Scientists call them atmospheric brown clouds (ABCs). They're noxious plumes of soot, sulfates, and other chemicals that are darkening skies over Asia, threatening water and agriculture, disturbing the monsoon system, and shrinking glaciers and snowpacks - not to mention contributing to hundreds of thousands of deaths each year. Five ABC "hot spots" are East Asia, including Hong Kong, above; South and Southeast Asia; southern Africa; and the Amazon basin. Caused by autos, slash-and-burn agriculture, dung or wood fires, and coal-fired power plants, they're an added reason for tougher antipollution laws, say authors of a recent United Nations report.

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