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“Soot, or black carbon, may be responsible for 15 to 30% of global warming, yet it’s not even considered in any of the discussions about controlling climate change,” Mark Jacobson, an assistant professor of civil and environmental engineering at the US’s Stanford University, announced on the publication of new research in 8 February’s edition of the journal Nature. Jacobson says as well as releasing greenhouse gases, the burning of fossil fuels, leads to an, as yet, little considered notion – that soot mixes with other particles absorbing more sunlight and radiating twice as much heat as pure black carbon .

In tests, soot, 90% of which is produced by burning fossil fuels and biomass, was found to interfere with the reflectivity of aerosols, or organic particles, darkening their colour so that they absorbed more radiation and reduced their cooling effect significantly. A computer model known as GATOR-GCMM, designed for analysing urban and global pollution, was used to simulate the emissions, movement, transformations and removal of soot and other important airborne particles. The results of the simulation show that, five days after entering the atmosphere, particles of pure soot are very likely to end up in mixtures containing dust, sea spray, sulphate and other chemicals. Jacobson said that the findings were consistent with atmospheric field studies, including one revealing that more than 93% of all soot above the North Atlantic Ocean contained particles of sulphate.

Jacobson then programmed his computer to simulate how millions of tons of mixed soot would affect the Earth’s climate, and the result showed that a much higher mixing rate between soot and aerosol exists than was previously thought, and that black carbon may be responsible for more atmospheric heating than methane. Once technical hurdles have been overcome, direct measurement could confirm whether this is true.

As a result, Jacobson urges policy makers to consider looking at ways to reduce soot pollution worldwide, pointing out that technologies exist or can be developed to remove excess soot produced in fireplaces, truck engines and other sources of black carbon. One of the biggest misconceptions, the researcher points out, is that diesel fuel is better for the environment than petrol, because diesel cars get more mileage. Unlike diesel, modern gasoline engines emit virtually no soot, although both produce large amounts of carbon dioxide.

“Besides its impact on global warming, soot is bad for your health,” Jacobson said, 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. The largest source of mortality from air pollution is indoor burning of biomass and coal, ” (see related story).

If black carbon production were to stop, however, any detrimental effects would soon disappear, scientists say, as the substance has a far shorter lifetime in the atmosphere than carbon dioxide or methane.

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

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