Direct measurements of climate data such as temperatures are rare before the 20th century. Climate reconstructions for earlier centuries and millenia rely on a range of different climate indicators or proxies. In his Perspective, [Mann][1] explains how these proxies can be used in multiproxy climate reconstructions to obtaina reliable record of climatic variations over the last millenium. [1]: http://www.sciencemag.org/cgi/content/full/297/5586/1481
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Michael Mann (2002) studied this question.
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