The authors wish to thank Isaac Held, Jerry Mahlman, Brian Soden, and Ron Stouffer for many helpful discussions as well as constructive criticism of the manuscript before it was released for publication. In addition, the article was greatly improved as a result of the comments of two reviewers, Graeme Stephens and Simon Tett. Alex Hall was supported by a NASA Earth System Science Fellowship while this research was being carried out. Observed global-mean surface temperature data were compiled by the Climate Research Unit of the University of East Anglia (United Kingdom) and obtained from their Web site: http://www.cru.uea.ac.uk:80/cru/data/temperature.htm.
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