Dirty air not stopping global warming

HALIFAX, Nova Scotia, Oct. 31 (UPI) -- Sun-blocking soot particles from air pollution do not cause nearly as much atmospheric cooling as previously estimated, Canadian researchers reported Thursday.

The findings mean the use of manmade pollutants could not be used as an effective countermeasure to offset global warming.

"What this means for politicians is not, 'Well, if you have enough air pollution, you can reduce global warming, so we don't have to reduce greenhouse gas emissions, we can just pollute the atmosphere more,'" lead researcher Ulrike Lohmann, an atmospheric scientist at Dalhousie University, told United Press International. "The cooling effect attributed to air pollution is smaller than thought."

Industrial activity, vehicular traffic, forest fires and agricultural burning all produce tiny carbon- and sulfur-loaded soot called aerosols that cling to microscopic water droplets, forming clouds that reflect sunlight back into space. However, the researchers explained, the droplets attracted by the aerosols are very small and take longer to concentrate into rain clouds than the droplets formed by naturally occurring particle emissions.

Although previous studies had shown air pollution is shrinking cloud droplets worldwide, scientists have remained uncertain how much global cooling the pollutants could generate.

"A range of estimates from the scientific community included such large cooling that you would not have seen any warming in the last 50 years," Lohmann said. The physics of such tiny particles are hard to scale up to planetary models, she said, and "with models, you can only simulate all the processes that you understand and know."

Based on Japanese satellite observations, Lohmann and colleague Glen Lesins write in the Nov. 1 issue of the journal Science, manmade pollutants block out about 40 percent less sunlight than previously estimated.

"There seems to be a leveling-off effect that models could not predict. Up to a certain point, it doesn't matter how much aerosols you put in, you can't reduce cooling any more," Lohmann told UPI. "Prior models didn't simulate this leveling-off well enough and overestimated the cooling effect."

Although aerosols still blot out a significant amount of solar radiation, that amount is not enough to prevent the global warming expected by previous research. Armed with the Canadian findings, atmospheric scientist Steve Ghan, of the Pacific Northwest National Laboratory in Richland, Wash., said researchers now can create more accurate models of global climate.

"These aerosols only have a short lifetime in atmosphere, while carbon dioxide has a lifetime of a hundred years or so," Ghan explained. Carbon dioxide is a greenhouse gas that absorbs solar heat and warms up the Earth. "So while we might be able to compensate for greenhouse warming by adding some more aerosols to the atmosphere now, in 100 years the greenhouse gas concentrations that have accumulated will dominate any conceivable aerosol levels. It's not a long-lasting solution."

Moreover, since soot results in cloud droplets that do not easily rain out, rainfall is suppressed in places that need it most. "In a place that rains all the time, like the Northwest United States, or in places like the Sahara, you won't notice it much. But in the margins like the Middle East, that's where the effect will be most noticeable."

(Reported by Charles Choi, UPI Science News, in New York)