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A noxious cocktail of soot, smog and toxic chemicals is blotting out the sun, fouling the lungs of millions of people and altering weather patterns in large parts of Asia, according to a report released Thursday by the United Nations.

The byproduct of automobiles, slash-and-burn agriculture, cooking on dung or wood fires and coal-fired power plants, these plumes rise over southern Africa, the Amazon basin and North America. But they are most pronounced in Asia, where so-called atmospheric brown clouds are reducing sunlight in many Chinese cities and leading to decreased crop yields in swaths of rural India, say a team of more than a dozen scientists who have been studying the problem since 2002.

“The imperative to act has never been clearer,” said Achim Steiner, executive director of the U.N. Environment Program, in Beijing, which the report identified as one of the world’s most polluted cities, and where the report was released.

The brownish haze, sometimes in a layer more than a mile thick and clearly visible from airplanes, stretches from the Arabian Peninsula to the Yellow Sea. During the spring, it sweeps past North and South Korea and Japan. Sometimes the cloud drifts as far east as California.

The report identified 13 cities as brown-cloud hot spots, among them Bangkok, Thailand, Cairo, Egypt, New Delhi, Seoul, South Korea, and Tehran, Iran.

It was issued on a day when Beijing’s own famously polluted skies were unusually clear. On Wednesday, by contrast, the capital was shrouded in a thick, throat-stinging haze that is the byproduct of heavy industry, coal-burning home heaters and the 3.5 million cars that clog the city’s roads.

Last month, the government reintroduced some of the traffic restrictions that were imposed on Beijing during the Olympics.