May 31, 2007

Warmer world gets wetter

Satellite observations suggest climate models are wrong on rainfall.

by Harvey Leifert
Nature News

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Global warming will increase worldwide precipitation by three times the amount predicted by current climate models, according to a study based on two decades' worth of satellite observations.

The discrepancy between the models and the data might mean that the models are wrong. Or it might be that two decades is not long enough to test their predictions. But researchers believe that the work is a step towards understanding the thorny issue of how global temperatures affect rainfall.

Warmer air holds more water. Satellite observations and climate models agree that each rise of 1 °C in global temperatures increases the amount of water vapour in the atmosphere by about 6.5%.

But climate models project that global warming will also bring weaker winds, leading to less water evaporating from the ocean and counteracting the effect of warming. Models predict that worldwide precipitation — which must match the amount of evaporation — will increase by only 1-3% for each degree of future global warming.

Uncertain forecast

To see how rainfall had changed with the 0.4 °C warming of the past 20 years, Frank Wentz and his colleagues at Remote Sensing Systems in Santa Rosa, California, analysed data collected by US weather satellites from 1987 to 2006. The satellites measure atmospheric water vapour, surface winds and precipitation.

They report in Science that the amount of water in the atmosphere, evaporation and precipitation all increased at the same rate, by about 1.3% per decade¹ — or about 6.5% for every degree of warming. Surface winds increased, not decreased, with warming.

It is currently impossible to predict where additional precipitation will fall, says Wentz. Wet areas may get wetter, but drought-plagued regions might also get some relief.

The study is "a good first step" in the debate over future precipitation, says Gerald North of Texas A&M University, College Station, who studies rainfall patterns. But, he adds, the single 20-year data set is not enough to get a definite answer.

He also notes that it is difficult to measure evaporation from space — and even from the ground. "The satellite-inferred trends are not a measurement but an estimate with unknown and subtle error characteristics," he says.

Wentz agrees that two decades' worth of data are not definitive — but "it is all we have". The accuracy of climate models' predictions regarding precipitation has not been tested before, he adds.

John Bates, a remote-sensing expert at the US National Climatic Data Center in Asheville, North Carolina, says studying the global water budget is "notoriously difficult", but the latest climate models may be better able to account for precipitation.

He is looking forward "to seeing how those models compare with the ever improving satellite observations".

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