A Smut Above: Unhealthy Soot in the Air Could Also Promote Global Warming

Scientific American

Black carbon, commonly described as soot, may play a larger role in global warming than previously estimated, according to a new study.

Every year in the Northern Hemisphere about 7.5 million metric tons of black carbon, the equivalent of more than 100 times Earth’s total biomass, enters the air from internal combustion engines, forest fires and other sources. The fine material absorbs sunlight almost as well as carbon dioxide—a well-known greenhouse gas—and may contribute to accelerated snowmelts and increased global temperatures.

"We’re trying to figure out how to deal with the greenhouse gas problem " says Sarah Doherty, an atmospheric scientist at the University of Washington in Seattle and co-author of the study. The new findings suggest that black carbon mitigation should be part of that strategy.

Scientists have known about the warming potential of black carbon for years, but Doherty's study, published yesterday in the Journal of Geophysical Research: Atmospheres, suggests previous calculations underestimated its impact. Doherty and her colleagues used a compilation of dozens of climate models to look at the life span of atmospheric black carbon. By imputing observational data recorded around the world, they studied the effect this pollutant might have on the Northern Hemisphere.

By itself, black carbon is a warming agent; however, it is not usually emitted in a pure form. Instead, black carbon is expelled into the atmosphere combined with other compounds such as sulfates, which affect their heat-absorbing and reflective properties. Black carbon could be seen as chocolate chips in baking, Doherty says by way of analogy. "In one case you put them in molasses cookies, and the other case you put them in sugar cookies." The result: cookies with different tastes and textures.

For example, black carbon from diesel engines is known to cause atmospheric warming because it is mixed with sulfates, says study co-author Tami Bond, an atmospheric scientist at the University of Illinois at Urbana–Champaign. Forest fires in the lower latitudes, however, are actually beneficial sources of black carbon because it is coupled with organic aerosols and ends up reflecting light and heat, causing the surrounding area to cool.

Atmospheric black carbon’s impact on climatic warming also varies due to its altitude. The team discovered that the consequences of black carbon and whether it promotes or combats climate change depend on its position—above, below or in the middle of cloud cover: Above the clouds, where the particulates absorb heat more readily, they produce a warming effect; below the cloud deck heat absorption is less significant.

Such disparities cause problems for scientists trying to describe the atmospheric effects accurately. According to the study, black carbon compounds have the potential to decrease the world's average temperature by 0.5 degree Celsius or warm it by 1.08 degrees C, depending on how it was produced. Previous studies lacked the observational data included in Doherty’s analysis, making earlier estimates much less accurate. Thus, policymakers have to consider the source of black carbon when drafting mitigation ordinances, Doherty says.

Although researchers still debate the overall influence of black carbon on the atmosphere, scientists do agree that it has become a serious health issue for populations on the ground. Inhaling black carbon could aggravate serious chronic illnesses such as asthma.

Because black carbon only remains in the atmosphere for about a week, decreasing emissions translates into immediate health benefits. The rapid change could also prove a victory for world leaders trying to rally support for climate policy. "Once you stop the emissions, it’s gone. It’s a public health win," Bond says. "We can begin [combating climate change] by doing things for climate that people want to do because it has other health benefits."

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