Global warming braked less than expected by haze

Alister Doyle, Reuters 18 Jun 09;

OSLO (Reuters) - 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.

COOLING

"The direct aerosol effect may have contributed to a cooling in the mid 20th century and may have masked a considerable degree of current global warming," he wrote.

That could potentially lead "to more rapid warming in the future owing to stricter controls on aerosol emissions."

More than 190 governments have agreed to work out a new U.N. pact to fight global warming at a meeting in Copenhagen in December, mainly by cutting use of fossil fuels.

Myhre said his study reconciled big differences between satellite data and estimates based on computer models. Satellites, for instance, were not good at gauging dimming over bright surfaces such as deserts. Clouds also block sunlight.

"This reduces the range of uncertainty ... about the scattering of solar light," Myhre said.

Human-caused aerosols add to natural particles including dust from volcanic eruptions, sandstorms from the Sahara desert or salt from sea spray that dominated before widespread use of fossil fuels began in the 18th century.