| Contents Notes |
Summary -- Overview -- Introduction to technical chapters -- The instrumental record -- Documentary and historical evidence -- Tree rings -- Marine, lake, and cave proxies -- Ice isotopes -- Glacier length and mass balance records -- Boreholes -- Statistical background -- Climate forcings and climate models -- Large-scale multiproxy reconstructions techniques -- References -- Appendixes: A. Statement of task. B.R code for figure 9-2. C. Biographical sketches of committee members. Overview -- Introduction to technical chapters -- The instrumental record -- Documentary and historical evidence -- Tree rings -- Marine, lake, and cave proxies -- Ice isotopes -- Glacier length and mass balance records -- Boreholes -- Statistical background -- Climate forcings and climate models -- Large-scale multiproxy reconstructions techniques -- References -- Appendixes: A. Statement of task. B. R code for figure 9-2. C. Biographical sketches of committee members. 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 for 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. |
