Scientists studying variations in tropical heat and rainfall since the mid-1980s have found a strong link between warm periods and more extreme downpours. The observed rise in the heaviest rains is about twice that produced by computer simulations used to assess human-caused global warming, said the researchers. Other studies have already measured a rise in heavy rains in areas as varied as North America and India, and climatologists have long forecast more heavy rains in a world warmed by accumulating greenhouse gases. But this analysis, using 20 years of NASA satellite measurements, is the first to find a strong statistical link between warmth and extreme downpours, the researchers said. The study was published Thursday in the online journal Science Express. The authors were Richard Allan of the University of Reading in England and Brian Soden at the University of Miami. A general relationship between warming and heavier rainfall is widely accepted; the new paper is important "because it uses observations to demonstrate the sensitivity of extreme rainfall to temperature," said Anthony Broccoli, the director of the Center for Environmental Prediction at Rutgers University. "Flash flooding is produced by the extreme rain events," he said. "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 percentage points, according to World Bank experts.