The Pew Center on Global Climate Change has put together some resources for examining the link between anthropogenic climate change extreme weather events—floods, droughts, and heat waves. No single extreme weather event can be tied directly to global warming, but there is rise in the probability of such events over time. For example, whereas a so-called "hundred-year-flood" used to occur on average every 100 years, now we will get one on average twice a century or four times a century. The chances of such a flood occurring in a given year will have changed from 1/100 to 1/50 or 1/25. These kinds of odds will likely rise as the world gets warmer.
Scientific American provides a good overview in Global Warming and the Science of Extreme Weather.
Until recently scientists had only been able to say that more extreme weather is "consistent" with climate change caused by greenhouse gases that humans are emitting into the atmosphere. Now, however, they can begin to say that the odds of having extreme weather have increased because of human-caused atmospheric changes—and that many individual events would not have happened in the same way without global warming. The reason: The signal of climate change is finally emerging from the "noise"—the huge amount of natural variability in weather.
Scientists compare the normal variation in weather with rolls of the dice...
The simple statistical concepts involved have been a source of great confusion for the general public, a great number of whom have no interest in Reality in any case. Nevertheless, the Pew Center tries to explain what's going on.
Why can’t scientists say whether climate change “caused” a given weather event?
Climate is the average of many weather events over of a span of years. By definition, therefore, an isolated event lacks useful information about climate trends. Consider a hypothetical example: Prior to any change in the climate, there was one category 5 hurricane per year, but after the climate warmed for some decades, there were two category 5 hurricanes per year. In a given year, which of the two hurricanes was caused by climate change? Since the two events are indistinguishable, this question is nonsense. It is not the occurrence of either of the two events that matters. The two events together – or more accurately, the average of two events per year – define the change in the climate. (read more)