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Stanford Report, December 12, 2001

Diesel soot added to list of global warming culprits


It's dirty, smelly, causes respiratory and cardiovascular disease and shortens life, true, but now there's another reason to hate diesel exhaust: Its soot exacerbates global warming. Reducing soot emissions will slow global warming faster than will reducing carbon dioxide, methane or other greenhouse gases, says a Stanford pollution expert.

"If you want to control global warming, the first thing to go after is soot," says Mark Z. Jacobson, associate professor of civil and environmental engineering. "But you should not neglect carbon dioxide. Controlling fossil-fuel soot will not only slow global warming but also will improve human health."

Jacobson presented his findings Dec. 11 at this year's San Francisco meeting of the American Geophysical Union (AGU), an international scientific society with more than 35,000 members dedicated to advancing the understanding of Earth and its environment.

Why is soot worse than greenhouse gases like carbon dioxide and methane? A particle made primarily of elemental black carbon, soot warms the air by absorbing sunlight and radiating the heat to the air. Greenhouse gases, in contrast, do not absorb sunlight; they warm the air by absorbing the Earth's heat and radiating it to the air. Soot does this as well.

Soot may be the second-leading cause of global warming after carbon dioxide, says Jacobson. But controlling soot will cool climate faster than will controlling carbon dioxide because soot has a very short lifetime in the air -- weeks to months -- whereas carbon dioxide has a lifetime of 50 to 1,000 years. That means soot leaves the atmosphere quickly and no longer has a warming effect.

Yet the 1997 Kyoto Protocol failed to consider this climate-changing culprit, created by burning diesel fuels, jet fuel, coal, wood and other biomass. Most past climate-change models took into account neither soot nor its interactions with other atmospheric aerosols or clouds. Jacobson's computer model, developed over a 12-year period, does.

Jacobson says that net global warming to date is due to warming by greenhouse gases and soot, significantly offset by cooling due to reflective particles, such as sulfate and nitrate from multiple pollution sources. Eliminating all fossil-fuel soot could reduce more than 40 percent of net global warming to date in three to five years, he says. Cutting fossil-fuel carbon dioxide emissions by a third would have the same effect, but only after 50 to 200 years.

Removing soot also would provide enormous health benefits, Jacobson says. Studies have estimated that, in industrialized nations, health costs due to soot are $160,000 to $2 million per ton emitted. Worldwide, about 5 million tons of soot are emitted each year from fossil fuels, experts estimate.

Diesel fuel powers almost all commercial trucks, buses and tractors worldwide and 33 percent of the passenger vehicles sold in Europe last year, Jacobson says. In 1997, tax laws in 14 European countries favored diesel over gasoline. Though only one American passenger vehicle in 1,000 runs on diesel, annual diesel emissions from all ground transportation sources in the United States are 75 to 80 percent of those in Europe.

Many people mistakenly believe that diesel vehicles are better for the environment because they travel 30 percent more miles per gallon than do gasoline-powered vehicles. But diesel vehicles emit about 18 percent more carbon per gallon than do gasoline vehicles. More important, soot is a much more efficient warming agent per unit mass than is the worst greenhouse gas. That translates into greater global warming with diesel than with gasoline over the next 100 to 150 years, Jacobson says. Only after that time, with the cumulative effect of the greater fuel efficiency of diesel vehicles, does diesel overtake gasoline in terms of climate benefit. But never do the health costs of diesel fall below those of gasoline, Jacobson says.

"In addition, within a few decades oil supplies are predicted to run out, so all oil will be burned, whether as diesel or gasoline," Jacobson says. "Since the replacement is likely to be hydrogen fuel, possibly produced from a renewable energy source, the use of diesel from now until that time serves only to exacerbate health and climate problems."

Ways to address global warming due to soot, Jacobson says, include tightening standards to reduce particulate emissions by a factor of four to eight, requiring industry to come up with better particle traps and switching from diesel fuel to gasoline or hydrogen fuel cells.

Are these recommendations feasible? Definitely, Jacobson says. He cited industry's reaction in 1970 to clean air amendments calling for 90 percent reductions in three pollutants: hydrocarbons, carbon monoxide and oxides of nitrogen. "The automobile industry argued that such regulations would cost too much, that people would lose jobs and that companies would go out of business," he says. "But in five years the industry developed one of the most effective inventions to date for controlling pollution -- the catalytic converter -- and today emissions of all those species are less than one-tenth of what they were in the 1970s per mile driven.

"Historically, regulation of air pollution has led to invention, and invention has led to patents, profits and reduced health costs to society."

Jacobson's research is supported by grants from NASA, the Environmental Protection Agency, the National Science Foundation, the David and Lucile Packard Foundation and the Stanford University Office of Technology Licensing. He has recently written his second textbook, Atmospheric Pollution: History, Science, and Regulation (Cambridge University Press, expected March 2002). SR