June 21, 2009

Global warming braked less than expected by haze

Air pollution, dust and other tiny particles that can bounce sunlight back into space are braking global warming less than previously believed, a Norwegian study said.

The report, which helps understand how climate change works, said scientific estimates of light-reflecting airborne particles had underestimated a fast build-up of black airborne soot, which has the opposite effect by soaking up heat.

"The black carbon, or soot, emissions have increased fastest," said Gunnar Myhre of the Center for International Climate and Environmental Research, Oslo (Cicero) of the report in Friday's edition of the journal Science.

Soot comes from burning vegetation, such as forest fires to clear farmland from the Amazon to Indonesia, and from burning fossil fuels.

Adjusting for soot and other smaller factors, airborne particles dim sunlight by about 0.3 watts per square meter, less than 0.5 watts estimated by a U.N. panel of climate scientists in a 2007 overview.

That offsets roughly a tenth of the heat-trapping impact of greenhouse gases, mainly from burning fossil fuels, of about 2.7 watts per square meter, Myhre told Reuters.

The main sunlight-reflecting particles from human activities include sulphates emitted by burning fossil fuels in factories, power plants or cars.

Myhre said that tougher pollution laws might lead to cleaner air and so accelerate warming that the U.N. panel says will cause more floods, droughts, rising sea levels, heatwaves and extinctions of plant and animal species.

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