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Diesel dilemma. Diesel engines are fuel-efficient, but the soot they produce may offset that advantage in terms of global warming.

A Quicker Climate Fix?

Reducing soot emissions, especially from diesel engines and power plants, may slow global warming more quickly than cutting carbon dioxide production, a new simulation suggests.

Carbon dioxide is the major cause of global warming, and it has been the focus of the 1997 Kyoto Protocol and other attempts to corral climate change. Scientists have paid less attention to soot--particles made of so-called black carbon and organic carbon--because earlier research indicated that their effects were relatively minor. But a 2001 study led by atmospheric scientist Mark Jacobson of Stanford University in California suggested that if soot is coated with other pollutants, such as sulfuric acid, its effect could be drastically greater. Such "aged" particles absorb more solar radiation, and they could cause more warming than previously thought.

Short of waiting, the only way to find out was to run a computer simulation. So Jacobson built the first computer model that incorporates many of soot's effects on the atmosphere. The model provides a conservative estimate of soot's impact, he says, because the simulated concentrations of soot were generally lower than real-world values over the last 40 years. Even so, warming decreased more rapidly when soot emissions were reduced than when carbon dioxide emissions were cut by the same proportion, Jacobson reports in the October issue of the Journal of Geophysical Research--Atmospheres. He says that's because soot is washed out of the atmosphere within a few months, whereas carbon dioxide persists up to 200 years.

The finding suggests that focusing exclusively on carbon dioxide is misguided, and that cutting soot could better prevent the short-term consequences of global warming, Jacobson says. For example, European Union standards favoring fuel-efficient diesel cars over gasoline cars may lead to more warming, because diesel engines release 40 to 250 times as much soot as gasoline engines. And the biggest source of soot in Europe and the United States--construction and farm equipment--is barely regulated, he says.

The study makes a good case for paying more attention to soot, says atmospheric scientist Donald Wuebbles of the University of Illinois, Urbana-Champagne. However, the model depends heavily on assumptions about the way soot particles form, he says. "At this point there are still large uncertainties."

Related sites
Jacobson's site
Wuebbles's site
Journal of Geophysical Research home page (subscription required for content)