The rapid melting of glaciers in the Himalayas threatens the water supply of more than a billion people.
Himalayan glaciers, which supply drinking water to more than a billion people, may be threatened not only by warming from greenhouse gases, but also by soot generated far below on the plains of India, Pakistan, Nepal and Bangladesh.
Researchers believe that high levels of soot over the vast plains to the south and west of the Himalayas may be speeding the melting of glaciers in two ways.
The first involves changes in weather. According to the researchers' models, beginning each year in April, high soot levels in the atmosphere absorb sunlight, warming the air.
As the monsoon season starts in May, the warmed air draws in extra moisture and pulls it over the Tibetan Plateau. This process drops increasing amounts of rain on the glaciers, according to the models, speeding their melt.
As the glaciers thaw, the dark ground beneath the snow absorbs more heat, which also encourages the melting effect.
William Lau of NASA Goddard Space Flight Center in Greenbelt, Md., presented simulations predicting this process at a meeting of the American Geophysical Union in San Francisco yesterday.
Soot may also be accelerating the melt by falling directly on glaciers. The black soot absorbs sunlight, pulling heat onto the ice surface.
Susan Kaspari of Central Washington University in Ellensburg and colleagues have documented a threefold increase since 1860 in the amount of soot deposited on Mount Everest, a finding that was also presented at the meeting.
In contrast, Kaspari found that levels of dust have not increased over time. "The threefold increase shows that black carbon is getting transported to high elevations, and it is related to humans," Kaspari told Discovery News.
The fact that the soot made it onto the high peaks supports Lau's hypothesis that changing weather patterns are drawing soot-laden air into the mountains.
"In addition to greenhouse warming, we have a very strong local feedback process in this region that may have a strong impact on the glacier melt," Lau said.
While it is certain that the Himalayan glaciers are receding -- by more than 20 percent in area since the 1960s -- the researchers do not yet have direct evidence that the soot is accelerating the loss of glacial ice.
"All we can really say right now is, yes, the increase (in soot deposition) is happening, but whether or not this black carbon is sufficient enough to affect the glaciers can't be said," Kaspari said. That potential connection will be left for future research.
Jeffrey Kargel of the University of Arizona in Tucson pointed out that while the role of black carbon in the Himalayas raises lots of questions, the importance of heat-trapping carbon dioxide and other greenhouse gases in shrinking the glaciers should not be overlooked.
"I do want to make sure we keep an eye on the 800-pound gorilla in the room and around the world, and that is greenhouse gases," Kargel said.
"We can't fail our responsibilities, in my view, to control emissions of the gases as well as the particulates," he added.