Extreme Rains to Be Supercharged by Warming, Study Says

Mason Inman
for National Geographic News
August 7, 2008
Global warming could make extreme rains stronger and more frequent than previously forecast, a new study suggests.

Such a scenario could make floods fiercer, damage more crops, and worsen the spread of diseases such as malaria, scientists say.

Rainfall patterns are already shifting as Earth warms under a blanket of humanmade greenhouse gases, experts say.

Study co-author Richard P. Allan, an atmospheric scientist at the University of Reading in Berkshire, United Kingdom, said previous studies have shown that "wet regions are becoming wetter, and dry regions drier."

The study team analyzed satellite images of rainfall over tropical oceans over nearly two decades, from 1988 to 2004.

The researchers found that during El Niño years, which tend to be warmer, rain fell in heavier showers. An El Niño is a climate event where the flow of abnormally warm surface Pacific waters temporarily changes global weather patterns.

"This is something that climate models had predicted," Allan said. "But getting the data from observations is very important."

Many previous rainfall pattern studies have relied on measurements from rain gauges. Such gauges are sparsely distributed across land, Allan said, whereas satellites can see large areas as a whole.

Global Warming Forecast

Although our planet is warming overall, Earth's climate still varies between warmer and wetter El Niño years and cooler and drier La Niña years.

Looking at these changes in rainfall can give scientists a good estimate of what will happen with continued global warming, according to Allan and his co-author, Brian Soden of the University of Miami in Florida.

With continued global warming, the changes in Earth's rainfall patterns will be worse than previously forecast, Allan and Soden write.

"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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