'Brown clouds' changing weather, United Nations report says

The huge smog-like plumes, caused mainly by the burning of fossil fuels and wood, are known as "atmospheric brown clouds." When mixed with emissions of carbon dioxide and other gases blamed for warming Earth's atmosphere, they are the newest threat to the global environment, according to a report commissioned by the U.N. Environment Program.

Brown clouds are caused by an unhealthy mix of particles, ozone and other chemicals that come from cars, coal-fired power plants, burning fields and wood-burning stoves and fireplaces. First identified by the report's lead researcher in 1990, the clouds were depicted Thursday as being more widespread and causing more environmental damage than previously known.

The clouds have been found to be more than a mile thick around glaciers in the Himalayan and Hindu Kush mountain ranges. They block sunlight and absorb radiation, leading to new worries not only about global climate change but also about extreme weather conditions.

"All these have led to negative effects on water resources and crop yields," the report says.

It says brown clouds are dimming the light by as much as 25% in some places, including Karachi, Pakistan, New Delhi, Shanghai and Beijing.

The phenomenon complicates the climate change scenario, because the brown clouds also help cool Earth's surface and mask the effect of global warming by an average of 40%, according to the report.

The enormous cloud masses can move across continents within three or four days. Although they also form over the eastern United States and Europe, winter snow and rain tend to reduce the effect in those areas.

An international response is needed to deal with "the twin threats of greenhouse gases and brown clouds and the unsustainable development that underpins both," said the lead researcher, Veerabhadran Ramanathan, a professor of climate and ocean sciences at UC San Diego.

Health problems associated with particulate pollution, such as cardiovascular and respiratory diseases, are linked to nearly 350,000 premature deaths in China and India every year, said Henning Rohde, a University of Stockholm scientist who worked on the study.

Soot levels in the air were reported to have risen alarmingly in 13 megacities: Bangkok, Beijing, Cairo, Dhaka, Karachi, Kolkata, Lagos, Mumbai, New Delhi, Seoul, Shanghai, Shenzhen and Tehran.