Washington: Nobody disputes that glaciers in the Himalayas are thinning. Now a physicist at the Lawrence Berkeley National Lab, Surabhi Menon has set out to isolate the impacts of the greenhouse gases such as carbon dioxide from other particles in the air that may be causing the melting.
The Himalayan glaciers are often referred to as the third polar ice cap because of the large amount of ice mass they hold. The glacial melt feeds rivers in China and throughout the Indian subcontinent and provide fresh water to more than a billion.
Menon, who did her BSc and MSc from Bombay University, India, in 1991 and 1992, and her collaborators found that airborne black carbon aerosols, or soot, from India is a major contributor to the decline in snow and ice cover on the glaciers.
Menon`s data shows black carbon emitted in India increased by 46 percent from 1990 to 2000 and by another 51 percent from 2000 to 2010. "Our simulations showed greenhouse gases alone are not nearly enough to be responsible for the snow melt," says Menon.
"Most of the change in snow and ice cover - about 90 percent - is from aerosols. Black carbon alone contributes at least 30 percent of this sum," adds Menon.
Menon and her collaborators used two sets of aerosol inventories by Indian researchers to run their simulations.
The actual contribution of black carbon, emitted largely as a result of burning fossil fuels and biomass, may be even higher than 30 percent because the inventories report less black carbon than what has been measured by observations at several stations in India.
However, these observations are too incomplete to be used in climate models. "We may be underestimating the amount of black carbon by as much as a factor of four," she says.
The findings are significant because they point to a simple way to make a swift impact on the snow melt.
"Carbon dioxide stays in the atmosphere for 100 years, but black carbon doesn`t stay in the atmosphere for more than a few weeks, so the effects of controlling black carbon are much faster," Menon says. "If you control black carbon now, you`re going to see an immediate effect."
Atmospheric aerosols are tiny particles containing nitrates, sulfates, carbon and other matter, and can influence the climate.
Unlike other aerosols, black carbon absorbs sunlight, similar to greenhouse gases. But unlike greenhouse gases, black carbon does not heat up the surface; it warms only the atmosphere.
Previous studies have shown that black carbon can have a powerful effect on local atmospheric temperature, said an LBNL release.
"Black carbon can be very strong," Menon says. "A small amount of black carbon tends to be more potent than the same mass of sulphate or other aerosols."
Black carbon, which is caused by incomplete combustion, is especially prevalent in India and China; satellite images clearly show that its levels there have climbed dramatically in the last few decades.
These results were published online in Atmospheric Chemistry and Physics.