The soot factor. (Global Warming).

A new computer model indicates that soot--blackened, unburned carbon--is a major factor in global warming due to the greenhouse effect, a fact that traditional global warming models have failed to take into account. Computer calculations by Mark Jacobson, an associate professor of civil and environmental engineering at California's Stanford University, have ranked soot second only to carbon dioxide (C[O.sub.2]) in overall global warming impact. The study, published in the 8 February 2001 issue of Nature, focused on how soot combines physically with other suspended particles in the atmosphere.

According to rough data compiled by the Intergovernmental Panel on Climate Change (IPCC), which produces worldwide scientific consensus statements on global warming, humans put about 11 million tons of soot into the atmosphere each year. About half comes from incomplete combustion of fossil fuels, and half from biomass burning (wildfires, which the panel considers nonanthropogenic, are excluded from the data).

Jacobson says most earlier models considered soot separately from other aerosols such as sulfates (another product of combustion), soil, and sea salt, naturally put into the atmosphere by ocean waves. They also modeled soot in one condition or another; that is, they treated soot as not interacting with anything else, or as only one size distribution. In contrast, Jacobson studied 18 different size distributions that were interacting with other aerosols, such as sulfate.

This combining changes how soot affects solar radiation. In general, dark particles increase warming by absorbing solar radiation reradiating it toward the earth. Light-colored particles reflect more radiation back into space, producing net cooling. Scientists call this effect of greenhouse gases "radiative forcing" and measure it in watts per square meter (W/[m.sup.2]); gas that exerts a forcing of 1 W/[m.sup.2] will warm the earth as much as a 1 W heater placed over each square meter. Jacobson's is the first study to attempt a realistic prediction of how mixed soot would affect radiative forcing.

The study, Jacobson says, calculated the total radiative forcing of pure soot produced by combustion and of soot mixed with other aerosols as 0.55 W/[m.sup.2]. For comparison, the IPCC says the forcing of C[O.sub.2] is 1.56 W/[m.sup.2], and of methane, 0.47 W/[m.sup.2]. Jacobson calculates that, taking into account changes in soot over time each ton of soot causes twice as much warming as calculated in previous estimates.

That finding is "probably legitimate," says Francis Bretherton, an emeritus professor of atmospheric science at the University of Wisconsin at Madison and a chapter author of the 1990 report Climate Change: The IPCC Scientific Assessment. "It's very plausible," he says, "although not unequivocally confirmed by hard evidence." Bretherton estimates that soot may contribute about 20% of overall greenhouse warming. He adds that, although Jacobson's results are another step in understanding the greenhouse effect, they should not divert attention from the major cause of global warming: C[O.sub.2] and the other greenhouse gases.

While soot may play a bigger role than expected in warming, it is more tractable than the greenhouse gases. Although C[O.sub.2] lasts for decades in the atmosphere, rain removes most soot within a week or two. And while C[O.sub.2] is an inevitable product of combustion, soot results from incomplete combustion, and efficient engines produce far less of it. Finally, unlike C[O.sub.2] methane, soot exacerbates asthma and other diseases, so reductions could produce health benefits while reducing global warming.