New study with PNNL help: Soot causing climate change

January 17, 2013 

The tiny black particles released into the atmosphere by burning fuels are far more powerful agents of global warming than had been estimated, some of the world's most prominent atmospheric scientists reported in a study issued Tuesday.

The scientists included Steve Ghan, a climate researcher at Pacific Northwest National Laboratory in Richland.

The particles, which are known as black carbon and are the major component of soot, are the second most important contributor to global warming, behind 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 most recent major report by the U.N. 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 findings suggest "that if we can reduce black carbon emissions across the planet, which would benefit human health too, we can delay some of the warming due to increasing greenhouse gases," Ghan said in a statement.

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 cookstoves and kerosene lamps in poorer nations. Natural phenomena like forest fires also produce it.

Black carbon already is 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. The program seeks to reduce the production of black carbon to combat 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 change, said Mark Z. Jacobson, an atmospheric scientist at Stanford University and one of the study's authors.

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 required that we rethink how we measure and model black carbon in climate simulations," said Ghan, who led the writing of the report's section on how black carbon directly absorbs sunlight in the atmosphere. "We changed our estimate of how much sunlight it absorbs by a factor of two in the process, probably because of underestimating emissions of black carbon."

It also took into account some of the complicated secondary climate effects that occur when black carbon interacts with chemicals, clouds and the Earth's surface. When black carbon settles on glaciers or Arctic ice, it renders them darker, and they absorb more heat and melt more quickly.

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