Science

Rainy days.
Annual average rainfall over the oceans for 2006 in units of mm/hour as derived from satellite data.

Rain, Rain, Here to Stay

Those captivated by cloudbursts will find a silver lining in global warming. An analysis of 20 years of satellite data indicates that rising temperatures will bring increased precipitation. The finding challenges well-established climate models and could help researchers more accurately predict dramatic weather events such as El Niños.

Climatologists have long agreed that as the planet heats up, the atmosphere will hold more water--almost 7% more for every additional degree Celsius. Complex computer models, however, predict that a rise in atmospheric water will only boost rainfall by 1% to 3% for every degree the temperature rises. Researchers have assumed the discrepancy must be because, although there is more water in the atmosphere, the rate of its precipitation and evaporation slows down.

But these models don't have a perfect track record. When used to simulate weather patterns over the past 2 decades, the models underestimate rainfall and miss dramatic weather events such as the 1998 El Niño. So physicist Frank Wentz and colleagues at Remote Sensing Systems, a satellite analysis company in Santa Rosa, California, ditched the models. Instead, they used real historical data collected from six satellites to see the relations between total atmospheric water, precipitation, evaporation, and global temperature. It was a good match. Precipitation and evaporation changed exactly in line with total atmospheric water--an increase of 6.5% for every degree Celsius that Earth's temperature rose. That means evaporation and precipitation don't slow down when the atmosphere gets wetter, the team reports online in Science.

Don't count on a drought-free future, however. The amount of annual rainfall varies greatly around the globe, the researchers note. And the increases due to global warming could range quite a bit. "In the tropics, you would get as much as 65 millimeters of water, whereas in the northern latitudes, it might only be a few millimeters," says Wentz.

The study is the first to question the accuracy of precipitation in current climate models, says climatologist Brian Soden of the University of Miami in Florida. "There are dozens of different climate models out there, and every single one of them predicts that precipitation will increase more slowly than this study suggests," he says. Plus, he notes, they all get the historical record wrong. Improving these models, Soden says, could help climatologists better predict future storms.

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