Mixing Politics and Science in Testing the Hypothesis That Greenhouse Warming Is Causing a Global Increase in Hurricane Intensity BY J. A . CURRY, P. J. WEBSTER, AND G . J. HOLLAND This complex hypothesis has been muddied frequently in recent public debate, yet can be clarified by laying bare the underlying causal chain and potential approach to verification. zyxwvutsrqponmlkjihgfe "Science is what we have learned about how to keep from fooling ourselves—RICHARD FEYNMAN T TzyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA he incidence of seven major hurricanes threaten- ing or directly affecting the United States during 2005, associated with warmer-than-average sur- face waters, has fueled the debate regarding the role of greenhouse warming in increasing hurricane in- tensity. The exceptional damage caused by Hurricane Katrina, estimated at exceeding $100 billion, and a death toll exceeding 1,300, has raised important policy issues on the vulnerability of the United States zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFED AFFILIATIONS: CURRY AND W EBSTERzyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA —School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia; HOLLAND—National Center for Atmospheric Research, Boulder, Colorado CORRESPONDING AUTHOR: Judith A. Curry, School of Earth and Atmospheric Sciences, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, GA 30332-0340 E-mail: curryja@eas.gatech.edu DOI: 10.1175/ BAMS-87-8-I025 In final form 7 April 2006 © 2006 American Meteorological Society to the intense hurricanes and rising sea level associ- ated with the emission of greenhouse gases. Prior to the 2005 North Atlantic hurricane season, Trenberth (2005) published a commentary in Science raising the issue as to whether the increase in North Atlantic hurricane activity since 1995 could be at- tributed to global warming. This paper motivated us to begin looking at global hurricane data. In August, Emanuel (2005) published a paper in Nature associat- ing the increase in sea surface temperature (SST) with an increase in maximum hurricane potential inten- sity and the destructive capacity of hurricanes, focus- ing on hurricanes in the North Atlantic and North Pacific. Webster et al. (2005; hereafter WHCC), in an article in Science, showed that since 1970 the total number of hurricanes has not increased globally, but the proportion of category-4 and -5 hurricanes had doubled, implying that the distribution of hurricane intensity has shifted toward being more intense. The timing of the publication of the Emanuel (2005) paper early in the 2005 North Atlantic hurricane season and the publication of WHCC between the landfalls of Hurricanes Katrina and Rita focused intense media attention on the topic of greenhouse warming and increasing hurricane intensity, although neither AMERIC AN METEO RO LO G IC AL SO C IETY A UG UST 2006 BAfft I 1025