March morning in 1994, and the explanatory diagram show a regional "haze" inland from the coast of California. While no chemical analysis is available, the haze is a widespread aerosol from sources that probably include smoke particles from biomass combustion and cities in the region. The enhanced albedo due to the haze causes sunlight to be reflected upward and thereby to fail to reach the ground. This constitutes a "direct climate forcing." The aerosol cloud is visible from the northern extremity of the Sacramento Valley on the left to the Bakersfield area of the San Joaquin Valley on the right, a distance of about 600 kilometers. The Sierra Nevada mountains and the coastal range bound the aerosol-laden valley. The photograph also shows coastal stratus clouds that extend along the coast and penetrate into the San Francisco Bay region. The albedo of these clouds, which can be influenced by anthropogenic aerosols, clearly controls the albedo of the oceanic portion of this view. (Shuttle photograph SS062-86-066, courtesy of the Earth Science Branch, NASA/Johnson Space Center, Houston, Texas) Cover art by Carrie Mallory. Ms. Mallory received her Bachelor of Fine Arts degree from the Cooper Union. She draws on the natural world and the effects of age on man-made objects for many of her themes. She has exhibited at a number of juried shows in the Northern Virginia area and has provided art for several NRC report covers. The art for this cover involved transferring an original photograph to an already cracked lithograph stone and adding texture with traditional litho crayons. As expected, the stone cracked further during the printing process, yielding only a few prints before disintegrating completely.

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change 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. This report has been reviewed by a group other than the authors according to procedures approved by a Report Review Committee consisting of members of the National Academy of Sciences, the National Academy of Engineering, and the Institute of 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. Harold Liebowitz 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. Harold Liebowitz are chairman and vice chairman, respectively, of the National Research Council. Support for this project was provided by the Department of Agriculture, the Department of Energy, the Environmental Protection Agency, the Office of Naval Research of the Department of Defense, the Air Force Office of Scientific Research, the National Aeronautics and Space Administration, the National Oceanic and Atmospheric Administration, and the National Science Foundation under Grant No. ATM-9316824. 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 above-mentioned agencies. Library of Congress Catalog Card Number 96-67382 International Standard Book Number 0-309-05429-X Additional copies of this report are available from: National Academy Press 2101 Constitution Avenue, NW Box 285 Washington, DC 20055 800-624-6242 202-334-3313 (in the Washington Metropolitan Area) B-705 Copyright 1996 by the National Academy of Sciences. All rights reserved. Printed in the United States of America

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change PANEL ON AEROSOL RADIATIVE FORCING AND CLIMATE CHANGE JOHN H. SEINFELD (Chair), California Institute of Technology, Pasadena ROBERT CHARLSON, University of Washington, Seattle PHILIP A. DURKEE, Naval Postgraduate School, Monterey, California DEAN HEGG, University of Washington, Seattle BARRY J. HUEBERT, University of Hawaii, Honolulu JEFFREY KIEHL, National Center for Atmospheric Research, Boulder, Colorado M. PATRICK MCCORMICK, Langley Research Center, National Aeronautics and Space Administration, Hampton, Virginia JOHN A. OGREN, Climate Monitoring and Diagnostics Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado JOYCE E. PENNER, Lawrence Livermore National Laboratory, Livermore, California VENKATACHALAM RAMASWAMY, Geophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration, Princeton, New Jersey W. GEORGE N. SLINN, Pacific Northwest Laboratories, Richland, Washington Staff DAVID H. SLADE, Senior Program Officer DORIS BOUADJEMI, Administrative Assistant

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change BOARD ON ATMOSPHERIC SCIENCES AND CLIMATE JOHN A. DUTTON (Chair), Pennsylvania State University, University Park ERIC J. BARRON, Pennsylvania State University, University Park WILLIAM L. CHAMEIDES, Georgia Institute of Technology, Atlanta CRAIG E. DORMAN, Department of Defense, Washington, D.C. FRANCO EINAUDI, Goddard Space Flight Center, Greenbelt, Maryland MARVIN A. GELLER, State University of New York, Stony Brook PETER V. HOBBS, University of Washington, Seattle WITOLD F. KRAJEWSKI, The University of Iowa, Iowa City MARGARET A. LEMONE, National Center for Atmospheric Research, Boulder, Colorado DOUGLAS K. LILLY, University of Oklahoma, Norman RICHARD S. LINDZEN, Massachusetts Institute of Technology, Cambridge GERALD R. NORTH, Texas A&M University, College Station EUGENE M. RASMUSSON, University of Maryland, College Park ROBERT J. SERAFIN, National Center for Atmospheric Research, Boulder, Colorado Staff WILLIAM A. SPRIGG, Director H. FRANK EDEN, Senior Program Officer MARK D. HANDEL, Senior Program Officer DAVID H. SLADE, Senior Program Officer ELLEN F. RICE, Reports Officer DORIS BOUADJEMI, Administrative Assistant THERESA M. FISHER, Administrative Assistant MARK BOEDO, Project Assistant

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change COMMISSION ON GEOSCIENCES, ENVIRONMENT, AND RESOURCES M. GORDON WOLMAN (Chair), The Johns Hopkins University, Baltimore, Maryland PATRICK R. ATKINS, Aluminum Company of America, Pittsburgh, Pennsylvania JAMES P. BRUCE, Canadian Climate Program Board, Ottawa, Ontario WILLIAM L. FISHER, University of Texas, Austin JERRY F. FRANKLIN, University of Washington, Seattle GEORGE M. HORNBERGER, University of Virginia, Charlottesville DEBRA KNOPMAN, Progressive Foundation, Washington, D.C. PERRY L. MCCARTY, Stanford University, California JUDITH E. MCDOWELL, Woods Hole Oceanographic Institution, Massachusetts S. GEORGE PHILANDER, Princeton University, New Jersey RAYMOND A. PRICE, Queen's University at Kingston, Ontario THOMAS C. SCHELLING, University of Maryland, College Park ELLEN SILBERGELD, University of Maryland Medical School, Baltimore STEVEN M. STANLEY, The Johns Hopkins University, Baltimore, Maryland VICTORIA J. TSCHINKEL, Landers and Parsons, Tallahassee, Florida Staff STEPHEN RATTIEN, Executive Director STEPHEN D. PARKER, Associate Executive Director MORGAN GOPNIK, Assistant Executive Director GREGORY SYMMES, Reports Officer JAMES MALLORY, Administrative Officer SANDI FITZPATRICK, Administrative Associate SUSAN SHERWIN, Project Assistant

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change Foreword As this report was receiving its final editing, Working Group I of the Intergovernmental Panel on Climate Change released its Summary for Policy Makers (IPCC, 1995b). The first section of the IPCC summary ("Greenhouse gas concentrations have continued to increase") documents the increase of greenhouse gases with arguments that are now almost universally accepted in the scientific community. The second section ("Anthropogenic aerosols tend to produce negative radiative forcings") quantifies the "direct" negative forcing of anthropogenic aerosols as a global average of 0.5 watts per square meter, and suggests that there is also an ''indirect" negative forcing of a similar magnitude. The remainder of the IPCC summary presents evidence that supports its view that aerosol radiative forcing plays a fundamental role in global climate change. The National Research Council's Panel on Aerosol Radiative Forcing and Climate Change agrees with the IPCC findings. The United States has taken a leading role in investigating the aerosol effect. Recent federal funding, at a level of about one-half percent of the U.S. Global Change Research Program, has supported efforts to provide preliminary estimates of the mechanisms, magnitudes, uncertainties, and environmental consequences of aerosol radiative forcing. As the following report points out, however, there is much to be done before the scientific community can confidently advise those charged with developing policy and legislation on the significance and timing of this climate-perturbing

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change problem. For example, currently even the composition and the spatial patterns of aerosol distribution are, in large part, tentative due to the paucity of basic measurements. Model descriptions of the process of aerosol formation, the environmental behavior of aerosols, and their effect on the dynamics of climate are all somewhat conjectural. The research that has been carried out in this country and abroad, however, is sufficient to support the main findings of the IPCC Working Group I and this panel: that aerosol radiative forcing of climate is not only an interesting scientific issue but also is likely to play a significant role in our future climate. John Seinfeld Chair Panel on Aerosol Radiative Forcing and Climate Change

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change Contents     EXECUTIVE SUMMARY   1 1   CLIMATE FORCING BY AEROSOLS   7     Atmospheric Aerosols   8     Aerosol Radiative Forcing of Climate   11     Evidence for Radiative Forcing by Anthropogenic Aerosols   13     Direct Forcing   13     Indirect Forcing   17     Evidence for Climate Response to Anthropogenic Aerosol Forcing   20     Radiative Forcing of Climate by Stratospheric Aerosols   23     Evidence for Climate Response to Stratospheric Aerosol Perturbation   25     Inferences from Stratospheric Aerosol Research   26     Climate Forcing by Key Tropospheric Aerosol Types   26     Conclusions   29 2   ELEMENTS OF A RESEARCH PROGRAM FOR AEROSOL FORCING OF CLIMATE   35     Global Climate Models   37     Atmospheric General Circulation Models   38     Atmospheric Chemical Transport Models   38     Recommended Research on Global Climate Modeling of Aerosol Radiative Forcing   42

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change     Process Research   43     Optical Properties   43     Recommended Process Research on Aerosol and Cloud Optical Properties   47     Aerosol Dynamics   47     Recommended Process Research on Aerosol Dynamics   51     Aerosol Sinks   52     Recommended Process Research on Aerosol Sinks   58     Aerosols and Ice Formation in Clouds   59     Recommended Process Research on Aerosols and Ice Formation in Clouds   61     Aerosol Process Models   62     Recommended Process Research on Aerosol Models   64     Field Studies   64     Closure Experiments   65     Multiplatform Field Campaigns   68     Recommended Field Studies   71     Satellite Observations and Continuous In Situ Monitoring   71     Satellite Remote Sensing of Aerosols   73     In Situ Monitoring of Aerosols   78     Recommended Surface-Based Monitoring Programs   83     Mobile Platforms   83     Recommended Mobile Monitoring Programs   85     Recommended Technology Developments   85     Summary of a Research Program on Aerosol Forcing of Climate   88 3   SENSITIVITY/UNCERTAINTY ANALYSIS AND THE SETTING OF PRIORITIES   89     Integration via Sensitivity/Uncertainty Analyses   90     An Example of Sensitivity Analysis: Direct Radiative Forcing   90     Frameworks for Research and Funding Priorities   94     An Example of Sensitivity and Uncertainty Analyses   98     Summary   104 4   THE PROPOSED ICARUS PROGRAM AND RECOMMENDED RESEARCH   107     The ICARUS Strategy   108     Organizational Structure of the ICARUS Program   111     Research Program   112     Global Climate Model Development   114     Process Research   115     Multiplatform Field Campaigns   117

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change     Satellite System Development   119     System Integration and Assessment   120     REFERENCES   123 APPENDIX A   ILLUSTRATIONS OF RECOMMENDED RESEARCH, EMPHASIZING RECENT LITERATURE   135 APPENDIX B   ACRONYMS AND OTHER INITIALS   159

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change List of Tables Table 1.1   Comparison of Climate Forcing by Aerosols with Forcing by Greenhouse Gases (GHGs): Fundamental Differences in Approach to Determination and Nature of Forcing   14 Table 1.2   Source Strength, Atmospheric Burden, Extinction Efficiency, and Optical Depth for Various Types of Aerosols   28 Table 1.3   Estimates of Direct Climate Forcing (W m-2) by Anthropogenic Aerosols   30 Table 1.4   Key Anthropogenic Aerosol Types, Associated Forcing Mechanisms, and Status of Understanding   32 Table 2.1   Global Models Currently Used to Study Aerosol Forcing: (A) Atmospheric General Circulation Models for Aerosol Forcing Calculations; (B) Global and Synoptic Models for Chemical Transport of Aerosols   39 Table 2.2   Satellite Instruments   76 Table 2.3   Aerosol Properties Needed at Continuous Monitoring Sites   80 Table 2.4   Categories of Sites to Monitor Intensive Properties   81 Table 3.1   Sensitivity Calculations for a Sulfate Aerosol Layer Below Clouds   92 Table 3.2   Sensitivity Calculations for an Aerosol Layer Above Lowest Cloud Layer   93 Table 3.3   Factors Contributing to Estimates of the Direct Forcing by Anthropogenic Sulfate (A) and Biomass Burning (B) Airborne Particles, Estimated Ranges, and Resulting Uncertainty Factors (for estimates of changes in reflected solar radiation)   100

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change List of Figures Figure 1   Organizational structure of the ICARUS program.   3 Figure 1.1   Estimated Northern Hemisphere and regional anthropogenic sulfur emissions over the past century.   10 Figure 2.1   General components of an integrated aerosol-climate research program.   36 Figure 2.2   Direct and indirect forcing mechanisms associated with sulfate aerosols.   44 Figure 2.3   Observations of continental haze by LITE (Lidar In-Space Technology Experiment).   75 Figure 2.4   Ship tracks off the coast of Northern California.   84 Figure 3.1   Sensitivity of aerosol forcing for an aerosol layer below cloud.   92 Figure 3.2   Sensitivity of aerosol forcing for an aerosol layer above lowest cloud layer.   93 Figure 3.3   Qualitative indications of current radiative forcing uncertainties for indirect effects (separately for marine and continental clouds) and for direct effects (separately for organic and inorganic aerosols) and a qualitative indication of the uncertainty goal (to be defined by USGCRP) for the first phase of ICARUS research.   95 Figure 3.4   Qualitative indication of relative ICARUS research priorities for different topics, with the differences from Figure 3.3 resulting from weighting the uncertainties of Figure 3.3 with USGCRP "strategic" and "integrating" priorities; here, the weighting has been by assumed amounts.   96

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A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change Figure 3.5   Qualitative indication of relative funding priorities (resource allocations) for the indicated broad research topics, with the differences from Figure 3.4 (research priorities) resulting from weighting these research priorities with costs to perform the research; here, the weighting has been by assumed amounts.   97 Figure 3.6   Plot of the uncertainties listed in Table 3.3A for sulfate aerosols, with a qualitative indication of the level to which the uncertainty could be set as a goal for the first phase of ICARUS research.   102 Figure 3.7   Qualitative indication of research priorities for direct radiative effects of sulfate aerosols, derived from Figure 3.6 (uncertainties) by weighting with such factors as mentioned in the text.   103 Figure 3.8   Qualitative indication of the relative costs to reduce the uncertainties shown in Figure 3.6, consistent with the research priorities shown in Figure 3.7, accounting for the cost of performing the research (e.g., a prorated portion of satellite costs to measure backscattered radiation).   103 Figure 3.9   Qualitative indication of the relative contributions from different processes to current uncertainty in the atmospheric lifetime of aerosol sulfate, with a qualitative indication of the level to which the uncertainty could be set as a goal for the first phase of ICARUS research.   104 Figure 3.10   Qualitative indication of funding priorities to reduce the uncertainties shown in Figure 3.9.   105 Figure 4.1   Organizational structure of the ICARUS program.   112