'Pollutants keeping global warming at bay'
(China Daily/The Guardian)
Updated: 2005-12-23 06:15

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, climate scientists reported yesterday.

The warning comes after researchers investigated the effect of fine particles known as aerosols on climate change. Aerosols particles smaller than one-hundredth of a millimetre are churned out from factory chimneys, from the burning of fossil fuels and forest fires, although sea salt and dust particles swept up by desert storms add to levels detected in the atmosphere.

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 brighter clouds made up of smaller water droplets, which reflect still more of the sun's warming radiation.

Cutting down on aerosols by improving air quality means that the Earth will in future be less shielded against the sun's rays.

Writing in the journal Nature, scientists at the 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 Met Office. The consequence is that as air quality improves and aerosol levels drop, future warming may be greater than we currently think."

Bellouin's study suggests that even by conservative estimates, climate models have got the impact of aerosols on the climate wildly wrong. "The discrepancy between the models and our observations is not good news," he said.

The scientists used images from a US satellite called Modis to look at how much sunlight aerosols in the atmosphere reflect back to space on cloud-free days. Using another satellite called Toms, they were able to separate readings for the effect of smaller aerosols produced by natural processes from those produced by human activity.

Scientists had assumed that the amount of sunlight reflected by aerosols from industry and fuel burning was tiny compared to the extra reflective cloud cover they caused, but Bellouin's research suggests the processes are equally important. Bellouin says climate scientists will have to plug 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 so-called 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 the 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 to people 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.

"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," he said.

The Guardian

(China Daily 12/23/2005 page1)