Why Are The Himalayas Melting? Blame Soot.

by Bradford Plumer | December 16, 2009

One of the biggest unpleasant climate shocks in recent years has been the discovery that the Himalayas are heating up much faster than the global average—a trend that most climate models had failed to predict, and something that's difficult to explain by pointing to greenhouse gases alone. And it's a critical issue, seeing as how Himalaya's glaciers provide water for more than 500 million people in countries like India, Pakistan, and Bangladesh, and they're expected to shrivel away by 2035 or so. But now recent studies have fingered a new co-culprit—black carbon:

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.

To be clear on this, greenhouse gases like CO2 are still largely responsible for the overall warming of the Earth. But, in certain regions, soot is making the problem much, much worse. Black carbon also appears to be helping sizzle the polar ice caps through the same mechanism—the dark particles swirl through the air thousands of miles northward, settle on the ice, and absorb more of the sun's rays. (One study has blamed black carbon for as much as 50 percent of the Arctic warming.)

Now, the good news is that unlike carbon-dioxide, black carbon doesn't linger in the atmosphere for very long, which means that if you clean it up, the climate problems it's causing can be ameliorated in fairly short order. And soot's fairly easy to tackle: The solutions involve particulate filters for diesel vehicles or cleaner cookstoves that either run on solar power or burn fuel cleanly. Plus, black-carbon pollution has been linked to all sorts of respiratory illnesses in the developing world (indoor cooking causes about two million deaths in India each year), so mopping it up is really a no-brainer. Even the Senate's foremost climate denier, James Inhofe, has co-sponsored a bill to aid soot-reduction projects abroad, and it looks like Copenhagen negotiators are trying to address soot in any new treaty, a move they hope can buy the world a bit of time to curb greenhouse gases.

(Flickr photo credit: Manny Pabla)

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