"This new way of integrating the aerosol, greenhouse gas and biosphere effects changes the picture from one where climate change most likely is a fairly tolerable thing to one where there is a fairly high risk of change sooner, and to a higher degree," said Professor Meinrat Andreae.

The Intergovernmental Panel on Climate Change (IPCC) predicts a rise in global temperatures from a doubling of carbon dioxide could be in the range of 1.5-4.5 degrees Celsius by the end of the century. But according to calculations by Andreae and his team, the upper figure could be as high as 6 degrees.

"That's quite a lot," the professor from the Max Planck Institute for Chemistry in Mainz, Germany said in an interview.

Andreae compared greenhouse gases to an accelerator that is speeding up global warming, while the aerosols act as a type of brake.

"The actual true force of the greenhouse gases has been masked by the effects of the aerosols. They put a brake on warming and we don't really know how strong that brake is," said Andreae, who reported his findings in the journal Nature.

Scientists have warned that severe climate change could lead to a rise in sea levels, flooding, severe droughts and the loss of crop and animals species.

Aerosols are small particles and droplets in the air from combustion processes, chemicals and smoke. As regulatory agencies issue new air purity controls, the amount of aerosols will diminish so their cooling effect will be smaller.

Aerosols stay in the atmosphere for about a week but greenhouse gases accumulate over about 50 years. The aerosol brake is going to come off faster than the decrease in greenhouse gases.

"Because one is cumulative and the other is not, the cumulative will always win out in the long run," said Andreae.

He admitted it was a situation of high scientific uncertainty. But if his calculations are correct, climate change in the 21 century could reach the upper extremes or exceed the IPCC estimates.