Asia’s Brown Cloud accelerates warming

Glaciers lost to Asian pollution

August 02, 2007

The report triggered an appeal from UN Environment Program 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. It spews 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.

Professor Ramanathan’s team used three unmanned aircraft fitted with 15 instruments to monitor temperature, clouds, humidity and aerosols. 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 500m, 1500m and 3000m. They discovered that the cloud boosted the effect of solar heating on the air around it by nearly 50 per cent because its particles are soot, which is black and thus absorbs sunlight.

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

The simulation estimated that, since 1950, South Asia’s atmosphere has warmed by 0.25C per decade at altitudes ranging from 2000m to 5000m above sea level — the height where thousands of Himalayan glaciers are located.

As much as half of this warming could be attributed to the effects of brown clouds, Professor Ramanathan said. "It is frightening, but I also look at the positive side, because it shows a way out of the conundrum," he said.

Roughly 60 per cent 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 said.

Professor Ramanathan’s data has been validated with measurements taken on the ground and in space by NASA

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