Humanity's carbon budget set at one trillion tonnes

• New Scientist, 29 April 2009 by Catherine Brahic
• http://www.newscientist.com/article/dn17051-humanitys-carbon-budget-set-at-one-trillion-tonnes.html
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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.

Good effort, but try harder

Both papers show that, because CO₂ takes so long to disappear from the atmosphere, governments need to aim for a global zero-carbon economy in the long run. This confirms results from earlier studies.

Regarding shorter-term goals, Meinshausen says that to have a good chance of staying below 2°C, global emissions must start falling after 2015. Achieving this will be no small feat: at present we emit between 1% and 3% more each year than we did the year before. That trend must be reversed within six years.

The researchers say that even the most ambitious climate plans tabled so far must be pushed further. The G8's aim is that by 2050 we will have cut emissions to half their levels in 1990, but even that may not be enough. Depending on the timing, the cutbacks would have to be closer to 70% below 1990 levels, studies suggest.

Journal references:

Nature, DOI: 10.1038/nature08019

Nature, DOI: 10.1038/nature08017