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  • Polluted air 'triggering drought' in northern China

Air pollution in the mountains of northern China is significantly reducing rainfall and causing drought, according to research published in Science this week (8 March).

Researchers, from Israel's Hebrew University of Jerusalem and the Chinese Academy of Meteorological Sciences, analysed long and short-term changes in precipitation using 50 years of rainfall records from the top of Mount Hua in China's northwestern Shaanxi province.

They found that on hazy days, precipitation in the region is cut by up to 50 per cent. There was also a smaller reduction in rainfall in the lowlands.

Since 1950 the volume of rain and snow at the top of the mountain has decreased by 20 per cent.

The researchers suggest an increase in air pollution ― which affects the formation of cloud droplets ― has triggered the shift.

Data collected locally shows that average visibility in 1950 stood at 30 kilometres ― twice that of today. Daniel Rosenfeld, lead researcher on the study, says this indicates the a greater presence of aerosols.

Aerosols are tiny particles suspended in the atmosphere. Some occur naturally, but they are also generated by human activities such as the burning of fossil fuels.

In this kind of high-altitude rainfall, known as orographic precipitation, moist air is deflected upwards by the mountain. This cools the air and causes the moisture in clouds to condense and form droplets, which then merge to create raindrops.

Cloud droplets form around aerosols. According to Rosenfeld, the higher number of aerosols in polluted air divide cloud droplets into smaller ones, which slows the formation and fall of rain.

"This is the first time a direct link between increasing pollution and decreasing precipitation has been observed," he said. "The finding is important since precipitation is one of the main sources of water in northern China."

Yao Zhanyu, co-author of the paper, told SciDev.Net that of all the natural disasters in China, droughts are the most serious. "In the western region, the annual average precipitation is about a fourth that of the world's average," he said.

Yao and Rosenfeld may soon be collaborating in a similar study to be conducted in Mount Lu in East China's Jiangxi Province.

Reference: Science 315, 1396 (2007)

Link to full paper in Science