An article in LIveScience reports on results of a study by NASA, using a model they created, that is one of the first that I have seen that attempts to quantify the one of of the results of global warming.
Researchers ran the model for a future climate scenario where carbon dioxide levels were double their current level and the Earth's surface was 5 degrees Fahrenheit warmer than it is now. The model's projections showed that continents warm more than oceans (a result which is expected because water needs to absorb more heat than land to raise its temperature) and that lightning occurs at a higher altitude where storms are usually more vigorous.
These effects would combine to cause more continental storms to be of the strongest kind we see today, though there would be fewer storms overall.
These conclusions are particularly bad news for the storm-prone portions of the central and eastern United States, where strong winds are a major source of weather-related casualties.
The western United States won't catch a break either—while it is expected to get drier, the storms that do form are likely to have more lightning, which could then trigger more wildfires.