Past studies of the number of tropical cyclones per year has shown no increase during the recent decades when the earth has experienced and increase in global mean temperature. Both the laws of physics and numerical models suggest, however, that hurricanes (strongest of the tropical cyclones) should increase in intensity with global warming. Kerry Emanuel (2005) (http://www.nature.com/nature/journal/vaop/ncurrent/abs/nature03906.html) defines an index of storm intensity, the Power Dissipation Index (PDI), as the cube of the maximum sustained wind speed at the 10-m height integrated over the lifetime of the storm. He finds that the PDI has more than doubled for Atlantic hurricanes in the last 30 years and nearly doubled for Western Pacific Hurricanes. The index for the combined set of tropical storms has nearly doubled. This finding should be of considerable interest and some concern for residents of hurricane-prone regions.
Reference
Emanuel, K., 2005: Increasing destructiveness of tropical cyclones over the past 30 years. Nature, 436, 868-688.