Black carbon pollution, or soot, produced by burning wood, coal, cow dung and diesel fuel, may be a much greater contributor to global warming than previously suspected, according to a study released this week.
The report concludes that the atmospheric warming effect of black carbon pollution is as much as three to four times the consensus estimate released last year in a report by the U.N.-sponsored Intergovernmental Panel on Climate Change.
The findings are of concern to areas such as the Indian subcontinent, where retreating glaciers in the Himalayas have the potential to flood densely populated areas and affect the drinking water of billions of people.
Unlike carbon dioxide, which traps solar energy radiating back from Earth's surface, black carbon particles absorb solar radiation as it enters Earth's atmosphere, increasing its heat. In addition, when they precipitate onto snowy areas, they increase heat absorption that leads to glacial melting.
The particles come from burning dung, wood, coal and other materials for household use, and travel in "brown clouds."
"In Los Angeles, it's what you see outside your door on the horizon," said V. Ramanathan, an atmospheric scientist with the Scripps Institution of Oceanography at UC San Diego. Ramanathan performed the study with Greg Carmichael, a chemical engineer at the University of Iowa.
The paper concluded that black carbon's warming effect in the atmosphere is about 0.9 watts per meter squared, compared with the climate change panel's consensus estimate of 0.2 to 0.4 watts.
The report, titled "Global and regional climate changes due to black carbon," confirms similar conclusions of three previous model studies released in 2002 and 2005.
The paper concludes that carbon pollution contributes to global warming at a level that is about 60% of carbon dioxide's warming effect, which makes black carbon the second most important contributor to global warming after carbon dioxide.
A mass of black carbon in the atmosphere causes about 300,000 times as much instantaneous warming as the same amount of carbon dioxide, said Mark Jacobson, a professor of civil and environmental engineering at Stanford University who worked on the 2002 study. But whereas black carbon disappears within a couple of weeks, carbon dioxide continues to build up and can take centuries to completely dissipate from the atmosphere.
About 25% to 35% of black carbon in the atmosphere comes from South and East Asia.
