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Soot Identified as Major Contributor to Global Warming

Soot, a prime culprit in smog and the bane of inner-city dwellers, has been fingered as perhaps the second-largest contributor to greenhouse gases in the atmosphere.

"Soot—or black carbon—may be responsible for 15 to 30 percent of global warming," says Mark Jacobson, an assistant professor of civil and environmental engineering at Stanford University.

The increased amount of greenhouse gases pumped into the Earth's atmosphere as the result of activities by humans is widely held to be accelerating the pace of climate change here on Earth.

Coal-coking furnaces contribute soot to the Earth's atmosphere.
Photograph by Stephen L. Ramer/NGS


The Earth's temperature could rise by 11°F (5.8°C) in the next 100 years according to a recent report issued by the United Nations Intergovernmental Panel on Climate Control (IPCC). Such an increase would likely lead to glaciers melting, a rise in sea levels flooding low-lying shorelines, persistent drought, changes in land cover, and extensive loss of biodiversity among the Earth's flora and fauna.

Soot Gets a Black Eye

Soot is made up of fine black carbon particles that are formed by incomplete combustion. It is almost exclusively produced as a byproduct of coal-burning power plants; diesel-burning cars, trucks, buses and tractors; jet fuel and forest fires; wood burning stoves and fireplaces; dung-fueled fires for heat and cooking; kerosene, and even candles are sources of soot.

Most studies and public policy have been based on the assumption that soot particles do not combine with other particle types.

"The lifetime of soot is anywhere from a week to a few weeks," says Jacobson. "And the thinking was that they didn't have time to combine with other particle types in that relatively short lifetime."

Instead scientists believed that the particles, once emitted into the atmosphere, float and swirl in the air until they fell to Earth from their own weight, or were washed out with rain.

Jacobson, who specializes in computer modeling of atmospheric pollution, developed a program to determine whether and how much soot combines with other atmospheric particles.

The model showed that within five days of entering the atmosphere, particles of pure soot could be found in mixtures containing dust, sea spray, sulfate, and other chemicals.

His simulations also showed that the effect of the black carbon mixtures on the Earth's climate is second only to carbon dioxide.

The study was published in the February 8 issue of the journal Nature.

Implications for Conservation

Identifying the types of pollution that contribute to climate change is the essential first step in adopting rational conservation initiatives, says Jacobson.

Because of the assumption that soot was not combining with other particles and chemicals once it reached the atmosphere, soot has not been addressed in international discussions about controlling climate change. The IPCC report, for instance, focuses primarily on the greenhouse gases carbon dioxide and methane. The treaty negotiated by the United Nations in Kyoto in 1997 to deal with global warming also points only to efforts to reduce greenhouse gases.

Jacobson argues that policy makers need to take a long hard look at soot reduction.

How do we reduce soot emissions? "That's an easy question," says Jacobson. "Get rid of a lot of old cars … in countries other than the United States and some in Europe the technology to control emissions is not being used."

"Tighten standards and the technology will follow. The catalytic converter [that controls car emissions] wasn't developed until after the Clean Air Act was passed in the 1970s. The technology exists to retrofit older coal-fired power plants."

Most important, says Jacobson, is phasing out dirty fuels like coal and diesel, and replacing them with alternative energies.

"Wind energy is selling for 3.5 cents per kilowatt hour, which is certainly affordable. But right now Germany has more wind power than the United States. The U.S. can do a lot more."

Many European countries actually subsidize the use of dirty fuels, says Jacobson.

"We encouraged the switch to diesel engines mistakenly believing that it was 'cleaner' because you get better gas mileage with diesel. In Europe many countries give tax credits to people who purchase diesel vehicles. But today's cars emit virtually no soot—although both produce large amounts of carbon dioxide."

"I don't want to sound like I don't think reducing greenhouses gases isn't important," he says. "It is. But soot may be comparable to methane in contributing to global warming, and it has a much larger impact on human health."

"The World Health Organization says something like 2.7 million people die each year from air pollution. Controlling soot should take priority over some of the lesser greenhouse gases like methane," concludes Jacobson. "It's just a bigger bang for the buck."

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