Pollution Reducing Rainfall in Semi-Arid Areas

Pollution from the burning of fossil fuels and biomass (living matter such as vegetation) inhibits precipitation over hills in semi-arid regions, according to a study announced on March 8 by a Chinese-Israeli research team.

The scientists, led by Daniel Rosenfeld of the Hebrew University of Jerusalem, concluded that the average precipitation on Mount Hua near Xian in central China has decreased by 20 percent along with increasing levels of human-made air pollution during the last 50 years. The precipitation loss was doubled on days that had the poorest visibility stemming from aerosol emissions.

The researchers said their findings explain the widely observed trends of decrease in mountain precipitation relative to the rainfall in nearby densely populated lowlands, which until now had not been directly ascribed to air pollution. The phenomenon the researchers discovered could have dire consequences for vital water resources in polluted regions of the world where hilly area precipitation makes a significant contribution to the regional water supply, such as the southwestern United States, central and northern China, and the Middle East.

The scientists studied observations of precipitation and visibility starting in 1954 at the top of Mount Hua. They linked the decreasing visibility at the mountain's summit with increasing air pollution that reaches to the clouds. The researchers said they were able to show that the trend of higher concentrations of aerosols is responsible for the observed decreasing trend of mountain precipitation.

Atmospheric aerosols absorb and emit heat, reflect light and, depending on their properties, can either cool or warm the atmosphere. For more information on aerosols, go to http://www.epa.gov/climatechange/science/recentac.html#aerosols.

Daniel Rosenfeld: http://earth.huji.ac.il/staff-main.asp?id=149

This article originally appeared in the 03/01/2007 issue of Environmental Protection.

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