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 today, 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."
Dr 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 Dr Bellouin's research suggests
the processes are equally important. Dr 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
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.
If the Met Office calculations are right, they suggest the atmosphere's
temperature is also more responsive to carbon dioxide than scientists
"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.
"And if that's the case, future climate change will be more than we
have expected with air quality improvements," he said.