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Humans Changing Rainfall Patterns, Study Says

John Roach
for National Geographic News
July 23, 2007
 
Humans have caused global precipitation patterns to change substantially over the past century, new research says.

About 1.8 inches (4.5 centimeters) more rain fell annually in Canada, Russia, and Europe in recent years than it did in 1925.

In the northern tropics and subtropics, such as Mexico and northern Africa, rainfall has decreased by nearly 2.8 inches (7 centimeters) per year.

And the southern tropics and subtropics such as Peru and Madagascar have seen increased rainfall of about 2.4 inches (6 centimeters).

Altogether humans account for about two-thirds of the precipitation increase in Canada, Russia, and Europe, a third of the drying out in the northern tropics and subtropics, and nearly all of the increase south of the Equator, the study says.

A significant driver behind the altered rainfall is greenhouse gas emissions, primarily from coal and oil burning, that contribute to global warming. (What is global warming?)

The study, led by climate researcher Xuebin Zhang of Environment Canada in Toronto, is the first to connect human activity with changing precipitation patterns.

A Significant Shift

Human-caused greenhouse gases emissions have previously been linked to several climate events, including rising sea and air temperatures around the world. (Related: "Global Warming "Very Likely" Caused by Humans, World Climate Experts Say" [Feburary 2, 2007].)

A global warming connection to rainfall, however, has proven more difficult to establish—partly because drier conditions in some regions cancel out wetter conditions elsewhere.

In the new study, the authors examined precipitation trends in different sections of land north and south of the Equator, rather than the globe as a whole.

This latitudinal approach shows a significant shift in global precipitation patterns over the past century.

The researchers then compared the observations to the changes in rainfall that multiple climate models predict should be attributable to human activity.

"Because we have this large number of simulations, we can average them all together in essence and filter out the effects of internal variability ... to obtain a best estimate," said study co-author Francis Zwiers, a climate scientist with Environment Canada.

The comparison, which appears in this week's journal Nature, shows that most of the changes in precipitation are due to human actions.

"You have this large-scale engine that moves moisture around the planet, and under greenhouse gas forcing, this engine essentially become more intense," Zwiers said.

However, the reasoning fails to fully explain why the region just south of the Equator is wetter. All things being equal, it should be drier, Zwiers noted.

The researchers suspect the increase is the result of a southward shift in the zone where trade winds of the Northern and Southern Hemispheres meet, but why the zone may have shifted remains unresolved.

Future Climate

Looking to the future, the study increases confidence in the prediction of climate models.

The models indicated a change should have occurred in the 20th-century rainfall, and the pattern shows up in the observations, Zwiers said.

However, the change predicted by the models was less than observed, which indicates the models, as a group, are not responding to greenhouse gases as much as they should.

Climate models' under-prediction of 20th-century change makes their accuracy for the 21st century a bit worrisome, Zwiers said. (Related: "Global Warming Models Underpredict Increase in Rainfall, Study Says" [May 31, 2007].)

"Maybe the projections for the future are not projecting a big enough change," he said.

The models predict the 20th-century trend should continue in the Northern Hemisphere, but most suggest the wet region just south of the Equator will revert to a drier pattern.

Any specific event cannot be pinned on human emissions of greenhouse gases, Zwiers said. Further studies of the climate models should reveal if increased greenhouse gases raise the risk of extreme rainfall events.

Others are not waiting for these scientific studies.

The findings were released as heavy flooding in the United Kingdom and China make headlines around the world.

"Extreme events such as we have seen in recent weeks herald the specter of climate change," Nick Reeves, the executive director of the Chartered Institution of Water and Environmental Management in London, England, told the Reuters news agency today.

"And it would be irresponsible to imagine that they won't become more frequent."

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