The haze of pollution that blankets southern Asia is accelerating the loss of Himalayan glaciers, bequeathing an incalculable bill to China, India and other countries whose rivers flow from this source, scientists warned yesterday.
In a study released by the British journal Nature, the investigators said 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 Program (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 to monitor temperature, clouds, humidity and aerosols.
Launched from the Maldives island of Hanimadhoo, the remote-controlled craft carried out 18 missions in March last year, flying in a vertical stack over the Indian Ocean.
The planes flew simultaneously through the Brown Cloud at heights of 500m, 1,500m and 3,000m.
They discovered that the cloud boosted the effect of solar heating on the air around it by nearly 50 percent.
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 0.25oC per decade at altitudes ranging from 2,000m to 5,000m above sea level -- precisely the height where thousands of Himalayan glaciers are located.