Dirty snow blamed in global warming

NASA scientists say soot might cause up to 25%


WASHINGTON - Dirty snow containing tiny amounts of soot might cause up to one-fourth of the global warming that scientists have attributed to greenhouse gases, NASA researchers reported yesterday.

Even though snow might still appear pristine to the human eye, soot causes it to absorb more sunlight and reflect less heat back into space, said James Hansen and Larissa Nazarenko, climate specialists at NASA's Goddard Institute for Space Studies in New York.

It also causes snow to melt faster, contributing to the most immediate danger from global warming, rising sea levels, Hansen and Nazarenko said in an article in the Proceedings of the National Academy of Sciences.

International commissions, such as the United Nations Intergovernmental Panel on Climate Change (IPCC), have not considered the role of dirty snow, the scientists wrote.

The good news is that ways are within reach to reduce the amount of soot from diesel engines and burned wood that finds its way into Earth's large snowpacks.

That could substantially reduce global warming relatively quickly, but the scientists also cautioned that the main force driving global warming is the buildup of greenhouse gases in the atmosphere.

The "dirty snow effect," as Hansen calls it, could add a new wrinkle to the debate over global warming and its causes.

U.S. industries and makers of gas-burning cars, which emit relatively little smoke, would like to embrace a theory that lays part of the blame for global warming elsewhere.

On the other hand, Hansen and Nazarenko could touch off a vigorous debate among scientists who study global warming. The two acknowledge the uncertainty of their conclusions because of the lack of accurate soot content measurements.

"I think that this is an important climate forcing that has been overlooked," Hansen said.

At concentrations measured in parts per billion, soot makes fresh snow less reflective than pristine snow and more inclined to absorb heat, he and Nazarenko wrote. The effect is compounded when dirty snow starts to melt, because wet snow is less reflective than dry snow.

Solutions to the dirty snow effect could be straightforward and within reach, Hansen said. In North America and Europe, the primary need is cleaner diesel engines.

"In the developing world, it is more a matter of replacing biofuels with cleaner fuels or burning them more cleanly," he said. "One of the most effective ways ... is electrification, so that fuels are burned at a power plant where it is easier to eliminate emissions."

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