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 International Standard Book Number X-XXX-XXXXX-X (Book) Additional copies of this report are available from the National Academies Press, 500 Fifth Copyright 2006 by the National Academy of Sciences. All rights reserved. Printed in the United States of America. THE NATIONAL ACADEMIES The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars The National Academy of Engineering was established in 1964, under the charter of the National Academy of The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad COMMITTEE ON SURFACE TEMPERATURE RECONSTRUCTIONS 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 LEAH PROBST, Research Associate DIANE GUSTAFSON, Administrative Coordinator BOARD ON ATMOSPHERIC SCIENCES AND CLIMATE ROBERT J. SERAFIN (Chair), National Center for Atmospheric Research, Boulder, Colorado KERRY A. EMANUEL, Massachusetts Institute of Technology, Cambridge PETER R. LEAVITT, Weather Information Inc., Newton, 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 IAN KRAUCUNAS, Associate Program Officer ROB GREENWAY, Senior Program Assistant Foreword Our understanding of climate and how it has varied over time are 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 to 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. 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 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 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 |