# How far to Armageddon? About half a trillion tonnes apparently...

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| 14:47 UK time, Thursday, 30 April 2009

Since the start of the 20th century, the Earth's climate is reckoned to have warmed by around 0.74°C, plus or minus 0.18°C. Common consensus suggests that a rise of more than 2°C from the same starting point would be undesirable, resulting in surging sea levels, more extreme weather events, water shortages, extinctions, generalised Armageddon, etc.

That leaves us roughly 1.26°C to play with (give or take a margin of error) before we get into the rough stuff. Scientists and policymakers are constantly searching for ways to quantify what this means in terms of reducing output of greenhouse gases.

The focus has long been on setting targets for reduction. For example the European Union recently announced a 'binding' commitment to reduce greenhouse emissions by 20% by 2020. But how these targets relate to keeping us below 2°C remains unclear, especially when binding commitments run up against the cold hard reality of things like a massive global recession and the need to maintain a competitive economy.

What we really need is an absolute measure of just how much carbon we can continue to chuck out before we bust the 2°C limit. Happily, a collection of scientists led by Myles R Allen of Oxford University's Atmospheric, Oceanic and Planetary Physics Department has just calculated exactly that number and published the results in the journal Nature.

By their reckoning, we've already burnt half a trillion (that's half of a million millions) tonnes of carbon and we can probably afford to burn another half trillion before sending temperatures beyond 2°C. (For anyone who thought we were in danger of running out of fossil fuels, we're not. Half a trillion tonnes of carbon represents just a quarter of our known reserves.)

It took roughly 250 years to burn the carbon we have already burnt, but Myles R Allen and friends think we could clear the next half trillion before 2050, based on current projections.

What conclusions can we draw from this report? First is that we probably need to: a) leave most of the coal in the ground; b) find ways to capture the emissions from the coal-fired power stations we have already planned to build; and c) find a way of reconciling this limit with the cold hard reality of things like a massive global recession and the need to maintain a competitive economy. Oh, hang on a second...

Related blogs: 'Since records began': a brief guide to who's taking the temperature

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