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.
Despite Pew's efforts, people generally will believe what they want of believe, with probability theory and evidence being irrelevant to ordinary "cognitive processes" involving the Mayan Calendar, Biblical Prophesy, Denial and other Defense Mechanisms, or Natural-Born Optimism. Ya' gotta love those Big Brains!
Nonetheless, Pew feels they must make the effort, and so do I. The evidence linking global warming and extreme weather events is already highly suggestive, and getting stronger all the time. This is one of those situations where by the time you've "proved" the link, which will require more time to pass, you are already fucked. To explain the concepts and lay out the evidence, Daniel G. Huber and Jay Gulledge have written a white paper called Extreme Weather And Climate Change: Understanding the Link, Managing the Risk.
Huber and Gulledge provide a long list of recent extreme weather events, and then tie them to the climate.
Taken in aggregate, this narrative of extreme events over recent decades provides a few snapshots of a larger statistical trend toward more frequent and intense extreme weather events. Rising frequency of both droughts and floods is an expected consequence of a warming climate, and both trends have been observed. Some areas will see more droughts as overall rainfall decreases and other areas will flood more regularly. Still other regions may not experience a change in total rainfall amounts, but might see rain come in rarer, more intense bursts, resulting in periodic flash flooding punctuating periods of chronic drought. Therefore, observed trends in heat, flooding, and drought in different places are consistent with global warming.
Over the past 50 years, total rainfall has increased by 7% globally, much of which is due to increased frequency of heavy downpours.
In the United States, the amount of precipitation falling in the heaviest 1% of rain events has increased by nearly 20 percent overall, while the frequency of light and moderate events has been steady or decreasing (Figure 1, top left).
Meanwhile, heat waves have become more humid, thereby increasing biological heat stress, and are increasingly characterized by extremely high night-time temperatures, which are responsible for most heat-related deaths. In the western United States, drought is more frequent and more persistent, while the Midwest experiences less frequent drought but more frequent flooding.
That's some of the evidence. Here's the bottom line—
When averaged together, changing climate extremes can be traced to rising global temperatures, increases in the amount of water vapor in the atmosphere, and changes in atmospheric circulation. Warmer temperatures directly influence heat waves and increase the moisture available in the atmosphere to supply extreme precipitation events. Expanding sub-tropical deserts swelling out from the equator are creating ever larger areas of sinking, dry air, thus expanding the area of land that is subject to drought. The expansion of this sub-tropical circulation pattern is also increasing heat transport from the tropics to the Arctic and pushing mid-latitude storm tracks, along with their rainfall, to higher latitudes.
As discussed above, no particular short-term event can be conclusively attributed to climate change. The historical record provides plenty of examples of extreme events occurring in the distant past and such events obviously occur without requiring a change in the climate. What matters is that there is a statistical record of these events occurring with increasing frequency and/or intensity over time, that this trend is consistent with expectations from global warming, and that our understanding of climate physics indicates that this trend should continue into the future as the world continues to warm...
Thus we can expect more extreme weather over time as the Earth warms. Remember, we are still in the early stages of a changing climate.
In short, you ain't seen nothin' yet. The "best" is yet to come.
Bonus Video — The news from Down Under (as of the end of 2010)
Another Bonus Video — Shorter and to the point.