Comments by William M. Gray (Colorado State University) on the recently   published paper in Science by Webster et al., titled "Changes in tropical   cyclone number, duration, and intensity in a warming environment." (September   2005, Vol. 309, pp. 1844-1846, www.sciencemag.org)
  Recent US major landfalling hurricanes Katrina and Rita and last year's four U.S. landfalling major hurricanes have spawned an abundance of questions concerning the role that global warming might be playing in these events. This idea has been given added credence by the September 2005 Science paper of Webster, Holland, Curry and Chang (Vol. 304, pp. 1844-1846) showing that the global number of Category 4-5 hurricanes have increased in the last 15 years (1990-2004) in comparison with the prior 15-year period of 1975-1989. They report 171 Category 4-5 hurricanes in the earlier 15-year period vs. 269 (56% increase) in the later 15 year period. Global mean surface temperature in the later period has been about 0.3C higher than in the earlier period. The authors' imply that their measured rise in global Category 4-5 hurricanes is likely related to these higher global temperatures.   Having been involved with hurricane research and forecasting for nearly 50 years, I feel I have an obligation to offer comments on this paper's primary finding on the recent rise of global Category 4-5 hurricanes. I do not agree that global Category 4-5 tropical cyclone activity has been rising, except in the Atlantic over the past 11 years. The recent Atlantic upsurge has explanations other than global temperature rise. 
William Gray