NASA scientists say soot, mostly from diesel engines, is causing as much as a quarter of all observed global warming by reducing the ability of snow and ice to reflect sunlight.
Their findings on how soot affects reflective ability, known as albedo, raise new questions about human-caused climate change from the Arctic to the Alps.
"We suggest that soot contributes to near worldwide melting of ice that is usually attributed solely to global warming," National Aeronautics and Space Administration scientists James Hansen and Larissa Nazarenko wrote in a paper published Monday in the Proceedings of the National Academy of Sciences.
"Soot is a more all-around 'bad actor' than has been appreciated," they wrote. Soot is a blackened material formed mainly from carbon particles that are, along with salts and dust, byproducts of burning fossil fuels and vegetation.
Hansen, director of NASA's Goddard Institute for Space Studies, and Nazarenko, a staff associate there, found soot is twice as potent as carbon dioxide in changing global surface air temperatures in the Arctic and the Northern Hemisphere.
Greenland may be an exception, they said, because it is downwind from Canadian forests and has little manmade pollution.
The biggest source of soot in developed countries is diesel fuel, but major sources elsewhere include burning wood, animal dung, vegetable oil and other biofuels.
Hansen told The Associated Press that the authors estimate the soot effect is equivalent to putting a 1-watt bulb, the size of a miniature Christmas tree bulb, over every two square yards in the Northern Hemisphere. The effect is greater in northernmost snow regions, and almost nonexistent in the tropics.
Levels of airborne soot as high as about 100 parts per billion were found in the Alps, enough to reduce the snow's ability to reflect light rather than absorb it from about 98 percent down to between 80 percent and 90 percent, Hansen said. In spring and summer, as the snow melts and some soot accumulates as crud on the surface, the remaining snow is even darker, he said.
The scientists suggest in their paper that the same pattern could occur in the Himalaya range of South Asia, where prevailing winds might deposit fossil fuel and biofuel soot carried in a brown haze from India.
Many scientists believe the burning of fossil fuels is causing an increase in atmospheric carbon dioxide, triggering what is called the greenhouse effect. A higher concentration of CO2 in the atmosphere would trap more of the sun's heat, possibly causing the Earth to warm.
Scientists thought until recently that only carbon dioxide and other greenhouse gases have global reach and effect. They now are finding the same thing with these microscopic, suspended particles of pollutants, generically known as aerosols, that settle on ground hours later.
Soot particles, which absorb toxic organic material, are minute enough to penetrate skin. Soot is the aerosol most responsible for the haze in rapidly developing countries such as India and China, the scientists said.
Michael Oppenheimer, a Princeton professor and expert on climate policy, called the study "an interesting early calculation" that could prove to be important.
"It means that - if it's right - we need to keep an eye on it," he said. "When we think about all these greenhouse gases, we ought also to think about controlling these particles that are also changing the climate."
The Bush administration in 2001 ordered pollution cuts from heavy-duty diesel engines and diesel fuel used in highway trucks and buses. This year it proposed requiring a 90 percent reduction in pollution from diesel-powered construction and other off-road equipment, starting with 2008 models.
By John Heilprin