x SURFACE TEMPERATURE RECONSTRUCTIONS FOR THE LAST 2,000 YEARS 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 over the last few months 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 NRC'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: Peter Huybers, Woods Hole Oceanographic Institution Carl Wunsch, Massachusetts Institute of Technology Connie Woodhouse, National Oceanic and Atmospheric Administration Lonnie Thompson, The Ohio State University David Chapman, University of Utah Ricardo Garcia-Herrera, Universidad Complutense de Madrid David Brillinger, University of California, Berkeley Robert Stine, University of Pennsylvania Alexander Flax, Independent consultant Claus Frohlich, PMOD Technologies Richard Muller, Lawrence Berkeley Laboratory Thomas Crowley, Duke University 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. Table of Contents Features of the Instrumental Record Uncertainties and Errors Associated with the Instrumental Record Limitations and Benefits of Historical and Documentary Sources Systematic Climate Reconstructions Derived from Historical Archives Results from Ice Isotopic Ratio Records Reconstructing Temperature Records from Glacier Records xiv SURFACE TEMPERATURE RECONSTRUCTIONS FOR THE LAST 2,000 YEARS More Detailed Background on Glacier-Length-Based Reconstructions 11 Principal Component Regression Validation and the Prediction Skill of the Proxy Reconstruction CLIMATE FORCINGS AND CLIMATE MODELS Climate Forcings Climate Model Simulations Anthropogenic Forcing and Recent Climate Change LARGE-SCALE MULTIPROXY RECONSTRUCTION TECHNIQUES Strengths and Limitations of Large-Scale Surface Temperature Reconstructions What Comments Can Be Made on the Value of Exchanging Information and Data? Summary Because widespread, reliable instrumental records are available only for the last 150 years or so, scientists estimate climatic conditions in the more distant past by analyzing proxy evidence from sources such as tree rings, corals, ocean and lake sediments, cave deposits, ice cores, boreholes, glaciers, and documentary evidence. For example, records of Alpine glacier length, some of which are derived from paintings and other documentary sources, have been used to reconstruct the time series of surface temperature variations in south-central Europe for the last several centuries. Studying past climates can help us put the 20th century warming into a broader context, better understand the climate system, and improve projections of future climate. Starting in the late 1990s, scientists began combining proxy evidence from many different locations in an effort to estimate surface temperature changes averaged over broad geographic regions during the last few hundred to few thousand years. These large-scale surface temperature reconstructions have enabled researchers to estimate past temperature variations over the Northern Hemisphere or even the entire globe, often with time resolution as fine as decades or even individual years. This research, and especially the first of these reconstructions published in 1998 and 1999 by Michael Mann, Raymond Bradley, and Malcolm Hughes, attracted considerable attention because the authors concluded that the Northern Hemisphere was warmer during the late 20th century than at any other time during the past millennium. Controversy arose because many people interpreted this result as definitive evidence of anthropogenic causes of recent climate change, while others criticized the methodologies and data that were used. In response to a request from Congress, this committee was assembled by the National Research Council to describe and assess the state of scientific efforts to reconstruct surface temperature records for the Earth over approximately the last 2,000 years and the implications of these efforts for our understanding of global climate change. Figure S-1 shows a compilation of large-scale surface temperature reconstructions from different research groups, each using its own methodology and selection of proxies, as well as the instrumental record (beginning in 1856) of global mean surface temperature. |