Subcontinental Smut

Greenhouse gases alone cannot explain the warming climate in the Himalayas. New studies are pointing to soot

By Davide Castelvecchi. From ScientificAmerican.com, December 15, 2009.

SAN FRANCISCO—The Himalaya Mountain region is warming up three to five times faster than the global trends—or about half a degree Celsius per decade—and many of its glaciers are rapidly losing mass. Greenhouse gases alone cannot explain this warming, however, and several new studies are pointing to an old form of pollution: soot.

A thick cloud of soot covers most of India, produced in part by millions of small cooking stoves, which typically burn wood. Soot, also known as black carbon, is made of particles less than a micron wide resulting from incomplete, inefficient combustion. (A micron is one millionth of a meter.) Globally, soot from sources such as forest fires and power stations is considered a major contributor to climate. The particles linger in the air, where they absorb sunlight and contribute to warming the atmosphere; they may also affect cloud formation and precipitation. But soot also eventually falls to the ground. When it lands on snow it can significantly darken it, so that glaciers absorb more sunlight and are warmed.

Using data from an international atmospheric observatory in Nepal,Teppei Yasunari of the NASA Goddard Space Flight Center in Greenbelt, Md., and his colleagues estimated the amount of soot that falls on a typical Himalayan glacier. The team’s computer simulations suggested that the soot can cause a decrease of between 1.6 and 4.1 percent in the glacier’s albedo—a measure of its sunlight-reflecting “whiteness”—and that the resulting heating can cause up to a 24 percent increase in the annual snowmelt, Yasunari reported here Monday at a meeting of the American Geophysical Union (AGU). The team made conservative assumptions when estimating the albedo reduction, Yasunari said, neglecting other potentially important factors. “Dust deposition, snow algae, wind and turbulence could bring further reductions,” he said.

Also at the AGU meeting, Yasunari’s co-author and Goddard colleague William Lau presented the results of a separate study today suggesting that soot heating the atmosphere over India could accelerate the glacier-melting effects of the warm currents that rise up to the Himalayan chain, in a “heat pump” effect.

Soot has already been implicated in the melting of the polar ice caps, and heating of the atmosphere over India was directly measured in 2007 by Veerabhadran Ramanathan of the Scripps Institution of Oceanography at the University of California, San Diego, and his colleagues. In that study the researchers also used a climate simulation to show that the soot could help melt the mountain range’s glaciers. Ramanathan, who also spoke at the meeting Monday, says that the study by Yasunari and colleagues included a more detailed, local model of an actual glacier. “It’s all looking at different pieces of the puzzle,” he sai