STANFORD (US)—The quickest and 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.

Eliminating soot could reduce warming above parts of the Arctic Circle in the next 15 years by up to 1.7 degrees Celsius.

Soot is second only to carbon dioxide in contributing to global warming, but climate models to date have mischaracterized its effects in the atmosphere,  says Mark Jacobson, professor of civil and environmental engineering at Stanford University.

Because of that, soot’s contribution to global warming has been largely ignored in national and international global warming policy legislation, he says.

“Controlling soot may be the only method of significantly slowing Arctic warming within the next two decades,” Jacobson says. “We have to start taking its effects into account in planning our mitigation efforts, and the sooner we start making changes, the better.”

To reach his conclusions, Jacobson used a computer model of global climate, air pollution, and weather that he developed over the last 20 years that includes atmospheric processes not incorporated in previous models.

He examined the effects of soot—black and brown particles that absorb solar radiation—from two types of sources.

He analyzed the impacts of soot from fossil fuels—diesel, coal, gasoline, jet fuel—and from solid biofuels, such as wood, manure, dung, and other biomass used for home heating and cooking in many locations. 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, ranking the effects of soot ahead of methane, an important greenhouse gas.

Soot emissions were also found to 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.

Details appear in the Journal of Geophysical Research–Atmospheres.

Immediate impact

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, leads to the conclusion that a reduction in soot output would start slowing the pace of global warming almost immediately, Jacobson says.

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.

The most immediate, effective and low-cost way to reduce emissions is to put particle traps on vehicles, diesel trucks, buses, and construction equipment to filter out soot particles from exhaust fumes.

Soot could be further reduced by converting vehicles to run on clean, renewable electric power.

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 says.

Using mathematical equations to describe the physical and chemical interactions of soot particles in cloud droplets in the atmosphere allowed Jacobson to include details such as light bouncing around inside clouds and within cloud drops, which are critical for understanding the full effect of black carbon on heating the atmosphere.

“The key to modeling the climate effects of soot is to account for all of its effects on clouds, sea ice, snow, and atmospheric heating,” Jacobson says. “Without treating these processes, no model can give the correct answer with respect to soot’s effects,” he explains.

Leaving out this scale of detail in other models has led many scientists and policymakers to undervalue the role of black carbon as a warming agent, Jacobson argues.

The strong global heating due to soot is supported by recent findings of Veerabhadran Ramanathan, a professor of climate and atmospheric sciences at the Scripps Institution of Oceanography, who measures and models the climate effects of soot.

“Jacobson’s study is the first time that a model has looked at the various ways black carbon can impact climate in a quantitative way,” says Ramanathan, who was not involved in the study.

Black carbon has an especially potent warming effect over the Arctic. When black carbon is present in the air over snow or ice, sunlight can hit the black carbon on its way toward Earth and also hit it as light reflects off the ice and heads back toward space.

“It’s a double-whammy over the ice surface in terms of heating the air,” Jacobson says. Black carbon also lands on the snow, darkening the surface and enhancing melting.

“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,” says 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.”

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