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Does dirty air cool the climate?

Study adds a factor to climate-change debate.

By Peter N. SpottsStaff writer of The Christian Science Monitor / June 30, 2005

Over the past several decades, industrial countries have made major strides in cleaning up pollutants roiling from smokestacks.

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But some researchers now say this progress could have a troubling side effect - accelerating the pace of global warming.

The reason: Tiny pollutant particles, once airborne, can reflect sunlight back into space, easing temperatures in what is known as aerosol cooling. By cleaning up industrial pollution, countries are reducing the effect of this cooling.

Nobody is recommending that nations halt efforts to curb pollution.

Still, when this factor is taken into account, global warming could outpace the level now forecast by climatologists, a team of European climate scientists reports in Thursday's edition of the journal Nature. Already, climate estimates sponsored by the United Nations foresee average temperatures rising by about 10 degrees Fahrenheit by 2100.

The study doesn't represent a formal forecast, but the team says it highlights the importance of pinning down the impact of twin changes in the atmosphere: As aerosol pollutants play an increasingly small role in affecting temperatures, the effect of changes in greenhouse-gas levels will play an even more significant role.

"Scientists must pay more attention to this issue as they do more complex modeling, and the public needs to be aware of the implications," says Meinrat Andreae, who led the research.

The team made calculations based on a simplified set of equations describing how the climate system works. Indeed, other researchers liken the effort to a somewhat sophisticated back-of-the-envelope calculation scientists often perform to see if a phenomenon is worth a second look.

The results, published a week before the G-8 summit in Scotland, are likely to add urgency to British Prime Minister Tony Blair's efforts to keep climate change a headline issue.

But Mr. Blair has been unable to convince President Bush to support mandatory targets and timetables for emissions reductions. The White House's preferred approach instead focuses on reducing the US economy's carbon intensity - the amount of carbon emitted per unit of economic output - by 18 percent over 10 years. It relies heavily on research and market forces to encourage adoption of technologies to achieve its goals.

The European research team's call for more research also comes as the United Nations sponsored Intergovernmental Panel on Climate Change (IPCC) is in the midst of preparing its next set of climate-change reports, set for release in 2007. Researchers had to submit results to journals by mid-May - deadlines that suggest a flurry of new studies relating to the global climate will come out over the next year.

Researchers are aiming to narrow uncertainties linked to climate sensitivity and man-made aerosols, because sensitivity determines how much warming could be expected as concentrations of carbon dioxide and other heat trapping "greenhouse" gases in the atmosphere grow. Even from the effects of what humans already have pumped skyward, many researchers believe the atmosphere's temperature will increase by 2 degrees by 2100. That number would rise with continued greenhouse emissions.