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Study raises hope of combating global warming by reducing soot

Black-carbon soot is the No. 2 global warming agent released into the atmosphere by human activities. A landmark study in California shows some success in controlling it.

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Between 1989 and 2008, clean-air rules in California virtually halved the concentrations of black-carbon soot in the state's skies, in effect reducing the state's carbon footprint by the equivalent of cutting carbon dioxide emissions by 21 million tons a year, according to new analysis.

That would represent about 5 percent of the state's CO2 emissions in 2009, according to the report. Though the data have yet to be fed into global climate models to see if California's results can be replicated elsewhere, they bolster the hope that focusing on black-carbon soot could be an effective way to begin to address global warming.

During the past decade, atmospheric scientists have focused increasing attention on black-carbon soot, the tiny particles found in Diesel exhaust as well as the emissions from wood and dung fires. The soot, which absorbs sunlight and re-radiates it as heat, has edged out methane as the second most-abundant greenhouse-agent released into the atmosphere by human activities. But unlike carbon dioxide or methane, soot takes only days or weeks to settle out of the atmosphere, compared with decades to centuries for methane and CO2.

This has led some scientists to conclude that by focusing near-term emissions reductions on global soot, in addition to other shorter-lived warming agents such as methane and ozone, humanity could slow the expected rate of global warming during the next several decades.

Last year, for instance, a study by NASA's Goddard Institute for Space Studies in New York suggested that such controls could reduce warming expected by 2050 by about 0.5 degrees Celsius, or about 1 degree Fahrenheit. That is a significant amount, given the international interest in ensuring that global temperatures don't increase by more than 2 degrees C by 2100, compared with preindustrial levels.

The study released Thursday marks the first time researchers have been able to measure the climate-related effects of long-term reductions in black-carbon soot over a region, the research team says. Until now, scientists have had to rely on limited field studies and computer simulations to estimate such effects.

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