Black carbon major suspect in warming trend

Black carbon major suspect in warming trend

24 November 2006

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Portland,OR, USA — East Sylvan Middle School sixth-grader Jack Edwards has two dominant impressions from studying Chinese in Beijing last summer: Heat and haze. “It always felt like you were walking through a big bowl of 2 percent milk,” Edwards says. “You knew the sun was there because it was really hot, but you couldn’t see it.”

The active ingredient in the haze is black carbon, or soot, a stew of fine particles belched from household stoves and heaters, vehicles and crop burning. China generates the largest share of these particles emitted worldwide each year, with India and the former Soviet Union not far behind.

Until recently, scientists overlooked the role of soot in global warming, concentrating on the more easily quantified gases that trap heat. But suddenly these aerosols — such as sulfur compounds that contribute to about 400,000 premature deaths each year in China — occupy the frontier of research on climatechange.

Researchers now suspect that black carbon, which absorbs the sun’s rays because of its dark color, is the second most powerful global-warming agent after carbon dioxide. The pervasive soot is a tough nut for scientists because it mixes with other poisons and disguises its planetary heating effect. Without the film of soot above him in Beijing, Edwards actually would have felt hotter.

China’s soot also matters here, in the Pacific Northwest.

Plumes of black carbon and other contaminants regularly pass over Oregon, dumping mercury that collects in fish. A dilute form of China’s milky haze taints once-pristine areas of the state. And soot’s warming effect helps shrink Cascade glaciers and turbocharge Pacific storms, spawning powerful waves that hammer the coast.

When Yun Qian was Edwards’ age, he enjoyed clear days and swam in a sparkling, fish-filled river at his hometown near Suzhou. The “Venice of China” would later become Portland’s sister city. But Qian, now 39 and a senior research scientist at the Pacific Northwest National Laboratory in Richland, Wash., returns on visits to his parents to find filthy water and hazy skies. The fish are either gone or he can’t see them.

Qian and other researchers have surveyed records kept by more than 500 weather stations across China between 1954 and 2001. They found that cloud-free days increased during the half-century. But as China increased fossil-fuel emissions almost ninefold, the amount of solar radiation reaching the earth decreased.

Their explanation: The milky haze that Edwards experienced absorbs and reflects heat, reducing solar radiation reaching the earth by 9 percent over the years and lowering crop yields.

“The air pollution problem has masked the effect of global warming over some areas,” Qian says. “Overall, the temperature is still going up across China, but when we looked at some very polluted areas, the rate of increase is much lower than other parts of theworld.”

The effect is deceptive. Black carbon, one of the major constituents of the haze, absorbs heat a half-mile or so above the earth, contributing to global warming while shading the ground below, says Michael Bergin, a Georgia Institute of Technology researcher. “You’re still putting more energy in the lower atmosphere than would have been there before,” Bergin says.

Incomplete combustion generates black carbon, whether in cook stoves, coal plants, diesel engines or field burning. It works in the opposite way from greenhouse gases, absorbing sunlight from above instead of trapping terrestrial radiation from below. Unlike greenhouse gases, which come mainly from economically developed nations, black carbon is generated primarily by less-developed countries, where people cook with wood, field residue and cow dung.

Black carbon affects not only global temperatures but regional weather and cloud formation. Soot-heated air makes the atmosphere unstable, NASA researchers say, by creating convection currents that bring rain to some areas and dry conditions toothers.

For years, scientists blamed China’s northern droughts and southern floods on dust storms from deserts that are expanding because of forest destruction and excessive grazing. But the NASA researchers fingered black carbon for the shift, which is the largest change in China’s precipitation trends since A.D. 950. Heated air also drifts to other regions of the world, changing weather in distant places.

These kinds of complex interactions draw scientists to particle research. “The uncertainty in all climate-change predictions right now is in knowing how particles interact with the climate,” says Steven Cliff, a University of California at Davis researcher. “Gases are pretty well-known.”

Black carbon falling on snow speeds melting of the ice caps, for example, and it can also help form clouds and make them brighter, scattering light. In contrast to soot, light-colored sulfur particles reflect heat, producing a slight climate-cooling effect that may be dominant across China.

The good news about particles is that they tend to float in the atmosphere for days or weeks at most, in contrast to greenhouse gases that can persist for a century or two. So reducing black-carbon emissions can have an immediate effect on temperatures as well as human health. It’s also cheaper to cut soot emissions than to control carbon dioxide.

China, which has 16 of the world’s 20 most polluted cities, is trying to clean up. It’s phasing out burning of raw coal in major cities in favor of briquettes, natural gas, electricity and renewable energy sources. Improved technology in large coal-fired plants temporarily lowered sulfur-dioxide emissions.

“There are two forces in play: growth and environmental controls,” says David Streets, an Argonne National Laboratory researcher. “And it’s not always clear which one is winning. There are a lot more vehicles on the road now, but the emission rates of individual cars are getting better.”

The Kyoto Protocol does not mention black carbon, an omission cited by President Bush in 2001 when he rejected the climate treaty. Streets proposes a new international compact in which the United States and other developed nations would reduce carbon dioxide, because they have emitted most of it and can better afford controls. China and other developing countries would cut black carbon emissions, because measures would be cheaper and also provide health benefits.

Another scientist, Nobel Prize winner Paul Crutzen, says radical measures may be needed because steps to address global warming are so pitiful. Crutzen, a researcher at Germany’s Max Planck Institute for Chemistry, proposes releasing sulfur particles in the upper atmosphere to cool the globe by reflecting sunlight and heat back into space. His plan is modeled partly on the 1991 eruption of Mount Pinatubo, which ejected sulfur that cooled the Earth slightly.

Other scientists are skeptical. “Given humans’ propensity for doing things that have unintended consequences, I would be very reluctant to go down that path,” says Richard “Tony” VanCuren, a UC Davis researcher in applied sciences.

In China, the Beijing 2008 Olympic Games have become a short-term cleanup incentive. Officials aim for clear skies during the landmark event to be held that August. “China does not want to have brown skies over the Olympics,” says Tom Cahill, a UC Davis researcher who is advising the Chinese government.

Officials may shut down coal-fired plants and other haze sources upwind of Beijing, as well as ban private vehicles in the city. Lest that sound Orwellian, Cahill harks back to 1984, when some California plants voluntarily suspended operations, he says, to enable mountain views during the Los Angeles Olympics.

“They’ve got some pretty aggressive dreams,” says Greg Carmichael, a University of Iowa atmospheric chemist. “As the Olympics get closer, clearly things will get better. But will that be sustainable? I don’t know.”

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