The Yale School of Forestry and Environmental Studies hosted a blog forum that asked eight climate experts "whether they think there is growing evidence that human-caused global warming is contributing to an increased incidence of extreme weather — and to cite specific recent examples in their answers":
Their responses varied, with some contending that rising temperatures already are creating more tempestuous weather and others saying that more extreme weather may be likely but that not enough data yet exists to discern a trend in that direction. Scientists in both camps said two physical phenomena — warmer air holds more moisture, and higher temperatures exacerbate naturally occurring heat waves — would almost by definition mean more extremes. But some argued that the growing human toll from hurricanes, tornadoes, floods, and heat waves is primarily related to burgeoning human population and the related degradation of the environment.
The scientists' succinct responses here.