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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.

By / January 15, 2013

Cyclists travel on the road on a hazy day in Huaibei, in central China's Anhui province, Monday, Jan. 14. Air pollution is a major problem in China due to the country's rapid pace of industrialization, reliance on coal power, explosive growth in car ownership and disregard for environmental laws.

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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.

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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.

During the past decade or so, however, field studies of soot's effect as a climate warmer typically yielded estimates two to three times higher than the effect seen in climate models, notes Veerabhadran Ramanathan, a professor of atmospheric and climate science at the Scripps Institution of Oceanography in La Jolla, Calif. 

With this new study, "we're coming closer to what we think black carbon is doing to the planet's climate," says Dr. Ramanathan, who was not a member of the team that pulled together the new analysis but has been studying the impact of soot on the climate for much of his career.

The soot comes from a mix of sources that varies by region. The study notes that roughly 90 percent of global soot emissions fall into several broad categories: diesel-fueled vehicles; use of coal to heat or cook in homes; small kilns and industrial boilers; burning wood or other biomass for cooking; and open burning of biomass, such as using fires to clear forests for farming.

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