As it tours through the atmosphere after its emission, carbon dioxide has a
long lifecycle about 50 to 200 years. Soot, however, sticks around for
much less time weeks to months. Thats why a reduction in soot emissions
has the potential to change global temperatures more quickly than a reduction
in carbon dioxide emissions, according to Mark Jacobson, associate professor
of civil and environmental engineering at Stanford University.
In terms of its speed and relative magnitude of its effect on global climate, controlling soot could possibly be the most effective method of controlling global warming, Jacobson says. In his study published in the October Journal of Geophysical Research-Atmospheres, Jacobson uses numerical model simulations to test various effects of aerosols on climate.
Soot, composed of black and organic carbon, absorbs a large amount of sunlight and reradiates the absorbed heat into the atmosphere. The magnitude of its effect on temperature is second to that of carbon dioxide and supersedes that of methane, Jacobson says. Not only does soot have a stronger effect on climate per unit mass than carbon dioxide, but it also carries with it deleterious health effects that carbon dioxide does not making soot a particularly attractive candidate for emissions reduction, he says.
Diesel vehicles are perhaps the most well-known sources of the soot in the atmosphere. In recent years, tax laws in all European Union countries, except the United Kingdom, have favored diesel over gasoline in cars because gasoline vehicles emit more carbon dioxide than diesel vehicles. As a major culprit in greenhouse gas increases, carbon dioxide has appeared to pose the largest controllable threat.
However, diesel cars without filters can emit 25 to 400 times more mass of soot per kilometer than similar gasoline cars. That, Jacobson says, is cause for rethinking the way we look at environmental regulations.
Particulate traps, now becoming more common on diesel vehicles, are an effective way to reduce soot, Jacobson says, but even vehicles using such traps emit more soot than gasoline cars. Vehicles with traps also use 3.5 percent more fuel. Jacobson adds that the leading sources of soot in the atmosphere today in developed countries are off-road vehicles and agricultural and construction equipment, not diesel cars and trucks.
Jim Hansen, a climatologist at the NASA Goddard Institute for Space Sciences, agrees that a reduction in soot will benefit both human health and climate. However, we must be careful not to oversell this climate effect, he says. In practice, if we pay attention to reducing black carbon emissions, probably the most cooling effect that we can get would be enough to balance the warming that we will get by reducing sulfate aerosols, referring to worldwide efforts to reduce acid rain and air pollution that result from sulfur emissions. Sulfate, a white aerosol, cools the atmosphere by reflecting sunlight.
Hansen stresses that a reduction in carbon dioxide and methane still must play the major role in solving the global warming problem. Indeed, Jacobson says that the bottom line of his research, funded by NASA, the Environmental Protection Agency, the National Science Foundation, the David and Lucile Packard Foundation, and the Hewlett-Packard Company, is that all emissions need to be taken seriously. You can reduce soot by any amount and youll get some benefit that will be a faster benefit than controlling carbon dioxide. But it is necessary to control soot and carbon dioxide, as well as methane.
Lisa M. Pinsker
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