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

A new study suggests that soot plays a major role in climate change – second only to carbon-dioxide emissions. Targeting soot could lead to quicker results in battling global warming.

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From diesel engines to cow-dung cook fires, soot from inefficiently burned fuel has supplanted methane as the second most significant global-warming agent that humans are pumping into the air, according to an exhaustive review of more than a decade's worth of research on black-carbon soot emissions.

Carbon dioxide from burning fossil fuel and from land-use changes remains in the No. 1 spot. But the direct effect of soot on air temperatures, as well as its indirect effect on ice and snow melt and on cloud formation and persistence, are knocking at the door.

Given the uncertainties in the estimates, black-carbon soot may even outpace CO2's warming effect, according to the 232-page study published today in the Journal of Geophysical Research–Atmospheres.

Soot remains in the atmosphere for around seven days – a far shorter time than CO2, which remains in the atmosphere for centuries. This means efforts to reduce soot may apply an important brake to warming in the short term with quick results, the researchers suggest.

Over the long term, however, countries still will have to solve the vexing political and economic challenges of tackling CO2 emissions.

"There's a lot of promise in reducing black carbon" and other relatively short-lived warming agents, such as methane, says Tami Bond, an associate professor of civil and environmental engineering at the University of Illinois at Urbana-Champaign and one of the study's three lead authors. "But there's also a lot of caution."

Properly done, moving to reduce black-carbon soot has immediate climate and public-health benefits, she says. But the uncertainties surrounding some of its climate effects remain large.

Rather than serving as an excuse for inaction, however, the uncertainties should to serve as a guide for research, she adds.

For example, the processes that produce soot also produce not only CO2 emissions but also other particles that can cool the atmosphere, she notes. In other words, emissions from any one source may contain competing influences on global warming.

The fingerprints "of human actions on climate are more complex than just the CO2 story," she says.

Concern over the climate effects of black-carbon soot date back to at least 1971, when interest began to grow in the role small particles, or aerosols, could play in Earth's climate system.

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