Notes: Scientific certainty about the contribution of global warming to severe storms varies by the type of storm.
Hurricanes: A strong consensus agrees that when hurricanes form, warm temperatures make them stronger because they feed off warm ocean waters. There is no agreement on whether warming will increase the total number of hurricanes.
Intense precipitation: There is also wide agreement that, by increasing the rate of evaporation, warming puts more moisture in the air, so when rain or snow events are more intense, leading to flooding, soil erosion, mud slides, etc. "When it rains, it pours."
Tornadoes: There is little agreement about the relation between warming and tornadoes, which are relatively very small and local events. While physics suggests that more moisture in the air may contribute to the formation of the supercell thunderstorms that give rise to tornadoes, the historic record is too unreliable to confirm this hypothesis. (The record actually shows no rise in big tornadoes, but a sharp rise in reports of small tornadoes, perhaps simply as an artifact of more reporting being done, not more tornadoes.)
For more, see Global Warming and the Science of Extreme Weather, by Jim Carey. Scientific American, June 29, 2011. On tornadoes, see Tornado Outbreak Raises Climate Questions, by Andrew Freeman, Climate Central, April 29, 2011.
Bite Source: Inspired by Jim Hansen, Scott Mandia and others.
Variation by Steve Sherwood (co-director of the Climate Change Research Center at the University of New South Wales in Australia, in Scientific American article above)
Image Source: Chris Beales