SURFACE TEMPERATURE RECONSTRUCTIONS FOR THE LAST 2,000 YEARS
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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.
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THE NATIONAL ACADEMIES
Advisers to the Nation on Science, Engineering, and Medicine
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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
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
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.
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.
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
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
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.