A new study confirms that black carbon -- more commonly known as soot -- is a significant player in global warming.
The work by Mark Jacobson, director of Stanford University's Atmosphere/Energy program and a fellow at the university's Woods Institute, argues that cutting emissions of black carbon may be the fastest method to limit the ongoing loss of ice in the Arctic, which is warming twice as fast as the global average.
The study also concludes that, over a 15-year period, cutting the black carbon produced by burning fossil fuels, vegetation, dung and other sources could reduce the warming the Earth has experienced since the Industrial Revolution -- about 0.8 degrees Celsius -- by 17 to 23 percent.
That is because black carbon lingers in the atmosphere for one to four weeks, compared to CO2's lifetime of centuries to millennia. And much of the technology needed to cut the world's black carbon output already exists, including pollution traps that can be installed on diesel engines and solar cookstoves that replace models powered by burning wood or dung.
"Because black carbon has a short lifetime, if you can remove it, you can have a fast climate impact," Jacobson said.
While cutting carbon dioxide is the only way to halt warming over the long term, experts said, earlier research has come to similar conclusions about the short-term advantages of limiting black carbon emissions.
They said the real strength of the Jacobson study -- now in press at the Journal of Geophysical Research - Atmospheres -- is that it relies on a new computer model of climate, air pollution and weather that accounts for several different ways black carbon influences the environment.
"I don't think many studies have realized this yet: Black carbon impacts global warming in at least four different ways," said V. Ramanathan, an atmospheric scientist at the Scripps Institution of Oceanography. "The Jacobson study is the first I know of that has included all of these effects."
Like carbon dioxide, black carbon absorbs sunlight and infrared radiation, trapping heat in the atmosphere -- including the boundary layer closest to Earth's surface. The tiny particles also serve as condensation nuclei for clouds and are trapped between cloud particles, where their ability to absorb heat helps dry up those clouds and allows more sunlight to reach Earth. And when soot falls on snow or ice, as in the Arctic, its ability to trap heat from the sun helps hasten melting.
"The lifetime of black carbon is pretty short, about one to two weeks on average," Jacobson said. "But even once it's removed [from the atmosphere], you've still heated the air around you. So the heating from black carbon lasts a little longer" than it lingers in the air.
Drew Shindell, a climate modeler at NASA's Goddard Institute for Space Studies, agreed that the new study's strength was its "very detailed" treatment of black carbon's contributions to warming.
Jacobson, the study's author, noted that the study also separated the effects of black carbon produced by burning fossil fuels and that produced by biofuels like wood or dung. The soot produced by each type of burning has a different makeup. The soot produced by burning fossil fuels has a stronger warming effect because it contains a higher ratio of black carbon to sulfate, which reflects sunlight to produce a cooling effect.
In contrast, soot produced by burning biofuels is often a product of dung- or wood-powered cookstoves used indoors and in densely populated areas. Cutting that source of black carbon could help reduce the estimated 1.5 million premature deaths per year attributed to biofuel soot, Jacobson said.
But Shindell said Jacobson's model was also the study's "strongest limitation." Many other studies on black carbon's climate influence have used models that have been used in reports by the Intergovernmental Panel on Climate Change.
The new study "is the kind of result I would like to see confirmed in the more road-tested climate models, but it's more than food for thought," Shindell said. "Still, I think that the results are consistent with some other analyses."
They include a paper Shindell published last year in Nature that found a one-two punch of decreasing sulfate emissions -- which help scatter light from the sun, cooling Earth -- and increasing black carbon emissions -- which have the opposite effect -- has "substantially contributed to rapid Arctic warming during the past three decades."
Jacobson said he agreed that "models need to be continuously tested," and noted that the model he used in this study is one he's been refining since 2002, which has formed the basis for several pieces of published research.
Global warming's quicker fix?
Meanwhile, Ramanathan said that Jacobson's conclusion that black carbon might be the second-largest contributor to warming -- behind only CO2 -- tracks with his own experiments based on observations of actual climatic conditions.
While the uncertainty in the results from Jacobson's model and his own experiments is large, Ramanathan said he "wouldn't rule out that black carbon is the second-largest global warmer."
Meanwhile, another group of researchers published a commentary yesterday in the journal Nature Geoscience that argues cutting black carbon, methane and other short-lived substances that influence climate could not only limit warming but improve climate models, if done correctly.
"Earth's climate can only be stabilized by bringing carbon dioxide emissions under control in the twenty-first century," they write. But reducing emissions of short-lived substances that help heat the planet "could significantly reduce the rate of warming over the next few decades."
And by carefully measuring and modeling the resulting changes in atmospheric composition, scientists could improve their estimate of how sensitive Earth's climate is to CO2, said lead author Joyce Penner, a professor of atmospheric science at the University of Michigan whose work focuses on improving global climate models and their ability to model the interplay between clouds and aerosol particles.
That would narrow estimates of how much warming the world can expect for a given level of CO2 in the atmosphere.
"Why should we go another 20 years without knowing whether we're on high [warming] curve or the low curve?" said Penner.
Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500