Humanity's carbon budget set at one trillion tons

No more than one-quarter: that's the proportion of existing reserves of oil, gas and coal that we can burn if we are serious about keeping the planet from warming by 2°C or more.

These are the conclusions of the most comprehensive efforts yet to pin down just how much carbon dioxide can be emitted into the atmosphere.

If governments are to stick to their pledge to avoid "dangerous" global warming – which most politicians and many scientists take to be no more than 2°C – the models come up with roughly the same answer. Humans must not inject more than 1 trillion tonnes of carbon into the atmosphere in total.

That, say teams led by Myles Allen of the University of Oxford and Malte Meinshausen of the Potsdam Institute for Climate Impact Research in Germany, will give us a 50:50 chance of limiting global warming to 2°C.

To improve the chances that the planet remains this side of 2°C, Meinshausen's study suggests we should emit no more than 750 billion tonnes of carbon in total. The risk of exceeding 2°C would then drop from 50% to 25%.

Halfway there

Industrial activity since the mid-18th century means we have already emitted 500 billion tonnes of carbon – half of the 1-trillion-tonne budget. "At some point in the last few years, we released the 500-billionth tonne of carbon," says Allen. We can afford to dump only 250 billion tonnes more – or perhaps 500 billion tonnes, if we are willing to run the higher risk.

So how much longer have we got? Don't let past emissions fool you, says Allen. "It took 250 years to burn the first 500 billion tonnes. On current trends we'll burn the next 500 billion in less than 40 years."

Busting the budget

That means that if we continue emitting carbon at the same rate as we are now, we will exhaust what Allen calls the trillion-tonne "carbon budget for the human race" by 2040. Anything that is emitted beyond that will commit the planet to more than 2°C of CO2-induced warming.

Meinshausen and colleagues calculate that we could exhaust the carbon budget within as little as 20 years. They also find that if we were to burn all the proven reserves of fossil fuels, this would inject nearly three times the carbon budget into the atmosphere.

To have a 75% chance of keeping to the 2°C target, "we can burn less than one-quarter of known economically recoverable fossil-fuel reserves between now and 2050", says Bill Hare of the Potsdam institute. "This means that whilst a lot of the oil and natural gas can be burned, certainly not much at all of coals reserves can."

None of these figures include "unconventional" fossil fuel reserves, such as tar sands.

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