Carbon, in a guise other than CO2, is causing a stir among climatologists.
Identified as BC or black carbon found in soot, the short-lived particulate
is suspected of causing global warming to an extent that could rival its
In the last few years, improvements in modeling have helped characterize the direct radiative performance of different aerosols — how much light these particles scatter or absorb. Soot, composed of mostly black carbon with graphite-like dark particles and light-absorbing organic matter, absorbs more than it reflects. This trait puts the particulate at odds with other aerosols.
"There has been much discussion and speculation in the climate research community about the role that black carbon particles may be playing in climate change," says Don Wuebbles, department head of atmospheric sciences at the University of Illinois. If black carbon is indeed a strong light absorber, it may counter the cooling effects of reflective aerosols — the coolest of all being sulfate. A new report in Nature says soot is doing just that.
The report makes way for arguments that black carbon, which once in the atmosphere is rained out after a few weeks, should be at the top of the Kyoto Protocol's list of climate factors to curb. Buses and trucks put soot into the atmosphere from diesel fuel, and soot also forms at the same time as other more reflective aerosols from fossil fuel and biomass burning.
In the Feb. 8 Nature, Mark Jacobson of Stanford University says that soot outplays the greenhouse gas methane in its warming role. "Black carbon may be the second most important component of global warming after carbon dioxide, in terms of direct forcing," he says. Jacobson studied soot's ability to mix with sulfate and other aerosols. Using a GATOR-GCMM computer model, he found that a core black carbon particulate most likely collects other aerosols around it like a dirty snowball. Previous studies had assumed the black carbon mixed externally with other aerosols like oil on water. But, whether mixed or on its own, soot from fossil fuels and biomass burning warms at a rate of 0.55 Watts per square meter, Jacobson says. That's more than methane, which absorbs 0.47 Watts per square meter, according to an assessment by the Intergovernmental Panel on Climate Change (IPCC). In comparison, carbon dioxide contributes heat to the atmosphere at a mean 1.56 Watts per square meter.
"If research investment were to be scaled by climate impact, these figures would suggest that resources at the level of about one-third of those now devoted to carbon-cycle research should go into black-carbon studies," wrote Meinrat O. Andreae of the Max Planck Institute for Chemistry in Mainz, Germany, in a Feb. 8 Nature News and Views article. "A staggering thought!"
On Jan. 21, IPCC released newest climate change assessment. Three years in the making with 123 lead authors and 516 contributing authors from around the world, the review gave black carbon a minor heating role among the much cooler aerosol gang: sulfate, organic carbon and other particulates produced from fossil fuels and biomass burning. The IPCC report shows black carbon produced from burning fossil fuels has a direct radiative effect of absorbing only 0.2 Watts per square meter Nature that's less than half the amount of warming to the atmosphere that Jacobson found for soot.
Before the IPCC report, soot had already made its debut as a controversial figure in the scientific arena. But the majority of studies showed human-induced aerosols mostly produce negative radiative forcing, reflecting light and cooling the planet. Existing regulations, then, not only decrease the harmful health effects of particulate air pollutants, but also their cooling influence.
"Jacobson's paper is significant in showing that most previous estimates probably underestimated the direct climate forcing by soot aerosols," says James Hansen of NASA's Goddard Institute for Space Studies. From his own estimation, which considers other climate forcing effects, Hansen says that black carbon is even more important as a climate forcing than is concluded in Jacobson's report. Hansen has suggested that controlling black carbon emissions not only improves air quality and health, but could also reduce Earth's global warming threat, an idea Jacobson supports.
"Soot has a lifetime of up to a week to a few weeks from its time of emissions, whether it is mixed with sulfate or pure," Jacobson says. "Since it is continuously emitted, though, it is always present in the atmosphere." The greenhouse gas methane can last for eight to 12 years, while carbon dioxide can last for centuries. But soot lasts only a few weeks, so the effects from reducing its volume could be seen right away.
Joyce Penner of the University of Michigan warns against isolating soot and its effects. She questions the importance of isolating the direct radiative forcing of soot away from the cooler aerosols. When sunlight hits aerosols, they directly absorb or reflect heat, changing the temperature of the air around them and creating a "semi-direct" effect on the lifespan of clouds. Aerosols can also dirty cloud droplets, causing an "indirect" impact that reduces a cloud's reflective properties. Although the role of aerosols on clouds is harder to model, Penner is concerned because reflecting aerosols, such as organic carbon, are emitted at the same time and place and by many of the same burning processes as soot. "The effects of black carbon need to be looked at in full — not in a piecemeal way."
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