SURFACE TEMPERATURE RECONSTRUCTIONS FOR THE LAST 2,000 YEARS

Committee on Surface Temperature Reconstructions for the Last 2,000 Years

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.
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Surface Temperature Reconstructions for the last 2,000 Years SURFACE TEMPERATURE RECONSTRUCTIONS FOR THE LAST 2,000 YEARS Committee on Surface Temperature Reconstructions for the Last 2,000 Years 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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Surface Temperature Reconstructions for the last 2,000 Years 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. International Standard Book Number-10: 0-309-10225-1 (Book) International Standard Book Number-13: 978-0-309-10225-4 (Book) International Standard Book Number-10: 0-309-66144-7 (PDF) International Standard Book Number-13: 978-0-309-66144-7 (PDF) Additional copies of this report are available from the National Academies Press, 500 Fifth Street, N.W., Lockbox 285, Washington, D.C. 20055; (800) 624-6242or (202)334-3313 (in the Washington metropolitan area); Internet, http://www.nap.edu. Copyright 2006 by the National Academy of Sciences. All rights reserved. Printed in the United States of America

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Surface Temperature Reconstructions for the last 2,000 Years 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. Ralph J. Cicerone 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. Ralph J. Cicerone and Dr. Wm. A. Wulf are chair and vice chair, respectively, of the National Research Council. www.national-academies.org

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Surface Temperature Reconstructions for the last 2,000 Years COMMITTEE ON SURFACE TEMPERATURE RECONSTRUCTIONS FOR THE LAST 2,000 YEARS GERALD R. NORTH (Chair), Texas A&M University, College Station FRANCO BIONDI, University of Nevada, Reno PETER BLOOMFIELD, North Carolina State University, Raleigh JOHN R. CHRISTY, University of Alabama, Huntsville KURT M. CUFFEY, University of California, Berkeley ROBERT E. DICKINSON, Georgia Institute of Technology, Atlanta ELLEN R.M. DRUFFEL, University of California, Irvine DOUGLAS NYCHKA, National Center for Atmospheric Research, Boulder, Colorado BETTE OTTO-BLIESNER, National Center for Atmospheric Research, Boulder, Colorado NEIL ROBERTS, University of Plymouth, United Kingdom KARL K. TUREKIAN, Yale University, New Haven, Connecticut JOHN M. WALLACE, University of Washington, Seattle NRC Staff IAN KRAUCUNAS, Study Director CHRIS ELFRING, Board Director AMANDA STAUDT, Senior Program Officer ELIZABETH A. GALINIS, Research Associate LEAH PROBST, Research Associate DIANE GUSTAFSON, Administrative Coordinator NORMAN GROSSBLATT, Senior Editor

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Surface Temperature Reconstructions for the last 2,000 Years BOARD ON ATMOSPHERIC SCIENCES AND CLIMATE ROBERT J. SERAFIN (Chair), National Center for Atmospheric Research, Boulder, Colorado M. JOAN ALEXANDER, NorthWest Research Associates/CORA, Boulder, Colorado FREDERICK R. ANDERSON, McKenna Long & Aldridge LLP, Washington, D.C. MICHAEL L. BENDER, Princeton University, New Jersey ROSINA M. BIERBAUM, University of Michigan, Ann Arbor MARY ANNE CARROLL, University of Michigan, Ann Arbor CAROL ANNE CLAYSON, Florida State University, Tallahassee WALTER F. DABBERDT, Vaisala Inc., Boulder, Colorado KERRY A. EMANUEL, Massachusetts Institute of Technology, Cambridge DENNIS L. HARTMANN, University of Washington, Seattle PETER R. LEAVITT, Weather Information Inc., Newton, Massachusetts JENNIFER A. LOGAN, Harvard University, Cambridge, Massachusetts VERNON R. MORRIS, Howard University, Washington, D.C. F. SHERWOOD ROWLAND, University of California, Irvine THOMAS H. VONDER HAAR, Colorado State University/CIRA, Fort Collins ROGER M. WAKIMOTO, National Center for Atmospheric Research, Boulder, Colorado 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 MARIA UHLE, Program Officer IAN KRAUCUNAS, Associate Program Officer CLAUDIA MENGELT, Associate Program Officer ELIZABETH A. GALINIS, Research Associate LEAH PROBST, Research Associate ROB GREENWAY, Senior Program Assistant DIANE GUSTAFSON, Administrative Coordinator ANDREAS SOHRE, Financial Associate

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Surface Temperature Reconstructions for the last 2,000 Years Foreword Our understanding of climate and how it has varied over time is advancing rapidly as new data are acquired and new investigative instruments and methods are employed. Thus in 2005, I suggested to the U.S. Congress that the National Research Council (NRC) could help answer questions about the data and methods that have been used in constructing records of Earth’s surface temperatures from times when there were no scientific instruments, using proxy indicators. How has temperature varied over the last 2,000 years? How certain is the answer to this question? Subsequently, this study was requested by Representative Sherwood Boehlert, chairman of the Committee on Science, U.S. House of Representatives. Chairman Boehlert asked for a clear and concise report in a relatively short period of time, and the NRC agreed to undertake the study quickly. An ad hoc committee was formed, with the group carefully composed to include the breadth and depth of expertise and perspectives needed to analyze all aspects of how surface temperatures are estimated and interpreted and to comment generally on climate science. The NRC asked the committee to summarize current scientific information on the temperature record for the past two millennia, describe the main areas of uncertainty and how significant they are, describe the principal methodologies used and any problems with these approaches, and explain how central is the debate over the paleoclimate temperature record to the state of scientific knowledge on global climate change. The committee has prepared a report that, in my view, provides policy makers and the scientific community with a critical view of surface temperature reconstructions and how they are evolving over time, as well as a good sense of how important our understanding of the paleoclimate temperature record is within the overall state of scientific knowledge on global climate change. The report does not make policy recommendations. I thank the members of the committee, who worked intensely to produce this careful report in a short period of time and contributed much personal time, insight, and energy.

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Surface Temperature Reconstructions for the last 2,000 Years The NRC staff and all those who contributed papers, data, graphics, and other information, as well as the independent experts who participated in the rigorous review process, were essential participants. Ralph J. Cicerone, President National Academy of Sciences Chair, National Research Council

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Surface Temperature Reconstructions for the last 2,000 Years Preface This committee was asked to describe and assess the state of scientific efforts to reconstruct surface temperature records for the Earth over approximately the last 2,000 years. (The full Statement of Task appears in Appendix A.) Normally, a technical issue such as surface temperature reconstructions might not generate widespread attention, but this case brings interesting lessons about how science works and how science, especially climate science, is communicated to policy makers and the public. The debate began in 1998 when a paper by Michael Mann, Raymond Bradley, and Malcolm Hughes was published in the journal Nature. The authors used a new methodology to combine data from a number of sources to estimate temperatures in the Northern Hemisphere for the last six centuries and later for the last 1,000 years. This research received wide attention, in part because it was illustrated with a simple graphic, the so-called hockey stick curve, that many interpreted as definitive evidence of anthropogenic causes of recent climate change. The research was given prominence in the 2001 report of the Intergovernmental Panel on Climate Change and then was picked up by many in the wider science community and by the popular media. Science is a process of exploration of ideas—hypotheses are proposed and research is conducted to investigate. Other scientists work on the issue, producing supporting or negating evidence, and each hypothesis either survives for another round, evolves into other ideas, or is proven false and rejected. In the case of the hockey stick, the scientific process has proceeded for the last few years with many researchers testing and debating the results. Critics of the original papers have argued that the statistical methods were flawed, that the choice of data was biased, and that the data and procedures used were not shared so others could verify the work. This report is an opportunity to examine the strengths and limitations of surface temperature reconstructions and the role that they play in improving our understanding of climate. The reconstruction produced by Dr. Mann and his colleagues was just one step in a long process of research, and it is not (as sometimes presented) a clinching argument for anthropogenic global warming, but rather one of many independent lines of research on global climate change.

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Surface Temperature Reconstructions for the last 2,000 Years Using multiple types of proxy data to infer temperature time series over large geographic regions is a relatively new area of scientific research, although it builds upon the considerable progress that has been made in deducing past temperature variations at single sites and local regions. Surface temperature reconstructions often combine data from a number of specialized disciplines, and few individuals have expertise in all aspects of the work. The procedures for dealing with these data are evolving—there is no one “right” way to proceed. It is my opinion that this field is progressing in a healthy manner. As in all scientific endeavors, research reported in the scientific literature is often “work in progress” aimed at other investigators, not always to be taken as individual calls for action in the policy community. With this as context, the committee considered the voluminous literature pertinent to its charge and received briefings and written contributions from more than two dozen people. We have organized our report knowing that we have at least two different audiences—the science community and the policy community. The principal conclusions of the committee are listed in the Summary and explained in the Overview using nontechnical language. More extensive technical justifications for the committee’s conclusions, including references, are presented in the chapters that follow. Finally, let me thank the members of the Committee on Surface Temperature Reconstructions for the Last 2,000 Years. The committee worked tirelessly to assess the status of this field of research so that the public can see exactly what is involved, what we currently know about it, and what the prospects are for improving our understanding. We have tried to make clear how this piece of the climate puzzle fits into the broader discussions about global climate change. Gerald R. North, Chair Committee on Surface Temperature Reconstructions for the Last 2,000 Years

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Surface Temperature Reconstructions for the last 2,000 Years 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: David Brillinger, University of California, Berkeley David Chapman, University of Utah Julia Cole, University of Arizona Thomas Crowley, Duke University Alexander Flax, Independent consultant Claus Fröhlich, PMOD Technologies Ricardo Garcia-Herrera, Universidad Complutense de Madrid Peter Huybers, Woods Hole Oceanographic Institution Richard Muller, Lawrence Berkeley Laboratory Robert Stine, University of Pennsylvania Lonnie Thompson, The Ohio State University Connie Woodhouse, National Oceanic and Atmospheric Administration Carl Wunsch, Massachusetts Institute of Technology Although the reviewers listed above have provided many 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 Andrew R. Solow, Woods Hole Oceanographic Institution, and Louis J. Lanzerotti, New

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Surface Temperature Reconstructions for the last 2,000 Years Jersey Institute of Technology. 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.

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Surface Temperature Reconstructions for the last 2,000 Years Contents     SUMMARY   1     OVERVIEW   5 1   INTRODUCTION TO TECHNICAL CHAPTERS   25      Concepts and Definitions,   25      Attribution of Global Warming to Human Influences,   27      Report Structure,   28 2   THE INSTRUMENTAL RECORD   29      Instrumental Data,   30      Features of the Instrumental Record,   30      Uncertainties and Errors Associated with the Instrumental Record,   33      Spatial Sampling Issues,   35 3   DOCUMENTARY AND HISTORICAL EVIDENCE   38      Types of Evidence,   38      Limitations and Benefits of Historical and Documentary Sources,   39      Systematic Climate Reconstructions Derived from Historical Archives,   40      Consequences of Climate Change for Past Societies,   43 4   TREE RINGS   45      Definition and Premises,   45      Field and Laboratory Methods,   47      Temperature Reconstructions,   48 5   MARINE, LAKE, AND CAVE PROXIES   53      Corals,   54      Marine Sediments,   58

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Surface Temperature Reconstructions for the last 2,000 Years      Lake and Peat Sediments,   60      Speleothems,   62      Summary,   64 6   ICE ISOTOPES   65      Physical Basis for Deriving Climate Signals from Ice Isotopic Ratio Records,   66      Calibration and Resolution,   67      Results from Ice Isotopic Ratio Records,   67 7   GLACIER LENGTH AND MASS BALANCE RECORDS   71      Reconstructing Temperature Records from Glacier Records,   72      More Detailed Background on Glacier-Length-Based Reconstructions,   72      Other Information Available from Glaciers,   75 8   BOREHOLES   77      Boreholes in Rock and Permafrost,   78      Limits on Borehole-Based Reconstructions,   80      Boreholes in Glacial Ice,   81 9   STATISTICAL BACKGROUND   83      Linear Regression and Proxy Reconstruction,   84      Principal Component Regression,   89      Validation and the Prediction Skill of the Proxy Reconstruction,   92      Quantifying the Full Uncertainty of a Reconstruction,   95 10   CLIMATE FORCINGS AND CLIMATE MODELS   98      Climate Forcings,   99      Climate Model Simulations,   105      Anthropogenic Forcing and Recent Climate Change,   107 11   LARGE-SCALE MULTIPROXY RECONSTRUCTION TECHNIQUES   110      Evolution of Multiproxy Reconstruction Techniques,   111      Strengths and Limitations of Large-Scale Surface Temperature Reconstructions,   115      Overall Findings and Conclusions,   117      What Comments Can Be Made on the Value of Exchanging Information and Data?,   118      What Might Be Done to Improve Our Understanding of Climate Variations Over the Last 2,000 Years?,   119     REFERENCES   120     APPENDIXES          A  Statement of Task   139      B  R Code for Figure 9-2   140      C  Biographical Sketches of Committee Members   142