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A number of estimates of global surface temperature sensitivity to a doubling of atmospheric carbon dioxide to 600 ppm are collected here and critically reviewed. The assumptions and formulations that lead to differences between certain models' estimates are explained in some detail. Based on current understanding of climate theory and modeling it is concluded that a state-of-the-art order-of-magnitude estimate for the global surface temperature increase from a doubling of atmospheric C02 content is between 1.5 and 3 K with an amplification of the global average increase in polar zones. It is pointed out, however, that this estimate may prove to be high or low by several-fold as a result of climatic feedback mechanisms not properly accounted for in state-of-the-art models.
Stephen H. Schneider (Sat,) studied this question.