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Soot is No. 2 global-warming culprit, study finds (+video)

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The effects vary by region as well. Eastern and southern Asia, major sources for the soot, can experience a warming effect from soot 10 times higher than the global average, the researchers estimate. Soot falling on ice and snow in the Arctic has accelerated warming there by speeding the pace at which snow and ice melt during the long hours of summer sun.

Indeed, the mid to high latitudes of the northern hemisphere have seen some of the most pronounced effects of warming from soot, the study notes, because that's where most of the world's population lives.

Scientists also have linked high soot levels to shifts in the regional distribution and intensity of rainfall during Asian monsoons.

Initially, calls from some climate scientists during the past decade to focus first on reducing emissions of shorter-lived but powerful warming agents, such as methane or soot, have been met with polite nods and a resumption of heated debates over CO2 emissions. But the increasing recognition of the adverse health effects of soot, as well as the experience from efforts to control soot, are changing that, some researchers say.

Indeed, "there are clear options" to cut soot emissions, Ramanathan says.

In California, for instance, black-carbon soot emissions fell by half between 1990 to 2008 in response to tighter air-quality regulations affecting diesel emissions, according to a study Ramanathan and colleagues from Scripps and Argonne National Laboratory published in early 2011.

The decline occurred even as "diesel consumption has increased significantly," he adds. The soot pollution "has come down to almost nothing" statewide.

The study on global black-carbon soot released Tuesday notes that focusing initially on diesel sources "appears to offer the most confidence in reducing near-term" warming.

Another opportunity lies in supplying cook stoves that burn biofuels like wood or dung more efficiently, Ramanathan adds.

In the end, putting an immediate focus on short-lived warming agents such as black carbon also cuts into CO2 emissions, notes Brenda Ekwurzel, a climate scientist with the Union of Concerned Scientists in Washington.

"Emitting black carbon also emits carbon dioxide, which has longer-term consequences," she says. "So going after black carbon also helps with the carbon-dioxide emissions associated with incomplete combustion of an ancient tree or critter."

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