Health & Science

08 September 2009 

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Pollution in China Affecting Rainfall

08 September 2009

 Pollution in China appears to be decreasing the amount of light rainfall the country receives, according to a new study. That's important because light rain is the kind that helps sustain crops. VOA's Steve Baragona has more.

Over the past 50 years, the total amount of rainfall in northeastern China has been decreasing, while the southeastern portion of the country has been getting more rain — mostly in the form of heavy downpours.

"People have been calling this trend the north drought and south flood," says Ruby Leung, a climate researcher at the U.S. Pacific Northwest National Laboratory. She notes that not all rain is good for agriculture.

"If you have very heavy rain, essentially this rain becomes runoff and it doesn't really soak into the soil to provide the moisture that is needed for the crops," she says.

Light rain decreasing

What crops need is light rain. Leung and her colleagues in the U.S., China and Sweden looked back at trends over the past 50 years, focusing on days when 10 millimeters or less of rain fell. They found that on average, eastern China has been losing about a week per year of this gentle rain every decade since 1956. The amount of precipitation falling as light rain has declined as well, by 1.3 percent per decade in the southeast and by 2 percent per decade in the northeast.

Leung and her team first asked whether the decrease was because the amount of moisture in the atmosphere has changed, and the answer was no.   

"In fact," she says, "we were not able to find any particular changes in the winds or the moisture that bring precipitation to China … in the last 50 years."

What has changed, she says, is the pollution. For example, the amount of sulfur produced by burning fossil fuels in China's rapidly industrializing east has increased nine-fold since the 1950s.

Origins of a raindrop

Raindrops begin forming when moisture condenses on sea salt, dust, sulfur pollution, or whatever particles happen to be floating in the atmosphere. Those drops have to reach a certain size before they're heavy enough to fall as rain. But with so many more particles in the air, and the same amount of moisture, Leung says the size of each drop is smaller, and, "They cannot grow to a big enough size that can start to form rain."

That's what her group observed by comparing recent satellite data from the skies over polluted cities to those over pristine oceans, and in computer models her group developed. The study appears in the Journal of Geophysical Research.  

Their findings fit with research done by atmospheric scientist V. Ramanathan at the Scripps Institution of Oceanography. He showed how brown clouds of pollution have affected rainfall patterns, and he adds, "We know climate change and greenhouse gases and global warming is impacting the rainfall pattern around the world. But what is not general knowledge is that air pollution is also impacting rainfall. That whole knowledge is just evolving over the last 10 years."

And, he says, it's not just industrial pollution from places like China's big cities that is contributing to changes in weather patterns. He studied pollution over Africa and India caused by burning dirty fuels like wood, crop residues, and cow dung, and found similar effects there.

Ruby Leung says that while air pollution seems to be playing a significant role in the loss of light rain in China, it may not be the only cause. She says her next step will be to look at other factors such as changes in land use.

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