The Soot Factor. (Global Warming)

Article excerpt

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. …