Did global warming cause the Midwest flooding?

This post is by James Wang, Ph.D., a climate scientist at Environmental Defense.

The record floodwaters in Iowa and other parts of the Midwest are claiming lives, destroying homes and crops, contaminating drinking water, and -- as the AP puts it -- spreading "a noxious brew of sewage, farm chemicals, and fuel that could sicken anyone who wades in." The cost in human anguish is incalculable.

But why is it happening? Is it just a freak of nature? One causal element, as reported in Thursday's Washington Post, may be human reengineering of the landscape. Mary Kelly, who heads up EDF's rivers and deltas program, gives a good overview of these issues.

Another element may be global warming, which increases the probability of extreme weather events like torrential rain.

Global warming intensifies the "hydrological cycle" -- the process in which water evaporates into the air, forms clouds, and then rains back down on the Earth.

Higher temperatures cause evaporation to occur more quickly. This can cause very dry conditions on land, even drought. But there's another side to it.

The greater amount of water vapor that a warm atmosphere can hold causes wetter clouds to form, so the rain, when it comes, can be unusually heavy -- heavy enough to cause flooding. This intensification of the hydrological cycle causes some seasons to be very wet while others are very dry.

We can't say for sure that global warming caused the unusually heavy rain in the Midwest -- or any specific weather event. But we can say that the probability of torrential rainfall is increased due to global warming.

The IPCC's 2007 report [PDF] says:

• "The frequency of heavy precipitation events has increased over most land areas, consistent with warming and observed increases of atmospheric water vapour."
• "It is very likely that hot extremes, heat waves and heavy precipitation events will continue to become more frequent."

Global warming doesn't fully explain the catastrophe in the Midwest, but it likely plays a role. The sooner we can bring emissions under control, the better.