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.
In Europe and the U.S., diesel fuel, wood-burning fireplaces and barbecues are major sources of black carbon. Forest fires also are large sources of black carbon emissions.
Black carbon emissions in Western Europe and the U.S. have decreased about 300% in the last 30 years because of more-efficient coal combustion, a move away from wood-burning fireplaces, and cleaner, more efficient technology.
“The positive side of this discouraging story is we know how to cut down black carbon,” Ramanathan said. “We have reduced it. So this is something we can do now.”
Ramanathan said the new figure was higher than previous estimates because the study took into account the atmospheric range of black carbon, which can rise several miles into the atmosphere and is more effective at heating the higher it travels. It also explores the increased heating effect that occurs when black carbon particles mix with sulfates and other organic particles in the atmosphere. Because the other particles in the brown cloud reflect light, these reflections are bounced around and eventually absorbed by the black carbon particles, further magnifying their heating effect.
The report’s data was drawn from NASA satellites and ground stations and from field studies near the Indian Ocean and California coast. Those numbers were then extrapolated to create a global figure for the black carbon warming effect.
“There’s an uncertainty in the actual number, but what it does seem to confirm is really the main point,” Jacobson said. “Black carbon warming is much stronger than the IPCC consensus number had estimated.”
The study was funded by the California Energy Commission, the National Science Foundation, the National Oceanic and Atmospheric Administration and NASA. It was published Sunday in the online edition of Nature Geoscience.
The report concluded that black carbon pollution, which scientists blame for the premature deaths of more than a million people, is one of the major contributors to the retreat of the Himalayan glaciers. Black carbon particles that land on snow absorb more solar radiation and accelerate the melting of ice, previous studies have said.