...please don't read this if you already have a headache...


Pollution is protecting us from harsh rays of the sun
By Ian Sample in London
December 23, 2005

CUTTING air pollution could trigger a greater surge in global warming than previously thought, suggesting future rises in sea level and other environmental consequences have been underestimated.

Scientists have issued the warning after investigating the effect of aerosols on climate. Aerosols - particles smaller than 100th of a millimetre - are churned out from factories, the burning of fossil fuels and forest fires, although sea salt and dust particles from desert storms add to them.

Because the particles are so light they remain aloft for long periods, where they cool the earth by reflecting radiation from the sun back out to space. Higher levels of aerosols lead to the formation of clouds made up of smaller water droplets, which reflect still more of the sun's radiation. Cutting down on aerosols by improving air quality means the earth will be less shielded against the sun's rays.

Writing in the journal Nature, scientists at the British Meteorological Office and the US Government's National Oceanic and Atmospheric Administration report that climate models used to predict future global warming have badly underestimated the cooling effect of aerosols.

"We found that aerosols actually have twice the cooling effect we thought," said Nicolas Bellouin, a climate modeller at the British Meteorological Office. "The consequence is that as air quality improves and aerosol levels drop, future warming may be greater than we currently think."

Scientists had assumed that the amount of sunlight reflected by aerosols from human activity was tiny compared with the extra reflective cloud cover they caused, but Bellouin's research suggests they are equally important. Scientists will have to feed the new information into their models before they can be sure of the implications for global warming.

One possibility is that while the latest study shows scientists have underestimated the direct effect of aerosols reflecting the sun's rays, they may have overestimated the indirect effect they have on cloud cover, meaning the overall error of climate models would not be serious.

Earlier this year, Peter Cox at England's Centre for Ecology and Hydrology in Winfrith, Dorset, warned that if the cooling effect of aerosols turned out to be greater, it could trigger faster global warming.

"It's quite a bizarre thing, because the last thing you want to suggest is that it would be a good idea to have dirty air, but as far as climate change is concerned, that's right. Everyone would be getting asthma, but the environment would be cooler," said Professor Cox.

"That said, the direct effects of air quality, particularly in urban areas, are so important to human health that it would be crazy to think of anything other than health damage."

If the Met Office calculations are right, they suggest the atmosphere's temperature is more responsive to carbon dioxide than scientists believed.

"If the cooling influence of aerosols is larger, it implies that the warming from the carbon dioxide must be larger than we think to match the warming we've seen in the past 100 years," Professor Cox said.

"And if that's the case, future climate change will be more than we have expected with air quality improvements."

The Guardian