How to Slice a Global Carbon Pie?

In its draft form, the fought-over paragraph declared that, to have the best chance of not exceeding the international target for global warming of 3.6 degrees Fahrenheit, society can burn no more than about 1 trillion tons of carbon, in the form of fossil fuels, and spew the resulting gases into the atmosphere.  More than half that carbon budget has been used already.  Moreover, the draft made it clear that if countries want to be safe and take account of other gases that are warming the planet, the carbon budget would be even less than a trillion tons.  At the rate things are going, we will exceed the budget in 30 years or fewer.

“It was inconveniently simple,” Dr. Reto Knutti, a key member of the Intergovernmental Panel on Climate Change,  would say a few days later.

How to Slice a Global Carbon Pie?