A new study finds that reducing carbon-rich soot emissions could be the fastest and most economical way to slow climate change and protect human health.
Those dust-like particles released in the exhaust of diesel-powered vehicles and wood fires rank second only to carbon dioxide as a major cause of global warming.
Stanford University professor Mark Jacobson developed the first computer models to measure the presence of soot in the atmosphere. He calculates that soot accounts for between 15 and 20 percent of global warming.
His study, presented this week at the American Chemical Society meeting in Colorado, describes how black carbon - the main component in soot - heats up clouds when it mingles with the rain drops suspended within them.
"And it turned out that there is more heating when the black carbon was inside the drop than between the drops and there was more heating when the black carbon was between the drop than outside of the cloud. So the bottom line was you get this enhancement of the heating of the cloud by the black carbon presence in the cloud drops."
Jacobson says climate models that ignore this cloud absorption phenomenon underestimate the effects of black carbon in the atmosphere. His research found that airborne soot quickly burns off cloud cover.
"If you look at satellite images over really polluted areas such as in China and India you can actually see an absence of clouds."
While carbon dioxide can remain in the atmosphere for 40 or 50 years, carbon soot stays around only for a week or 10 days before settling out, and has no continuing warming effect.
"Soot is a solar absorber, whereas carbon dioxide is primarily a heat absorber. Now, per unit mass, black carbon is about a million times more powerful in warming the air than is carbon dioxide. But because soot, black carbon in soot, are so powerful and warming and because they are very short-lived, that is actually important for control strategies for global warming."
Case in point: The Arctic is warming faster than anywhere on the planet. The white sea ice, which normally reflects sunlight and heat back into space, is giving way to darker areas of open water, which absorbs heat faster, and so accelerates the warming. The Arctic could be ice free within 30 years, according to recent studies.
Jacobson says reducing the amount of soot in the atmosphere can reverse this trend. "And you can slow down the loss of the Arctic ice. And so it may be the only way to prevent or slow down the elimination of the Arctic. And that has implications, of course, not only for climate feedbacks, but also for wildlife such as polar bears which rely on ice floes to survive."
Jacobson says technology exists to address the problem. Wood burning cooking and heating devices - used widely in the developing world - can be upgraded to burn more cleanly. We can switch away from diesel fuels, or use more efficient, low-emission diesel engines.
And there are other measures that can help reduce the volume of soot released into the atmosphere. "You can certainly put on particle traps on vehicles, off road equipment, passenger vehicles and buses, trucks. But in terms of controlling CO2 and soot, which I think really we want to do, it is really changing the energy infrastructure and the vehicle infrastructure."
That would mean a move to electric or hydrogen vehicles, powered by clean energy sources. Jacobson says that with concerted national and international efforts, soot levels in the atmosphere could be reduced by 90 percent in five to 10 years.