Revised Global Warming Forecast: Even More Rain

Four children try to cross a busy street to go home on a rainy day in India. CREDIT: Nikhil Gangavane, dreamstime

Global warming may bring down even more rain worldwide than previous studies have predicted, a new report says.

Many climate models have predicted that as the Earth warms, “the wet will get wetter and the dry will get drier,” said lead author of the new study, Frank Wentz of Remote Sensing Systems. This exaggeration of current rainfall patterns would mean that very rainy places such as Bangladesh could be soaked even more, while places such as the American Southwest would see even fewer drops than it currently does.

But by examining satellite data from the past 20 years, Wentz and his colleagues found that these predictions don’t seem to match what observations have shown, underestimating the rain that fell over that period and indicating that something was amiss in the way the models portrayed the water cycle.

Vertical conveyor belt

Wentz described the water cycle as a “vertical conveyor belt”: winds help evaporate moisture from the surface and transport it, like boxes on a conveyor belt, upward in the atmosphere where it then eventually falls as precipitation.

“As the planet warms, there’s going to be more water, and that means more boxes on the conveyor belt,” he explained.

With more water being transported, more rainfall might be expected to follow, but the previous models predicted a decrease in global winds, which would fail to circulate the moisture upward and leave the air humid and stagnant.

“What the models predict is that the conveyor belt’s going to slow down,” he told LiveScience. “So you’ll have more boxes on this, but it’ll be moving more slowly.”

But looking at the satellite data, Wentz and his team found that winds actually slightly increased over the past two decades, and so did precipitation and evaporation—and all by about the same percentage for each degree of warming during the time period. Their work is detailed in the May 31 online edition of Science Express.

Predictions accurate?

Wentz and his colleagues wondered what might be causing this discrepancy between the previous predictions and observations and what it meant about those models’ ability to predict rainfall.

“We don’t know for sure, but it certainly starts putting in doubt some of these climate predictions of this muted increase in precipitation,” he said. “Something in the model was not realistic.”

While models generally predict trends in temperatures and the amount of water vapor in the atmosphere fairly well, precipitation is much harder to predict because it varies so much in short periods of time and over short distances—afternoon rain showers have a way of disappearing just a quickly as they pop up.

“I just think it’s a much harder process for the models to represent,” Wentz said.

By extrapolating his research into the future, Wentz says that the world could see more rain than the models expect, which could be good for the dried out Southwest United States, but would be bad news for people already inundated in wetter climates.

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