Overview

In response to a request from Congress, this report assesses 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. Because widespread, reliable temperature records are only available for the last 150 years or so, scientists estimate temperatures in the more distant past by analyzing proxy evidence, which includes tree rings, corals, ocean and lake sediments, cave deposits, ice cores, boreholes, and glaciers. Starting in the late 1990s, scientists began using sophisticated methods to combine proxy evidence from many different locations in an effort to estimate surface temperature changes during the last few hundred to few thousand years. This report concludes that large-scale surface temperature reconstructions are important tools in our understanding of global climate change that allows us to say, with a high level of confidence, that global mean surface temperature was higher during the last few decades of the 20th century than during any comparable period during the preceding four centuries. Less confidence can be placed in large-scale surface temperature reconstructions for the period from A.D. 900 to 1600, although available proxy evidence indicates that temperatures at many, but not all, individual locations were higher during the past 25 years than during any period of comparable length since A.D. 900. Very little confidence can be assigned to statements concerning the hemispheric mean or global mean surface temperature prior to about A.D. 900, primarily because of the scarcity of precisely dated proxy evidence.

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9786610742400

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