Was the active hurricane season of 2004 in the US a prelude of higher hurricane frequency due to global warming (or some other cause, for that matter)? Local sea surface temperatures exceeding 26ºC generally are required for occurrence of hurricanes. Atlantic hurricanes (the ones that come ashore in the south and southeast US) are suppressed in years when an El Nino is active in the tropical Pacific Ocean. Trenberth provides statistics on a measure of regional storm activity, accumulated cyclone energy (ACE), over the past 50 years and finds that for the decade of 1995-2004 ACE has been above normal every year except the two El Nino years (1997 and 2002). During this period there were 13.6 topical storms, 7.8 hurricanes, and 3.8 major hurricanes per year, compared to 8.6, 5.0, and 1.5 per year, respectively, for the preceding 25-year period. Despite this statistical evidence, Trenberth asserts there is no theoretical evidence for a human contribution to the increased number of hurricanes. However, there is evidence that human factors play a role once a hurricane forms: a warmed environment creates more intense hurricanes with more rainfall.
Reference
Trenberth, K., 2005: Uncertainty in hurricanes and global warming. Science, 308, 1753-1754.