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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
gaseous cousin.
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."
Christina Reed
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