These particles, which are known as black carbon
and are the major component of soot, are the second most important
contributor to global warming, behind only carbon dioxide, wrote the 31
authors of the study, published online by The Journal of Geophysical Research-Atmospheres.
The new estimate of black carbon’s heat-trapping power is about double
the one made in the last major report by the United Nations’ Intergovernmental Panel on Climate Change,
in 2007. And the researchers said that if indirect warming effects of
the particles are factored in, they may be trapping heat at almost three
times the previously estimated rate.
The new calculation adds urgency to efforts to curb the production of
black carbon, which is released primarily by diesel engines in the
industrialized world and by primitive cook stoves and kerosene lamps in
poorer nations. Natural phenomena like forest fires also produce it.
Black carbon is already a central target of one of the few international
climate initiatives championed by the United States, the Climate and Clean Air Coalition to Reduce Short-Lived Climate Pollutants,
which has been supported by Secretary of State Hillary Rodham Clinton.
The program seeks to reduce the production of black carbon to combat
both climate change and air pollution and respiratory disease on the ground.
Although some scientists have long believed that black carbon is a major
force in climate change, the vast majority of previous mathematical
models had predicted that the particles had only a modest impact. That
view should now change, said Mark Z. Jacobson, an atmospheric scientist
at Stanford University and one of the study’s authors, calling the old
models “overly simplistic.” He said that many of his co-authors had
previously hewed to the lower estimates.
Veerabhadran Ramanathan, a professor of climate science at the Scripps Institution of Oceanography
in San Diego who has long campaigned to control black carbon, described
the study as highly authoritative. “The fact that it’s written by a
very large group of modelers gives it enormous credibility,” he said.
“It was lonely before. I’m now glad to be right in the middle.”
The group reached its conclusions after factoring in a new series of
measurements about the amount of black carbon accumulating in the
atmosphere and how much heat from the sun it absorbs. It also took into
account some of the complicated secondary climate effects that occur
when black carbon interacts with chemical, clouds and the earth’s
surface.
For example, when black carbon settles on glaciers or Arctic ice, it
renders them darker, and they absorb more heat and melt at a faster
rate.
Still, some scientists said the paper mostly underlined how much
remained to be studied about the warming effects of these particles.
“The paper makes a good case that our models are underestimating the
effect, but what it does for me is to underscore all the various
uncertainties,” said Christopher D. Cappa, an associate professor of environmental science at the University of California at Davis.
In a study
published last year in the journal Science, Dr. Cappa and his
colleagues studied atmospheric samples containing black carbon and concluded
that they absorbed less sunlight than might be predicted from
laboratory experiments, in part because black carbon is coated with
atmospheric chemicals.
Carbon dioxide, the leading greenhouse gas, remains in the atmosphere
for decades and is distributed nearly uniformly across the earth’s
atmosphere. By contrast, black carbon generally only persists in the air
for a week to 10 days, so its presence across the globe is far more
variable. And its effect varies greatly depending on whether it is above
or below the clouds, Dr. Cappa said.
But the short-lived nature of black carbon also makes it a ready target
for efforts to rein in climate change. Any reduction in carbon dioxide
production today will take years to have a tangible effect on global
warming because so much of the gas is already in the atmosphere. But
preventing the release of a ton of black carbon, particularly in just
the right place — say, upwind from a glacier — could have a strong and
nearly immediate impact.
Mrs. Clinton has also been a strong supporter of the Global Alliance for Clean Cookstoves,
a public-private partnership whose goal is to replace 100 million
primitive stoves in poor countries with modern versions that produce
less black carbon.
On another front, a greater emphasis on black carbon as a warming agent
could affect elements of climate policies in many countries. Most
notably, to meet national fuel efficiency
standards, many carmakers are making more diesel cars because they get
better gas mileage and produce less carbon dioxide.
But diesel engines also produce relatively heavy emissions of black
carbon, Dr. Jacobson said, which partly cancels out the benefit.
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