Humans Changing Rainfall Patterns, Study Says

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.

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