Kristof today:

"The available scientific evidence indicates that it is likely that global warming will make - and possibly already is making - those hurricanes that form more destructive than they otherwise would have been," declares an analysis by five climate scientists at www.realclimate.org.

Hurricanes derive their power in part from warm water, and so forecasting models show future hurricanes becoming more severe as sea surface temperatures rise. One summary of 1,200 simulations published in the Journal of Climate last year showed that rising levels of greenhouse gases could triple the number of Category 5 hurricanes.

Moreover, there's empirical evidence that hurricanes have already become more intense (but not more frequent). Nature magazine this summer reported a new study by Kerry Emanuel, a hurricane guru at M.I.T., indicating that by one measure hurricanes have almost doubled in intensity over the last 30 years.