But according to IPCC scientists, atmospheric soot has relatively little effect on world climate.

Jacobson disagrees.

``Only a handful of studies have considered the impact of soot on global warming,`` he says, ``and most of those were based on the premise that soot never mixes with other particles in the atmosphere.``

But scientists have known for many years that floating soot particles actually do combine with dust and chemicals in the air, notes Jacobson.

This is a crucial point, he says, because mixtures containing black carbon absorb more sunlight and radiate twice as much heat as do particles of pure black carbon. Therefore, soot in its mixed state has the potential to make a significant contribution to global warming.

But what is the actual mixing state of soot? Do particles of black carbon normally float around by themselves, radiating relatively little heat into the atmosphere? Or do they routinely mix with other particles, causing atmospheric temperatures to rise?

Computer model

To find out, Jacobson used a sophisticated computer model known as GATOR-GCMM, which he designed for the purpose of analyzing urban and global pollution.

``For the Nature study, I used the model to simulate the emissions, movement, transformations and removal of soot and other important airborne particles,`` he says.

The results of the simulation show that, just five days after entering the atmosphere, particles of pure soot are very likely to end up in mixtures containing dust, sea spray, sulfate and other chemicals.

These findings are consistent with several atmospheric field studies, including a 1999 survey that found that more than 93 percent of all soot above the North Atlantic Ocean contained particles of sulfate.

Jacobson then programmed his computer to simulate how millions of tons of mixed soot would affect the Earth`s climate.

The results were dramatic.

``These black carbon mixtures turn out to be one of the
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Contact: Mark Shwartz
mshwartz@stanford.edu
650-723-9296
Stanford University
7-Feb-2001