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Airborne Soot Is Significant Factor in Global Warming, Study Says

There's more of it and it's a much bigger part of the problem than previously thought, scientists find. Others call for more research.

May 15, 2003|Gary Polakovic | Times Staff Writer

Soot in the sky is more plentiful worldwide than experts previously thought and is contributing to the rapid heating of the Earth's atmosphere, according to new research by a team of U.S. scientists.

Microscopic carbon particles in air pollution have long been linked to respiratory ailments, but scientists are trying to improve their understanding of how smoke in the air interacts with sunlight and chemicals to influence global warming.

The team of researchers at NASA, Columbia University and Lawrence Berkeley National Laboratory determined that soot probably contributes twice as much to global warming as was estimated two years ago by an international panel of experts.

A rapid rise in worldwide temperatures over the last 50 years may be largely due to smoky particles in the air, the study says.

Furthermore, the scientists warn that unless soot is reduced as rapidly as other air pollutants, including sulfates and nitrates that deflect sunlight from the planet, the globe could warm even more rapidly.

The study appeared this week in the journal Proceedings of the National Academy of Sciences.

"The study indicates there is a lot more black carbon in the atmosphere than we thought," said Dorothy Koch, a coauthor of the paper and an atmospheric scientist at NASA's Goddard Institute of Space Studies.

"All black carbon does is absorb sunlight. If you put more into the atmosphere, you increase the warming."

But other scientists say too little is known about sooty pollutants to reach firm conclusions.

"Soot is the wild card," said Stanford University climatologist Stephen Schneider. "And this stuff can get pretty uncertain. It's very complex. It could have any number of effects."

Among the outcomes, soot particles might help form clouds that reflect sunlight, or concentrate in clouds, heat them up and evaporate them to contribute to global warming.

Also, the effect depends on how high soot accumulates in the atmosphere and the landscape below it, Schneider said.

Understanding airborne soot is critical to determining how Earth's climate has been altered since the Industrial Revolution and how it might change in the future.

Other pollutants, such as carbon dioxide or aerosol sulfates, can offset one another, leaving smoke in the air as a key variable, the researchers concluded.

About 1 million tons of so-called black carbon is floating above the Earth.

About 40% comes from fossil-fuel power plants and factories and diesel-powered vehicles in industrialized countries.

The rest comes from smoke from forest burning or stoves using wood or dung to cook and heat homes in developing nations.

Rather than rely on computer models or emissions estimates, the scientists used an alternative approach that relied on a global network of 100 sun- and sky-scanning devices, known as Aeronet.

It enabled them to zero in on soot particles by measuring how they affect rays of light.

They found the amount of sunlight absorbed by soot was two to four times greater than previously assumed.

The researchers concluded that carbon accumulates inside other types of floating specks, acting almost as an inescapable spider web for any sun rays that enter the particle.

The scientists also warned that reductions in gases that contribute to smog, haze and acid rain must be done in concert with cuts in sooty particles, or else global warming could be accelerated.

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