Downside of cleaner air: more warming

Fewer pollutants means average temperatures may rise 8 degrees F. by 2100, new research suggests.

New measurements of tiny particles in Earth's atmosphere contain a sobering message: All those hard-won efforts to cut air pollution may unwittingly accelerate global warming.

The result: The planet is likely to warm more and faster than current projections suggest, according to a team of British and American scientists.

The group has produced the most precise estimates yet of how tiny particles, known as aerosols, could affect the world's climate. Aerosols, which include pollutants, have a cooling effect on the atmosphere, and the team's work suggests that the cooling effect is strong - nearly as strong as the top estimates of the United Nation's Intergovernmental Panel on Climate Change (IPCC).

Thus, the dwindling presence of aerosols means that global average temperatures could rise faster than previously estimated and reach toward the high end of projections for the end of the century.

Those estimates currently range from 2.7 to 7.9 degrees F., depending on how emissions of greenhouse gases and other factors play out in coming years.

The results, published in the current edition of Nature, imply "future atmospheric warming greater than is presently predicted, as aerosol emissions continue to decline," suggests the team, led by Nicolas Bellouin at Britain's Meteorological Office in Exeter.

Aerosols occur naturally as dust blown from deserts, wind-whipped sea salt, and emission from volcanoes. They also come from burning fossil fuels. But scientists have had a tough time discerning aerosols' precise role in affecting climate.

The IPCC has been trying to get a clear picture of aerosols' impact for at least 10 years, "but the results always come up very uncertain," says James Coakley, an atmospheric scientist at Oregon State University in Corvallis. This latest research, showing aerosols' large cooling effect and a narrow range of uncertainty, represents "a shot across the bow. The paper will be controversial."

The research comes as the international community is trying to figure out what collective steps to take to combat global warming after 2012, when the Kyoto Protocol's first commitment period ends.

Much of what scientists understand about aerosols' effects on climate has come from computer models. The new study, though, used data from the Terra and Aqua satellites of the National Aeronautics and Space Administration, combined with ground-based and aircraft measurements. The satellites give the overall amount of aerosols in the atmosphere, while the other measurements help establish the range of particle sizes and their reflective properties. They also help give estimates over land, where current satellite sensors have difficulty.

But measurements of aerosols' "direct" effect on climate don't tell the whole aerosol story, Dr. Coakley notes. The particles also have indirect effects.

For one, they can act as seeds for cloud formation. For another, their size can help determine whether clouds are more effective at trapping heat or at reflecting sunlight back into space. And black-carbon soot, which comes from burning fossil fuels inefficiently, can heat the surrounding air, drying it and suppressing cloud formation.

Teasing out these important secondary effects and factoring them into estimates will probably have to wait until two new satellites, Calipso and Cloudsat, join Terra and Aqua next year, Coakley says.

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