Image: Courtesy of DAVID PARSONS / DOE / NREL

Though it pours ominously out of chimneys, forest fires and the exhaust pipes of diesel-run vehicles (right), soot has received little attention from scientists studying global warming. Results published today in the journal Nature, however, suggest that soot, 90 percent of which comes from burning fossil fuels and biomass, may be a leading cause of rising world temperatures. "Soot¿or black carbon¿may be responsible for 15 to 30 percent of global warming," says Stanford University researcher Mark Z. Jacobson, the author of the report. "Yet it's not even considered in any of the discussions about controlling climate change."

The conventional model of global heat balance holds that greenhouse gases warm the earth by trapping infrared radiation, while aerosol particles in the atmosphere reflect sunlight back into space, reducing the amount of heat the planet absorbs. The aerosols, in this view, cool the earth in the same way that light-colored clothing keeps you cooler on a hot day than dark-colored clothing. But according to the new findings, soot in these atmospheric aerosols may cancel out the sulfate that makes them such effective cooling agents by darkening the aerosols so that they soak up more radiation.

Jacobson notes that of the few previous studies that considered the impact of soot on global warming, most assumed that soot doesn't mix with other particles in the atmosphere. His own research, based on computer simulations, suggests quite the opposite, indicating that within five days of entering the atmosphere, particles of pure soot will probably end up in mixtures. Simulating how millions of tons of mixed soot would affect climate yielded dramatic results. "These black carbon mixtures turn out to be one of the most important components of global warming," Jacobson observes, "perhaps second only to carbon dioxide." Thus, reducing soot emissions could be one effective way to counter global warming, he says.

More work will be needed to substantiate Jacobson's claims, but considering that soot has been linked to cancer and respiratory illnesses, citizens of the world would probably do well to cut back on soot emissions anyway. "The largest source of mortality from air pollution is indoor burning of biomass and coal," he remarks. "Reduction of such burning, therefore, will not only mitigate global warming but also will save lives and improve people's health.