I am indebted to S. Manabe for his continuing support and encouragement regarding this and other projects. R. Avissar, T. Crowley, L. Donner, C. Jackson, I. Held, J. Mahlman, S. Manabe, J. Miller, D. Robinson, and B. Soden provided helpful comments on this paper, as did an anonymous reviewer. S. Porter, R. Schneider, and M. Stute kindly provided compilations of paleodata. C. Raphael provided her assistance in preparing the figures for electronic submission.
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