In a study released by the British journal Nature, the investigators say the so-called Asian Brown Cloud is as much to blame as greenhouse gases for the warming observed in the Himalayas over the past half century.

Rapid melting among the 46,000 glaciers on the Tibetan Plateau, the third-largest ice mass on the planet, is already causing downstream flooding late summer. But long-term worries focus more on the danger of drought, as the glaciers shrink.

The new report triggered an appeal from UN Environment Programme (UNEP) chief Achim Steiner, who urged the international community "to ever greater action" on tackling climate change.

Researchers led by Veerabhadran Ramanathan, a professor of atmospheric sciences at Scripps Institution of Oceanography in California, used an innovative technique to explore the Asian Brown Cloud.

The plume sprawls across South Asia, parts of Southeast Asia and the northern Indian Ocean, spewed from tailpipes, factory chimneys and power plants, forests or fields that are being burned for agriculture, and wood and dung which are burned for fuel.

Emissions of carbon gases are known to be the big drivers of global warming, but the role of particulate pollution, such as brown clouds, is unclear.

Particulates, also called aerosols, cool the land or sea beneath them because they filter out sunlight, a process known as global dimming.

But what they do to the air around them has been poorly researched.

Some aerosols absorb sunlight and thus warm the atmosphere locally, while others reflect and scatter the light.

Ramanathan's team used three unmanned aircraft of the type often used as for observation work by the military and police, but this time fitted them with 15 instruments to monitor temperature, clouds, humidity and aerosols.

Launched from the Maldives island of Hanimadhoo, the remote-controlled craft carried out 18 missions in March 2006, flying in a vertical stack over the Indian Ocean.

The planes flew simultaneously through the Brown Cloud at heights of 500 metres (1,625 feet), 1,500 metres (4,875 feet) and 3,000 metres (9,750 feet).

They discovered that the cloud boosted the effect of solar heating on the air around it by nearly 50 percent. This was because its particles are soot, which is black and thus absorbs sunlight.

The researchers then crunched data from greenhouse gases and from the brown clouds in a US computer model for climate change.

The simulation estimated that, since 1950, South Asia's atmosphere has warmed by 0.25 degrees Celsius (0.5 degrees Fahrenheit) per decade at altitudes ranging from 2,000 to 5,000 metres (6,500 to 16,500 feet) above sea level -- precisely the height where thousands of Himalayan glaciers are located.

As much as half of this warming can be attributed to the effects of brown clouds, Ramanathan told AFP by phone from the Himalayas, where he was setting up a glacier monitoring station.

"It is frightening, but I also look at the positive side, because it shows a way out of the conundrum," said Ramanathan.

Roughly 60 percent of the soot in South Asia comes from biofuel cooking and biomass burning, which could be eased by helping the rural poor get bottled gas or solar cookers, he explained.

Ramanathan's data has been validated with measurements taken on the ground at Hanimadhoo and in space by the NASA Earth-monitoring satellite CALIPSO.

UNEP's Steiner said the Asian Brown Cloud and greenhouse gases were linked by a common dependence on carbon fuels.

"The new findings should spur the international community to ever greater action, in particular at the next crucial climate change convention meeting in Indonesia this December," he said

"For it is likely that in curbing greenhouse gases, we can tackle the twin challenges of climate change and brown clouds and in doing so, reap wider benefits, from reduced air pollution to improved agricultural yields."

Himalayan glaciers provide headwaters for Asia's nine largest rivers, a lifeline for the 1.3 billion people who live downstream.

In June, a conference of glaciologists in the Nepalese capital of Kathmandu heard that on present trends the glaciers could disappear within the next 50 years.

"If temperatures continue to rise as it is, then there will be no snow and ice in the Himalayas in 50 years' time," said Surendra Shrestha, UNEP's regional director.

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