Black carbon, the color agent in soot, could be the second most important contributor to climate change after carbon dioxide and a key to preventing warming, at least in the short-run, a new study suggests.
Black carbon is a type of aerosol ¡ª a small particle suspended in the atmosphere ¡ª that is produced in diesel exhaust and when wood, coal or other types of solid fuel are burned.
Like other aerosols, soot particles absorb and scatter the sun's radiation; black carbon is the absorbing component of soot. V. Ramanathan of the Scripps Institution of Oceanography in San Diego, California, and his colleagues reviewed recent studies of black carbon's warming contribution in the March 24 issue of the journal Nature Geoscience.
They also used data from satellites, aircraft and surface instruments to arrive at their own estimate of just how much warming black carbon causes. Their estimates are well above those in the most recent report from the Intergovernmental Panel on Climate Change and suggest that black carbon has a bigger warming effect than previously thought.
Black carbon can mix with other aerosols to form what are called atmospheric brown clouds, which have been observed in regional hotspots over China, India, Southeast Asia, Africa and parts of Central and South America. These brown clouds absorb incoming solar radiation and prevent it from reaching the surface, warming the atmosphere in the process.
Ramanathan's study found that black carbon had a warming effect of about 0.9 watts per meter squared (the average light bulb in a lamp is about 60 watts, so while this effect is a fraction of the heat from a light bulb, that little heater is widespread ¨C there is effectively one on every square meter, about the area of the top of an office desk, across Earth's surface). The most recent IPCC assessment made a lower estimate of the warming effect, between 0.2 and 0.4 watts per meter squared.
Ramanathan and co-author Greg Carmichael of the University of Iowa said these estimates are conservative because they don't take into account the amplification of black carbon's warming effect that occurs when it mixes with other aerosols and because they don't account for warming differences at different altitudes.
But atmospheric scientist Dorothy Koch of NASA's Goddard Institute for Space Studies, who was not affiliated with the study, cautioned that the case wasn't that simple: "There's a lot of uncertainty with black carbon and that makes people kind of reluctant to put it on the table with the greenhouse gases," Koch told LiveScience.
Black carbon doesn't stay in the atmosphere for long, which means it might have a high effect in the short term, but greenhouse gases that stay in the atmosphere longer have a bigger long-term impact. There is also uncertainty as to just how much of the black carbon in the atmosphere comes from human activities, Koch said.
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