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UNDERSTANDING
~ 1 11V12~1=
CHANGE
FEEDBACKS
Panel on Climate Change Feedbacks
Climate Research Committees
Boa rcl 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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THE NATIONAL ACADEMIES PRESS · 500 Fifth Street, N.W. · Washington, D.C. 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.
This study was supported by Contract No. NASW-01008 between the National Academy of
Sciences and the U.S. Global Change Research Program (through the National Aeronautic and Space
Administration) and Grant No. ATM-0136349 between the National Academy of Sciences and the
National Science Foundation. 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.
International Standard Book Number 0-309-09072-5 (Book)
International Standard Book Number 0-309-52744-9 (PDF)
Library of Congress Control Number 2003115368
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: A vortex street near the Canary Islands. A vortex street often occurs when cloud formations
over the ocean are disturbed by wind passing over land or another obstacle. In this Moderate
Resolution Imaging Spectroradiometer (MODIS) image from July 5, 2002, marine stratocumulus
clouds have arranged themselves in rows, or streets, which are usually parallel to the direction of
wind flow. Downwind of obstacles, in this case, the Canary Islands off the west African coast,
eddies create turbulent patterns called vortex streets. Marine boundary layer clouds have a large
effect on Earth's energy balance and understanding them is important to predicting the response of
climate to human activities.
Copyright 2003 by the National Academy of Sciences. All rights reserved.
Printed in the United States of America
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THE NATIONAL ACADEMIES
Advisers to the Nation on Science, Engineering, and Medirine
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
~ . .
. ~ nglneermg.
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-academ ies.org
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v
PANEL ON CLIMATE CHANGE FEEDBACKS
DENNIS L. HARTMANN (Chair), University of Washington, Seattle
ALAN K. BETTS, Atmospheric Research, Pittsforct, Vermont
GORDON B. BONAN, National Center for Atmospheric Research, Boulder, Colorado
LEE E. BRANSCOME, Environmental Dynamics Research, Palm Beach Gardens,
Florida
ANTONIO ]. BUSALACCHI, JR., University of Maryland, College Park
AMANDA LYNCH, University of Colorado, Boulder*
SYUKURO MANABE, Princeton University, New Jersey
DOUGLAS G. MARTINSON, Lamont-Doherty Earth Observatory, Columbia
University, Palisades, New York
RAYMOND NAIJAR, Pennsylvania State University, University Park
EUGENE M. RASMUSSON, University of Maryland, College Park
A.R. RAVISHANKARA, NOAA Aeronomy Laboratory, Boulder, Colorado
DANIEL R. SAREWITZ, Columbia University, Center for Science, Policy, and
Outcomes, Washington, D.C.**
DIAN ]. SElDEL, NOAA Air Resources Laboratory, Silver Spring, Maryland
GRAEME L. STEPHENS, Colorado State University, Fort Collins
LYNNE D. TALLEY, Scripps Institution of Oceanography, University of California at
San Diego, La Jolla, California
JOHN M. WALLACE, University of Washington, Seattle
ANDREW J. WEAVER, University of Victoria, British Columbia, Canada
STEVEN C. WOFSY, Harvard University, Cambridge, Massachusetts
ERIC F. WOOD, Princeton University, New Jersey
.,,
· A,
Consultant
MICHAEL J. PRATHER, University of California, Irvine
NRC Staff
CHRIS ELFRING, BASC Director
PETER A. SCHULTZ, Study Director
ROB GREENWAY, Project Assistant (from February 2002)
CARTER W. FORD, Project Assistant (until December 2001)
*Through February 2002.
**Through August 2003.
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BOARD ON ATMOSPHERIC SCIENCES AND CLIMATE
ERIC J. BARRON (Chair), Pennsylvania State University, University Park
FREDERICK R. ANDERSON, Cadwalader, Wickersham & Taft,
Washington, D.C.
RAYMOND J. BAN, The Weather Channel, Inc., Atlanta, Georgia
ROBERT C. BEARDSLEY, Woods Hole Oceanographic Institution,
Massachusetts
ROSINA M. BIERBAUM, University of Michigan, Ann Arbor
RAFAEL L. BRAS, Massachusetts Institute of Technology, Cambridge
MARY ANNE CARROLL, University of Michigan, Ann Arbor
KERRY A. EMANUEL, Massachusetts Institute of Technology,
Cambridge
CASSANDRA G. FESEN, Dartmouth College, Hanover, New Hampshire
MARGARET A. LEMONE, National Center for Atmospheric Research,
Boulder, Colorado
JENNIFER A. LOGAN, Harvard University, Cambridge, Massachusetts
MARIO J. MOLINA, Massachusetts Institute of Technology, Cambridge
WILLIAM J. RANDEL, National Center for Atmospheric Research,
Boulder, Colorado
ROBERT J. SERAFIN, National Center for Atmospheric Research,
Boulder, Colorado
ROGER M. WAKIMOTO, University of California, Los Angeles
JOHN C. WYNGAARD, Pennsylvania State University, University Park
Ax Off cio Members
ANTONIO J. BUSALACCHI, JR., University of Maryland, College Park
ERIC F. WOOD, Princeton University, New Jersey
NRC Staff
CHRIS ELFRING, Director
ELBERT W. (JOE) FRIDAY, JR., Senior Scholar
PETER A. SCHULTZ, Senior Program Officer
LAURIE S. GELLER, Senior Program Officer
AMANDA STAUDT, Program Officer
DIANE L. GUSTAFSON, Administrative Associate
ROB GREENWAY, Project Assistant
ELIZABETH A. GALINIS, Project Assistant
ROBIN MORRIS, Financial Officer
Vl
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PREFACE
The National Research Council (NRC), the operating arm of the
National Academies, has over the years produced an extensive body of work
focused on issues relevant to observing and understanding changes in
climate. Several NRC reports have provided advice to guide the evolution of
U.S. research priorities and, most recently, the Climate Change Science
report (NRC, 2001a) concluded that
Maintaining a vigorous, ongoing program of basic research, funded
and managed independently of the climate assessment activity, will be
crucial for narrowing these uncertainties.... The ability of the United
States to assess fixture climate change is severely limited by the lack of a
climate observing system, by inadequate computational resources, and
by the general inability of government to focus resources on climate
problems. Efforts are needed to ensure that U.S. efforts in climate
research are supported and managed to ensure innovation, effectiveness,
and efficiency.
Although these statements are addressed toward the entirety of the
climate research enterprise, the report also specifically discussed the
importance of reducing the uncertainties associated with climate change
feedbacks such as water vapor, clouds, and snow cover. This report is an
attempt to look in detail at that challenge and identify ways to improve our
understanding of climate change feedback processes.
'Decade-to-Century-Scale Climate Variability and Change: A Science Strategy (NRC,
1998a); The Atmospheric Sciences Entering the Twenty-First Century (NRC, 1998b);
Adequacy of Climate Observing Systems (NRC, 1999a); Global Environmental Change:
Research Pathways for the Next Decade (NRC, l999b); Improving the Effectiveness of
U.S. Climate Modeling (NRC, 2001b); The Science of Regional and Global Change:
Putting Knowledge to Work (NRC, 2001 c).
vi'
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. . .
V111
UNDERSTANDING CLING TE CHANGE FEEDBA CKS
Over the past decade we have learned much about the complex natural
processes that influence climate variability and change, and our ability to
model climate has increased significantly. In addition, we have gained a
better appreciation for the important connections between physical,
biological, and social sciences in the climate system. We have also begun to
better identify those parts of the climate system that are particularly
important and not well understood, and therefore limit our ability to project
the future evolution of Earth's climate. One of these critical areas is our
understanding of the role of feedbacks in the climate system and their role in
determining climate sensitivity. The ultimate goal of climate feedbacks
research is to enable accurate predictions of the response of Earth's climate
to specified natural or human-induced influences on the climate system.
Clearly, humans play a major role in some feedback loops, particularly
those involving biological and land surface processes, and it can be difficult
to distinguish between the effects of human activities and natural processes.
To maintain a sharp focus for this study, however, we have not addressed
those feedbacks that depend on a human response to either amplify or damp
an initial perturbation. We recognize, however, that research into climate
change must ultimately embrace natural and human-induced feedbacks
holistically. Furthermore, we do not address uncertainties associated with
natural or human-induced climate forcing. The question addressed here is,
"If humans provide specified inputs to atmospheric composition or changes
in land surface that force changes in the climate system, how do natural
climate processes influence our ability to project the response of climate to
that forcing?"
Although this report focuses in particular on climate change feedbacks,
this research cannot be separated from other efforts to understand the climate
system. Many of the research approaches and techniques used to better
understand climate feedbacks will also shed light on other aspects of the
climate system. Moreover, climate feedbacks research will take place in the
context of continuing efforts to detect climate change, attribute climate
change to specific causes, and make practical projections of future climates.
For these reasons we have taken a broad view of what constitutes a feedback
process, including in our definition not only the processes that affect the
equilibrium response of global mean surface temperature to a specified
forcing but also natural processes whereby climate changes feed back on the
rate of warming, the magnitude of the climate forcing, or the spatial pattern
of climate change.
The Panel on Climate Change Feedbacks interpreted its charge (see
Executive Summary) as a request for broad guidance on the key avenues of
research to be pursued to better understand climate feedbacks and their role
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PREFA CE
IX
in climate sensitivity. Given our resources, we did not attempt to formulate a
detailed research strategy, which would take significantly more time and
community input than our process allowed. Rather, we sought to review the
science of climate feedback processes broadly and call attention to those
areas where additional focus might bear fruit in the near term. Thus, this
report should be viewed as a starting point for the formulation of a long-term
research strategy for addressing the issue of climate feedback processes and
their role in climate sensitivity. Comprehensive disciplinary plans have in
some cases already been carried out by groups of scientists convened by the
agencies. Where this work is relevant to the report's discussion we have
referred to it.
The Panel's deliberations included some critical and difficult issues that
did not make it into the final report. These include the scientific definition of
uncertainty, its quantitative evaluation, and its relation to the development of
policy options regarding climate change. I, like many scientists, believe that
directed, high-quality research will increase understanding, and thus reduce
uncertainty that is associated with incomplete knowledge of the climate
system. My belief is that this improved knowledge can then be used to
support decision making for the benefit of society, although this view is not
universally accepted. In my view the proper and useful role of scientists is to
use the scientific method to seek after natural truth as best we can. This
being said, it is difficult to argue in a quantitative way that increased
understanding will lead quickly to narrowing of the range of estimates of
how much global warming will result from a doubling of atmospheric
carbon dioxide. Indeed, new discoveries may widen the apparent
possibilities. In this report we focus our attention on the key research
questions necessary to better understand the feedback processes that we
believe to be the most important. Larger questions are left to other groups to
address.
The primary funding for this study was provided by the U.S. Global
Change Research Program, with additional funds from the National Science
Foundation. Representatives from NSF and other relevant agencies,
including the National Oceanic and Atmospheric Administration, the
National Aeronautics and Space Administration, and the Department of
Energy provided encouragement and input to the study, in particular through
a meeting with the Panel in February 2002.
In closing, I wish to thank the members of the Panel on Climate Change
Feedbacks for their efforts in preparing this report. I also thank all those who
participated in the Panel's information-gathering workshop in Boulder,
Colorado, during August 2001 and in the Panel's meeting with federal
agencies during February 2002 in Washington, D.C. Peter A. Schultz did an
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x
UNDERSTANDING CLIMA TE CHANGE FEEDBACKS
excellent job as study director and contributed greatly to this report. Chris
Elfring made major contributions during the review and revision process.
Dennis Hartrnann
Chair
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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:
D. James Baker, Academy of Natural Sciences, Philadelphia,
Pennsylvania
Peter Gent, National Center for Atmospheric Research, Boulder.
Colorado
Anthony DelGenio, Goddard Institute for Space Studies, New York,
N.Y.
Isaac Held, National Oceanic and Atmospheric Administration,
Princeton, New Jersey
George Hornberger, University of Virginia, Charlottesville
Richard Lindzen, Massachusetts Institute of Technology, Cambridge
Berrien Moore, University of New Hampshire, Durham
Gerald North, Texas A&M University, College Station
Edward Sarachik, University of Washington, Seattle
Tom Wigley, National Center for Atmospheric Research, Boulder,
Colorado
James Anderson, Harvard University, Cambridge, Massachusetts
Although the reviewers listed above have provided constructive
comments and suggestions, they were not asked to endorse the report's
Xl
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. .
Xll
UNDERSTANDING CLING TE CHANGE FEEDBACKS
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
Goody, Harvard University, and Kuo-Nan Liou, University of California,
Los Angeles. 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 panel and the
institution.
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CONTENTS
EXECUTIVE SUMMARY
The Need for Climate Feedbacks Research, 2
Priorities in Climate Feedback Research, 7
1 INTRODUCTION
15
2 CLOUD, WATER VAPOR, AND LAPSE RATE FEEDBACKS 21
Water Vapor, 21
Lapse Rate Feedback, 24
Cloud Feedbacks, 26
Why Has Progress on Cloud, Water Vapor, and Lapse Rate
Feedbacks Been so Elusive? 32
Developing a Scientific Strategy, 35
3 SEA-ICE FEEDBACKS
Overview of Sea-Ice Feedbacks, 43
Developing a Scientific Strategy, 44
4 OCEAN HEAT UPTAKE AND OCEAN CIRCULATION
FEEDBACKS
Mixing, Ocean Heat Uptake, and Climate Feedbacks, 50
Thermohaline Circulation Feedbacks, 53
TERRESTRIAL HYDROLOGY AND VEGETATION
FEEDBACKS
Terrestrial Hydrology, 60
Vegetation Feedbacks, 65
Developing a Scientific Strategy, 69
. . .
X111
41
48
59
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xlv
UNDERSTANDING CLIMA TE CHANGE FEEDBACKS
6 ATMOSPHERIC CHEMICAL FEEDBACKS
An Example of the Multiple Dimensions of Climate-Air Chemistry
Feedback, 78
Overview of Factors Controlling Climate, Chemistry, and Aerosol
Interactions, 8 1
Developing a Scientific Strategy, 84
76
7 BIOGEOCHEMICAL FEEDBACKS AND THE CARBON CYCLE 88
Terrestrial Carbon Feedbacks, 90
Marine Biogeochemical Feedbacks, 97
8 MODES OF VARIABILITY
9 RECOMMENDATIONS
Key Observations Needed to Monitor and Understand Climate
Change Feedbacks, 111
Evaluating Progress in Understanding Climate Feedbacks, 113
REFERENCES
APPENDIXES
A Climate Change Feedbacks Workshop
B Biographical Sketches of Panel Members
C Acronyms
107
110
118
137
144
151
