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Brown clouds of pollution that stretch over the Indian Ocean during the dry season are warming the climate just as much as greenhouse gases, say scientists.

According to a study, published last week (2 August) in the journal Nature, these 'atmospheric brown clouds' could be directly responsible for the melting of glaciers in the Asian Himalayan and Hindu Kush mountain ranges, and may lead to water shortages in southern and eastern Asia.

The research was conducted by a group at the Center for Clouds, Chemistry and Climate at the Scripps Institution of Oceanography at the University of California San Diego, United States.

Atmospheric brown clouds are made up mostly of soot — suspended carbon particles — produced by people burning biomass and fossil fuels. This pollution gathers over the northern Indian Ocean and south Asia from the dry season of November–May every year.

The soot particles that make up these pollution blankets can both absorb and disperse light, and therefore both warm and cool the climate.

Until now, scientists were unsure of the net effect of these particles. The theory was that 'brown clouds' were cooling the lower atmosphere and therefore masking some of the warming effects of greenhouse gases.

But the Scripps team found that, rather than cooling, the brown clouds were warming the atmosphere and, perhaps more significantly, by around the same amount as greenhouse gases.

The authors write that their findings have "substantial implications" for the high mountain ranges in northern India and Nepal.

They say the brown clouds have most likely contributed to the increase in air temperature in the region, which in turn is in part responsible for the observed melting and receding of glaciers.

The authors write that this could have an effect on the amount of water that flows down many major Asian rivers, such as the Yangtze, the Indus and the Ganges, reducing the amount of water available to the people who live along them.

The study is based on data collected from the clouds in March last year using unmanned aircraft, supplemented with ground-based and satellite observations.

Link to full paper in Nature

Reference: Nature 449, 575 (2007)