Climate’s smoky spectre

Published online on July 1, 2009, in Nature.

With their focus on greenhouse gases, atmospheric scientists have largely overlooked lowly soot particles. But black carbon is now a hot topic among researchers and politicians. Jeff Tollefson investigates.

Steve Warren spent his spring break island-hopping with a couple of friends, but they didn’t go to bask in the sun. Instead, his team from the University of Washington in Seattle toured the Canadian Arctic, digging pits in the snow and collecting hundreds of samples to take back to the lab. The targets of their expedition, hidden in all the whiteness, were specks of something called black carbon.

These dark particles, the major constituents of soot, are the legacy of incomplete combustion in diesel engines, coal power plants, agricultural burning and wildfires far to the south. Prevailing winds sweep black carbon and other pollutants into the Arctic, where they circulate in a dirty yellow haze until storms wash them out of the air. Warren’s team was collecting those that fell among the snow flakes.

The aerosol haze has long plagued the Arctic, but scientists are only now taking stock of a different and potentially uglier dimension of soot. As its name would suggest, black carbon absorbs sunlight. These particles heat the atmosphere while aloft; when they settle on the snow, they hasten its melting. This exposes the dark land and water, which absorb more of the sun’s energy and thereby drive up the region’s temperature. Recent research1 suggests that black carbon could be responsible for a large fraction of the Arctic warming. Soot also takes a toll elsewhere. In southeast Asia, studies suggest2 that it is choking the moisture supply for the Indian monsoons and contributing to the retreat of mountain glaciers that provide fresh water for more than a billion people.

At this point, scientists lack enough data to definitively conclude how strongly black carbon is affecting the climate. But some studies suggest that it could be second behind carbon dioxide in terms of its contribution to global warming. There is a crucial difference between the two pollutants, however: soot particles hang in the atmosphere for just a few weeks, whereas CO2molecules can remain in the air for centuries. This means that efforts to curb soot emissions could reap immediate climatic benefits. That possibility has recently pulled soot, which has conventionally been seen as a public-health issue, into the climate-policy arena.

“There’s an urgency about this: we still don’t have a viable way of cutting down CO2,” says Veerabhadran Ramanathan, an atmospheric scientist at Scripps Institution of Oceanography in La Jolla, California. By comparison, reducing soot emissions seems remarkably simple and cheap. “It’s not going to take 30 or 100 years to do it. If you halt the black carbon now, it will be gone in two weeks.”

Hazy data

Long before the current interest in black carbon, an accidental observation by Warren led him to do some pioneering work on the pollutant. In 1980, he and Warren Wiscombe of the National Center for Atmospheric Research in Boulder, Colorado, were having trouble developing a mathematical model of snow reflectance, or albedo. The two couldn’t make their calculations align with the latest albedo measurements in the Arctic because the snow in their study was reflecting less light than expected. “It turned out the snow was being collected downwind from a diesel generator,” Warren says.

Warren went on to collaborate with Antony Clarke at the University of Washington, who organized the first survey of black-carbon deposition in the Arctic, largely using samples collected by researchers who were going there for other reasons. On the basis of those data, Clarke concluded in 1985 that soot could have a measurable effect on the Arctic climate3. But his paper had little influence until Jim Hansen, the man known for alerting the world to the threat of CO2 pollution, pressed the issue years later. 

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