Nov. 13 (Bloomberg) -- A three-kilometer (1.9-mile) thick layer of soot and particles stretching from the Middle East to China is making Asian cities darker and glaciers in the Himalayas melt more quickly, a United Nations report said.

Known as the ``atmospheric brown cloud,'' the pollution has resulted from increased burning of gasoline, coal and biomass such as wood and manure, the UN's Environment Program said on its Web site. Glaciers have shrunk 5 percent since the 1950s in China as soot contributes to atmospheric warming.

Most affected are air quality and agriculture in Asia, where risks to human health and food production for about half the world's population are rising, the report said. With glaciers shrinking, the main water supply for the world's two most populous nations may be reduced.

The pollution makes skies from Beijing to Tehran darker by blocking sunlight as it is absorbed by particles linked with burning fossil fuels. Guangzhou, a city in southern China, has reported a 20 percent drop in sunlight since the 1970s.

The cloud may also be contributing to shifts in weather patterns, including drought in northern China and flooding the southern region of the country as well as affecting the seasonal monsoon, the UN said. At the same time, it may be having a cooling effect on the planet as some soot and biomass material reflects sunlight, the report said.

To contact the reporter on this story: Jeremy van Loon in Berlin at jvanloon@bloomberg.net.

Last Updated: November 13, 2008 07:22 EST