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University of Iowa News Release


March 24, 2008

Soot found to play larger role in global warming than previously thought

Soot from diesel engine exhaust and cooking fires -- widely used in Asia -- may play a larger role than previously thought in global warming, according to a University of Iowa researcher and his colleague.

Greg Carmichael, professor of chemical and biochemical engineering in the UI College of Engineering, and V. Ramanathan, atmospheric scientist at Scripps Institution of Oceanography at the University of California San Diego, presented their findings in the Sunday, March 23, online edition of the journal Nature Geoscience.

Titled "Global and regional climate changes due to black carbon," the paper says that black carbon soot has an atmospheric warming effect three to four times greater than previously estimated. Coal and cow dung-fueled cooking fires in China and India produce about one-third of black carbon; the rest is largely due to diesel exhaust in Europe and other regions relying on diesel transport. The article also noted that soot and other forms of black carbon could equal up to 60 percent of the current global warming effect of carbon dioxide, the leading greenhouse gas.

However, the researchers also noted that the findings might have a silver lining.

"Given black carbon's significance in global warming, a major focus on decreasing black carbon emissions offers an opportunity to mitigate the effects of global warming trends in the short term," Carmichael said.

Funded by the National Science Foundation, the National Oceanic and Atmospheric Administration and NASA, Carmichael and Ramanathan integrated observed data from satellites, aircraft and surface instruments about the warming effect of black carbon. They found that its warming effect in the atmosphere, is about 0.9 watts per meter squared (W/m-2), compared to estimates of between 0.2 W/m-2 and 0.4 W/m-2 that were agreed upon as a consensus estimate in a report released last year by the U.N.-sponsored Intergovernmental Panel on Climate Change.

Those estimates were based on computer model simulations that do not take into account the degree of black carbon's increased warming effect when it is mixed with other aerosols. Ramanathan and Carmichael said that the models also do not adequately represent the full range of altitudes at which the warming effect occurs. The most recent observations, in contrast, have found significant black carbon warming effects at altitudes in the range of two kilometers (6,500 feet), levels at which black carbon particles absorb not only sunlight but also solar energy reflected by clouds at lower altitudes.

The researchers noted that black carbon is both a pollutant and a global warming agent, a fact that could provide political opportunities.

"Black carbon's unique role as a leading air pollutant and a major global warming agent offers a chance to address two of our major environmental problems simultaneously by implementing policies for reducing black carbon," Carmichael said. "We have the capacity to do this, and we hope that our paper provides more traction to proceed."

Additional information on the study can be found by accessing the Scripps news release at the Scripps Institution of Oceanography Web site

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