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STANFORD UNIVERSITY — The quickest, best way to slow the rapid melting of Arctic sea ice is to reduce soot emissions from the burning of fossil fuel, wood and dung, according to a new study by Stanford researcher Mark Z. Jacobson.

His analysis shows that soot is second only to carbon dioxide in contributing to global warming. But, he said, climate models to date have mischaracterized the effects of soot in the atmosphere, meaning that soot’s contribution to global warming has been ignored in national and international global warming policy legislation.

“Controlling soot may be the only method of significantly slowing Arctic warming within the next two decades,” said Jacobson, a professor of civil and environmental engineering and director of Stanford’s Atmosphere/Energy Program. “We have to start taking its effects into account in planning our mitigation efforts, and the sooner we start making changes, the better.”

Jacobson examined the effects of soot — black and brown particles that absorb solar radiation — from two types of sources: Fossil fuels — diesel, coal, gasoline, jet fuel — and from solid biofuels, such as wood, manure, dung and other biomass. He also focused in detail on the effects of soot on heating clouds, snow and ice.

What he found was that the combination of both types of soot is the second-leading cause of global warming after carbon dioxide and ahead of methane, an important greenhouse gas. He also found that soot emissions kill more than 1.5 million people prematurely worldwide each year and afflict millions more with respiratory illness, cardiovascular disease and asthma, mostly in the developing world, where biofuels are used for home heating and cooking.

Jacobson’s study will be published this week in Journal of Geophysical Research–Atmospheres.

It is the magnitude of soot’s contribution, combined with the fact that it lingers in the atmosphere for only a few weeks before being washed out,  that leads to the conclusion that a reduction in soot output would start slowing the pace of global warming almost immediately. Greenhouse gases, in contrast, typically persist in the atmosphere for decades — some up to a century or more — creating a considerable time lag between when emissions are cut and when the results become apparent.

Jacobson found that eliminating soot produced by the burning of fossil fuels and solid biofuels could reduce warming above parts of the Arctic Circle in the next 15 years by up to 1.7 degrees Celsius. For perspective, net warming in the Arctic has been at least 2.5 degrees Celsius during the last century and is expected to warm significantly more in the future if nothing is done.

The most immediate, effective and low-cost way to reduce soot emissions is to put particle traps to filter out soot particles from exhaust fumes on vehicles, diesel trucks, buses and construction equipment. Soot could be further reduced by converting vehicles to run on clean, renewable electric power.

Jacobson found that although fossil fuel soot contributed more to global warming, biofuel-derived soot caused about eight times the number of deaths as fossil fuel soot. Providing electricity to rural developing areas, thereby reducing usage of solid biofuels for home heating and cooking, would have major health benefits, he said.

Soot from fossil fuels contains more black carbon than soot produced by burning biofuels, which is why there is a difference in impact.

Black carbon is highly efficient at absorbing solar radiation in the atmosphere, just like a black shirt on a sunny day. Black carbon converts sunlight to heat and radiates it back to the air around it. This is different from greenhouse gases, which primarily trap heat that rises from the Earth’s surface. Black carbon also can absorb light reflecting from Earth’s surface, which helps make it such a potent warming agent.

“There is a big concern that if the Arctic melts, it will be a tipping point for the Earth’s climate because the reflective sea ice will be replaced by a much darker heat-absorbing ocean below,” said Jacobson. “Once the sea ice is gone, it is really hard to regenerate because there is not an efficient mechanism to cool the ocean down in the short term.”

Jacobson is a professor of civil and environmental engineering and a senior fellow at Stanford’s Woods Institute for the Environment. His work was supported by grants from the U.S. Environmental Protection Agency, NASA, the NASA High-End Computing Program and the National Science Foundation.

BY JESS MCNALLYS

Science-writing intern at the Stanford News Service

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