Monday, March 12, 2007

Air pollution can create water shortages

Most Americans are now familiar with the idea that human activity is causing global warming. Nature reports that we may be impacting rainfall as well:
Pollution decreases rainfall

Air pollution is severely diminishing rainfall in Chinese mountains, researchers have found. The same effect is probably causing water shortages in many other highly polluted areas that depend on the nearby hills for their water.

Meteorologists have long assumed that pollution can suppress or delay this precipitation. Small particles known as aerosols are known to alter the size and properties of the water droplets that form in clouds, and can affect the weather in several ways. In some mountain ranges in the western United States, rainfall has decreased by 10-25% over the past 50 years. Tiny particles in air pollution are suspected to be the main cause, but a lack of data has hindered any robust testing of this notion.

Now, scientists have gathered enough data to quantify the effect. Daniel Rosenfeld, a meteorologist at the Hebrew University of Jerusalem, Israel, and his team used a 50-year record of continuous measurements made on top of Mt. Hua Shan, near Xi'an in central China, to show that orographic precipitation decreased by 30-50% when conditions were hazy. Overall, annual precipitation at Mt. Hua Shan decreased by around a third between 1970 and 2005, as pollution rose.

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