Tropical Warming Tied to Flooding Rains
by ANDREW C. REVKIN, New York Times, August 7, 2008

Scientists studying variations in tropical heat and rainfall since the mid-1980s have found a strong link between warm periods and a rise in the frequency of the most extreme downpours.

The observed rise in the heaviest tropical rains is about twice that produced by computer simulations used to assess how human-caused global warming could change rainfall, said the researchers.

Other studies have already measured a rise in recent decades in heavy rains in areas as varied as North America and India, and climatologists have long forecast more heavy rainstorms in a world warmed by accumulating greenhouse gases.

But this analysis, using satellite measurements, is the first to find a strong statistical link between warmth and extreme tropical downpours, the researchers said.

The study was published Thursday in the online journal Science Express. The authors were Richard P. Allan of the University of Reading in England and Brian J. Soden at the University of Miami.

While a general relationship between warming and more flooding rains is already widely accepted, the new paper is important “because it uses observations to demonstrate the sensitivity of extreme rainfall to temperature,” said Anthony J. Broccoli, the director of the Center for Environmental Prediction at Rutgers University.

“Such changes in extreme rainfall are quite important in my view, as flash flooding is produced by the extreme rain events,” Dr. Broccoli added. “In the U.S., flooding is a greater cause of death than lightning or tornadoes, and presumably poses similar risks elsewhere.”

In developing countries, cities with poor drainage routinely grind to a halt and see outbreaks of waterborne disease after extreme rainstorms. Such downpours have been estimated in some such countries to blunt economic growth by several percent, according to World Bank experts on disasters.

The new study analyzed 20 years of data from NASA satellites measuring tropical rainfall through several cycles of El Nino events. The periodic hot spells in the tropical Pacific Ocean, and contrasting cooler La Nina episodes, can influence weather from North America to Southeast Asia.

The rise in frequency of the heaviest rains (the top one percent of downpours) was accompanied by diminishing light rains, the scientists reported.

Overall, the work paints a portrait of a warming world producing more of the most destructive tropical flash floods than climatologists had realized, Dr. Soden said.

Many experts in disaster management have increasingly warned that global warming is likely to pose an outsized threat to poor countries around the tropics, which cannot handle weather extremes now, let alone what may be coming later in the century.

Dr. Soden agreed that wealthier places were likely more able to deal with such risks. “The better your infrastructure for dealing with extremes, the less vulnerable you are,” he said.

Link to article: http://www.nytimes.com/2008/08/08/science/earth/08rain.html