Less than a quarter of the world’s proven reserves of fossil fuels can be burnt if the world wants to avoid a high risk of dangerous levels of global warming, research suggests.

The risk of severe climate change is determined mainly by the total amount of carbon released into the air since the start of the Industrial Revolution, rather than by short-term emission rates, according to two studies published today in the journal Nature.

The authors argue that this means the world will eventually have to stop emitting carbon entirely to make up for what we have already pumped into the atmosphere.

The researchers concluded that humankind can emit no more than one trillion tonnes of carbon to avoid the likelihood of a global temperature rise of two degrees celsius, the level widely considered the minimum to avoid widespread harm.

We have burnt through more than half of that since industrialisation began 250 years ago and at current rates we will burn the other half within 40 years, say a team from Britain, Germany and Switzerland.

The scientists believe that their findings could simplify efforts to reduce global emissions by providing an easily understood “carbon budget” that could cut through a tangle of competing targets.

“A tonne of carbon is a tonne of carbon whether its released now or in 50 years' time,” said Myles Allen, head of the Climate Dynamics group at the University of Oxford and lead author on one of the papers.

“We find a simple and predictable relationship between the total amount of carbon we inject into the atmosphere and the peak projected warming in response.”

Dr Allen’s team modelled the warming that would result from burning a trillion tonnes of carbon (about 3.7 trillion tonnes of CO₂), using new data that show carbon does not decay in the atmosphere as simply as previously thought. They found that the most likely result was a warming of two degrees above pre-industrial levels, a number that was affected very little by the timing or rate of emissions.

They add that given the impact of other greenhouse gases and the uncertainty in the model — under which a trillion tonnes of carbon could produce warming of 3.9C — “we may have to accept an even lower limit to have any realistic chance of avoiding two degrees of anthropogenic warming”.

Another team, led by Malte Meinshausen, of the Potsdam Institute of Climate Impact Research, also found that total cumulative carbon emissions were the crucial factor in keeping global warming below two degrees.

They concluded that to keep the risk of warming greater than this to only 25 per cent, the world could from now on afford to release less than a quarter of the carbon stored in the planet's proven, economically recoverable reserves of fossil fuels.

The authors argue that their conclusions reinforce the case for early cuts in emissions. “No one could credibly suggest that we should carry on with business as usual to the 2040s and then somehow stop using fossil fuels, switch to 100 per cent carbon capture or just shut down the world economy overnight,” they write in a commentary paper.

The longer the world lets emissions rise, the harder it will be to reduce them enough to stay within a trillion tonnes. “If you let emissions rise beyond 2020 you’re facing 8 per cent a year reductions even to stay within the trillion target,” Dr Allen said.

Ultimately, rather than trying to stabilise emissions at a lower rate, we will need an “exit strategy” to move to a entirely zero carbon economy, he said. “Reducing emissions in the short term will only work if it’s in the context of a plan to phase out carbon emissions entirely.”

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