Extreme Rains to Be Supercharged by Warming, Study Says

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"The models seem to underestimate the response in extreme rainfall with warming," Allan said.

For every 1.8 degree Fahrenheit (1 degree Celsius) rise in global temperature, heavy rain showers became more common, with most intense category jumping 60 percent, says the study, which will be published tomorrow in the journal Science.

During the 20th century, Earth's average global temperature rose about 1.2 degrees Fahrenheit (0.7 degrees Celsius). But researchers predict that bump will be dwarfed by the warming to come.

The latest UN Intergovernmental Panel on Climate Change report predicts at least three times as much warming—about 3.2 to 7.1 degrees Fahrenheit (2 to 4 degrees Celsius)—by the end of the 21st century. (See [April 6, 2007].)

The uptick could drive a big jump in intense rainfall events, Allan and Soden argue.

Human Impact

Warmer air can hold more moisture. "So if the air is more moist, you get more heavy rainfall," Allan noted, adding that such extreme weather takes a toll on people.

With intense rains, "you can get flash flooding, and heavy rainfall can destroy crops," he said. "Those are the most immediate impacts."

Coupled with rising global temperatures, more frequent and intense rainfall has "major implications for infectious diseases," said Paul Epstein, a tropical disease expert at Harvard Medical School in Boston, Massachusetts.

"After floods one often sees clusters of vector-borne diseases—malaria, dengue fever, Japanese B encephalitis," Epstein said.

Floods often cause a jump in cholera and other water-borne diseases, as well as plague and other rodent-borne diseases, he added.

David Neelin, a climate scientist at the University of California in Los Angeles, takes a more cautious view of the study results.

"Rainfall changes remain among the hardest impacts of global warming to predict precisely," he said.

But, Neelin added, "Allan and Soden's results add fuel to the growing concern from a number of research groups [that] the extremes of rainfall may increase under global warming."

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