The Economist:
For the first time, the IPCC has set what is usually called a carbon budget. To have a two-thirds chance of keeping global warming below 2ºC, it says, “will require cumulative CO2 emissions from all anthropogenic sources to stay between 0 and about 1,000 [trillion tonnes]”.
The world has already blown through just over half that amount (531 trillion tonnes) by 2011. At current rates of greenhouse-gas emission, the rest of the budget will have been spent before 2040.