Black Carbon: The Dark Horse of Climate Change Drivers

Article excerpt

For decades, efforts to slow global warming have mostly aimed to limit heat-trapping emissions of carbon dioxide ([CO.sub.2]). Now scientists are pointing to a different class of warming agents they say also must be targeted to keep global temperatures in check. Dubbed, "shortlived climate forcings" (SLCFs), these other emissions--namely, black carbon particles, methane, hydrofluorocarbons, and tropospheric ozone--are even more powerful than [CO.sub.2], in terms of their warming potential. But they persist in the atmosphere for much shorter durations than [CO.sub.2], which can linger airborne for hundreds to thousands of years.(1)

Steve Seidel, vice president for policy analysis at the Pew Center on Global Climate Change, says the recent emphasis on SLCFs represents new policy thinking on climate change.

"We thought the Kyoto Protocol and its follow-on agreements would get us to where we need to be, but that's not working out the way we hoped it would," he says. "So, we're broadening the discussion and opening up new pathways for going forward."

More than three-quarters of the world's black cartoon Is thought to come from developing countries, discharged from cookstoves, open burning, and older diesel engines. This data visualization uses data from NASA's GEOS-5 Goddard Chemistry Aerosol and Transport (G0CART) climate model to show atmospheric concentrations of black carbon on 26 September 2009. Aerosol optical thickness ranges nonlinearly from 0.002 (transparent) to 0.02 (purple) to 0.2 (white). Animations of global black soot transport are available at http://tinyurl.com/64nbykb and http://tinyurl.com/69w9s6z.

Given the enormity of human emissions, many climate scientists believe [CO.sub.2] will one day become the dominant force behind climate change. But for now, [CO.sub.2] and the SLCFs are nearly on par in terms of their climate changing effects, according to Veerabhadran Ramanathan, a professor at The Scripps Institute of Oceanography.

In a report published in February 2011, the United Nations Environment Programme (UNEP) called attention to SLCFs, claiming their emissions must be cut together with [CO.sub.2] in order to prevent global temperatures from crossing a dangerous threshold.(2) Doing that would offer health benefits too, UNEP stated, because SLFCs are also toxic air pollutants. Particulate emissions from diesel exhaust--a major source of black carbon--have been linked to lung and heart disease as well as cancer.(3) But where it would take a transformation of the energy sector (at a cost of trillions of dollars over multiple decades(1)) to drop [CO.sub.2] emissions enough to influence the climate, cutting SLCFs to achieve a similar goal could be achieved with current technologies under policy frameworks that ate already in place, such as clean air regulations, according to Seidel.

Dark and Dirty

Among the SLCFs, black carbon garners the most attention because its climate and health effects are greater than those of the others, says Mark Jacobson, a professor in the Stanford University Department of Energy Resources Engineering. Evidence on black carbon's climate impacts has been building since at least the mid-1990s, when Ramanathan and colleague Paul Cruzan, a Nobel prize--winning atmospheric chemist from the Max Planck Institute for Chemistry, first speculated that "brown clouds" laden with the dark particles influence weather patterns over South Asia, a hypothesis that was supported by future research.(4)

But the way black carbon affects the climate is nuanced and hard to study, and it's only recently that the science has begun to mature to the degree that policies to limit emissions can be proposed on climatic grounds, says Drew Shindell, a scientist with the Goddard Institute for Space Studies at the National Aeronautics and Space Administration (NASA), who led the panel that produced the new report by UNEP "What we're seeing now with the UNEP document and other more recent papers are attempts to generate the first cohesive picture of black carbons effects on the climate and ways to address it," Seidel says. …