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Solar radiation reaching Earth is not constant but rather varies with time because of cyclical changes in the sun's activity. In their Perspective, [Lean and Rind][1] discuss the emerging evidence that this variability, albeit small, is translated into climatic variability on Earth. This issue is important because the solar variability signal must be distinguished from the anthropogenic signal to understand the human influence on climatic change. [1]: http://www.sciencemag.org/cgi/content/full/292/5515/234
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Lean et al. (2001) studied this question.
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