Warmer Seas, Wetter Air Make Harder Rains

The greatestincreases will occur over land in the tropics, according to the study.Heavier rain or snow will also fall in northwestern and northeasternNorth America, northern Europe, northern Asia, the east coast of Asia,southwestern Australia, and parts of south-central South America duringthe 21st century.

"The models show most areas around the worldwill experience more intense precipitation for a given storm duringthis century," says lead author Gerald Meehl. "Information on whichareas will be most affected could help communities to better managewater resources and anticipate possible flooding."

NCAR authorsMeehl, Julie Arblaster, and Claudia Tebaldi analyzed the results ofnine atmosphere-ocean global climate models to explain the physicalmechanisms involved as intensity increased. Precipitation intensityrefers to the amount of rain or snow that falls on a single stormy day.

Boththe oceans and the atmosphere are warming as greenhouse gases build inthe atmosphere. Warmer sea surfaces boost evaporation, while warmer airholds more moisture. As this soggy air moves from the oceans to theland, it dumps extra rain per storm.

Though water vapor increasesthe most in the tropics, it also plays a role in the midlatitudes,according to the study. Combined with changes in sea-level pressure andwinds, the extra moisture produces heavier rain or snow in areas wheremoist air converges.

In the Mediterranean and the U.S. Southwest,even though intensity increases, average precipitation decreases. Theauthors attribute the decrease to longer periods of dry days betweenwet ones. The heavier rain and snow will most likely fall in lateautumn, winter, and early spring, while warmer months may still bring agreater risk of drought.