To avoid dangerous climate change of 2C, the world can only burn another half a trillion tonnes of carbon, climate change experts warn

The world has already burned half the fossil fuels necessary to bring about a catastrophic 2C rise in average global temperature, scientists revealed today.

The experts say about half a trillion tonnes of carbon have been consumed since the industrial revolution. To prevent a 2C rise, they say, the total burnt must be kept to below a trillion tonnes. On current rates, that figure will be reached in 40 years.

Myles Allen, a climate expert at Oxford University who led the new study, said: "Mother Nature doesn't care about dates. To avoid dangerous climate change we will have to limit the total amount of carbon we inject into the atmosphere, not just the emission rate in any given year."

The scientists say their research could simplify political attempts to tackle global warming, which encompass a range of targets and timetables. Such proposals usually set future limits on the amount of carbon dioxide allowed to build up in the atmosphere, such as 450 parts per million (ppm), or as future emission rates, such as the UK's pledge to slash emissions 80% by 2050.

The new study effectively re-frames such targets as an available budget - to avoid dangerous climate change of 2C the world can only burn another half a trillion tonnes of carbon.

Writing in today's Nature, Allen and colleagues say a trillion tonnes of carbon burnt would be likely to produce a warming of between 1.6C and 2.6C, with a "most likely" 2C rise.

Chris Huntingford of the NERC Centre for Ecology and Hydrology said: "Research often reveals new complexities, but this analysis could actually simplify matters for policy makers. The relationship between total emissions and future warming can be inferred largely from quantities we can observe, and is remarkably insensitive to the timing of future emissions."

The key implication of the research, the scientists say, is that access to fossil fuels must somehow be rationed and eventually turned off, if the 2C target is to be met. "If country A burns it then country B can't," said Bill Hare, a climate expert with the Potsdam Institute in Germany. "It's like a draining tank."

The research also highlights that continued high rates of fossil fuel use in the next decade will demand extraordinary cuts in emissions in future decades to hit the 2C target. Allen said: "If you use too much [carbon] this year, it doesn't mean the planet will come to an end. It means you have to work even harder the next year."

A separate study, also published today in Nature, led by Malte Meinshausen at the Potsdam Institute, use a similar approach and sets a different carbon budget. They say the world can only emit 190bn tonnes of carbon between now and 2050 if it aims for a 2C rise. Emissions over 310bn tonnes in that time lead to a 50% chance of going over 2C.

The new research does not say anything about the likelihood of reaching the 2C target. They simply change the way progress towards the target is measured.

In an accompanying commentary article, the scientists behind both studies say: "These results are not incompatible with current proposals for near-term emission targets -- the small size of the cumulative emission budgets to 2050 reinforces the need for global CO2 emissions to peak around or before 2020 so that emission pathways remain technologically and economically feasible."

They add: "Having taken 250 years to burn the first half trillion tonnes of carbon we look set, on current trends, to burn the next half trillion in less than 40. No one could credibly suggest that we should carry on with business as usual to the 2040s and then somehow suddenly stop using fossil fuels, switch to 100% carbon capture or just shut down the world economy overnight."