Rainy days in the mountains are fewer near polluted cities.Punchstock

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.

Mountainous areas are well known to be wet: as air rises up over a hill it cools and condenses, creating rainy climes on the windward side. 'Orographic rain', as it is called, is a crucial component of water supply in dry regions from Israel to California.

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. The data is reported in Science¹.

Although the study does not provide definitive proof, it is a strong indicator that pollution reduces rainfall, says Ulrike Lohmann, an atmospheric physicist at the ETH Zurich in Switzerland.

"Extracting the human influence on precipitation is extremely interesting, but also extraordinarily difficult," she says. "Given strong pollution, China is probably the best place in the world to study this relation."

Where the wind blows

Aerosols affect the climate in many ways, sometimes affecting the weather far from the pollution source.

Different types of aerosols have different effects. The fine particles from diesel engines, biomass burning and fertilizers used in agriculture — as seen in this Chinese study — have a different properties than large particles such as mineral dust or sea salt. Larger aerosols contain more soluble material, which generally attracts water. This can help to coalesce raindrops, thus enhancing rainfall.

Pollution from Asia that is being blown eastward across the Pacific, for example, seems to be intensifying cloud cover, and possibly storm intensity, over the ocean².

Some people have attempted to take advantage of the relationship between aerosols and rain by using engineered particles to control rainfall. In Israel, this has been done for more than 40 years, says Lohmann. But most of these experiments have failed because the effects could not be proven statistically, she adds.

Clear waters

Clean water supply is a severe problem in dry regions around the world. In Israel, for example, decreasing rainfall in mountainous regions has led, according to unpublished data, to a 5% reduction in the availability of clean water. The country is forced to obtain more and more of its water from distillation of sea water, says Rosenfeld.

Rosenfeld hopes that efforts to improve air quality will reduce the concentrations of small aerosols. Because fine dust can also cause cancer, the European Union and the United States have both set limits for their emission.

He notes that changes in rainfall due to manmade influences are just as important to take note of as the change in temperature caused by rising emissions of carbon dioxide. "Hydrology and precipitation are at least equally important," he says.

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• References

  1. Rosenfeld D., et al. Science, 315 . 1396 - 1398 (2007).
  2. Zhang R., et al. PNAS Online edition (2007).