According to a new study in the journal Nature, soot may be the second biggest contributor to global warming -- just behind the infamous greenhouse gas, carbon dioxide (CO2).

"Soot -- or black carbon -- may be responsible for 15 to 30 percent of global warming, yet it's not even considered in any of the discussions about controlling climate change," says Stanford Professor Mark Z. Jacobson, author of the Feb. 8 Nature study.

Human beings produce most of the soot particles that pollute the atmosphere, observes Jacobson, an assistant professor of civil and environmental engineering.

"Soot consists primarily of elemental carbon," he says, "and 90 percent of it comes from the consumption of fossil fuels -- particularly diesel fuel, coal, jet fuel, natural gas and kerosene -- as well as the burning of wood and other biomass when land is cleared."

A reduction in worldwide soot emissions, he maintains, could prove beneficial in slowing down the disastrous pace of global warming.

Dire warning
Jacobson's findings come on the heels of a Jan. 21 report by the United Nations Intergovernmental Panel on Climate Control (IPCC), an organization made up of hundreds of scientists from around the world.

In its most dire forecast to date, the IPCC predicted that, by the end of the century, the average surface temperature of the Earth could increase by 10.4 degrees Fahrenheit, with catastrophic results: melted glaciers, flooded shorelines and long periods of drought that persist for hundreds of years.

The IPCC report pins most of the blame for global warming on human-produced greenhouse gases such as carbon dioxide and methane, which also are byproducts of fossil fuel burning.

But according to IPCC scientists, atmospheric soot has relatively little effect on world climate.

Jacobson disagrees.

"Only a handful of studies have considered the impact of soot on global warming," he says, "and most of those were based on the premise that soot never mixes with other particles in the atmosphere."

But scientists have known for many years that floating soot particles actually do combine with dust and chemicals in the air, notes Jacobson.

This is a crucial point, he says, because mixtures containing black carbon absorb more sunlight and radiate twice as much heat as do particles of pure black carbon. Therefore, soot in its mixed state has the potential to make a significant contribution to global warming.

But what is the actual mixing state of soot? Do particles of black carbon normally float around by themselves, radiating relatively little heat into the atmosphere? Or do they routinely mix with other particles, causing atmospheric temperatures to rise?

Computer model
To find out, Jacobson used a sophisticated computer model known as GATOR-GCMM, which he designed for the purpose of analyzing urban and global pollution.

"For the Nature study, I used the model to simulate the emissions, movement, transformations and removal of soot and other important airborne particles," he says.

The results of the simulation show that, just five days after entering the atmosphere, particles of pure soot are very likely to end up in mixtures containing dust, sea spray, sulfate and other chemicals.

These findings are consistent with several atmospheric field studies, including a 1999 survey that found that more than 93 percent of all soot above the North Atlantic Ocean contained particles of sulfate.

Jacobson then programmed his computer to simulate how millions of tons of mixed soot would affect the Earth's climate. The results were dramatic.

"These black carbon mixtures turn out to be one of the most important components of global warming," says Jacobson, "perhaps second only to CO2."

Equally surprising was the discovery that soot may be responsible for more atmospheric heating than methane -- another significant greenhouse gas.

Global commitment
In his Nature study, Jacobson urges policy makers finally to consider looking at ways to reduce soot pollution worldwide.

U.N. negotiators are currently struggling to ratify the 1997 Kyoto treaty on climate change, which -- if approved -- would require many countries to decrease their annual emissions of carbon dioxide.

Similar cutbacks in soot emissions could prove to be a very effective way to counter global warming, argues Jacobson. He points out that technologies exist or can be developed to remove excess soot produced in fireplaces, truck engines and other sources of black carbon.

"We can also make efforts to control biomass burning and reduce our reliance on soot-producing fuels, such as coal and diesel," he notes.

According to Jacobson, well-meaning policies have been put into place based on the misguided assumption that diesel fuel is better for the environment than gasoline -- simply because diesel cars get better mileage than those that run on gas.

For example, many European countries actually give tax credits to drivers who purchase diesel vehicles. The irony is that, unlike diesel, modern gasoline engines emit virtually no soot -- although both produce large amounts of carbon dioxide.

Currently, about one-fourth of all European cars run on diesel, as do most European and American trucks, buses and tractors.

"Besides its impact on global warming, soot is bad for your health," adds Jacobson, noting that soot exposure has been linked to respiratory illnesses and cancer.

"The World Health Organization reports that about 2.7 million people die each year from air pollution -- 900,000 in cities and 1.8 million in rural areas," he observes.

"The largest source of mortality from air pollution is indoor burning of biomass and coal," he notes. "Reduction of such burning, therefore, will not only mitigate global warming but also will save lives and improve people's health."

Jacobson is now working on more extensive computer simulations that he hopes will provide new data about the dramatic impact of soot on our climate. Those results are expected to be published later this year.

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