Andreae says aerosols are a "brake" on warming

The Earth could heat up more quickly as the cooling effect of this pollution is reduced, Professor Meinrat Andreae told a London conference on Monday.

He said aerosols - tiny particles of, typically, sulphur or carbon held in the air - helped to cool the planet.

But since these were expected to diminish in the coming decades, global warming would accelerate, he claimed.

The aerosols come from natural sources, such as volcanoes - but a substantial load also comes from human sources, such as the soot from oil and wood burning.

Aerosols are an important but uncertain agent of climate change. By absorbing or scattering radiation, they can either warm or cool the troposphere. They can also modify clouds and affect precipitation.

Professor Andreae, of the Max Planck Institute in Mainz, Germany, told the 13th World Clean Air and Environmental Protection Congress that the overall cooling effect was, in his view, currently dominant and offsetting the warming brought about by greenhouse gases.

"We've been in an accelerating car with one foot on the brake and one foot on the gas," he told the BBC.

The scientist said the "climate protection" provided by aerosols was likely to diminish in the future.

"The aerosol particles don't stay in the atmosphere for very long, so we don't expect their concentration - their effect - to grow over the next century.

"The greenhouse gases, on the other hand - carbon dioxide and methane - they keep accumulating in the atmosphere because they have long lifetimes.

"Whether we want it or not, the warming forces are going to overpower the cooling forces and the big question now is just how strong that effect is going to be."

Predictions of the rise in global temperatures may therefore have to be revised upwards, Professor Andreae argued.

The US space agency's Aura satellite was recently sent into orbit with a specific task to unravel aerosols' precise impact on the global climate.