"A doubling of carbon dioxide concentrations would cause a global temperature increase of around 3 Celsius," said Thomas Schneider of the Potsdam Institute for Climate Impact Research who led the report.

The findings broadly back up other Potsdam forecasts about the effects of a build-up of carbon dioxide emitted by power plants, cars and factories. Some sceptics dismiss such models as exaggerations.

Temperatures have already risen by 0.6 Celsius since before the Industrial Revolution in the 18th century. Many scientists project that higher temperatures will cause more heatwaves, droughts, floods and rising sea levels.

Greenhouse gas concentrations are likely to double from pre-Industrial levels this century unless the world drastically cuts energy use and shifts to clean wind or solar power.

The Potsdam scientists worked out 1,000 climate model versions, each with different assumptions of the behaviour of clouds, ocean currents and other factors.

They then checked the likelihood of the scenarios against climate shifts at the end of the Ice Age -- carbon dioxide trapped in air bubbles in ice and the chemical makeup of marine sediments which gives clues to temperatures.

Schneider said the study, published in the journal Climate Dynamics, indicated that the outer ranges of likely temperature rises were 1.2-4.3 Celsius if carbon dioxide levels doubled.

Still, he said the study meant temperatures were unlikely to rise by six or seven degrees, as some studies had suggested.

The European Union wants to limit any rise in temperatures linked to global warming to 2 Celsius -- a threshold it sees as triggering "dangerous" climate change.

Carbon dioxide levels were far lower at the end of the last Ice Age than in the 18th century. Today, concentrations are at their highest level for at least 650,000 years.