A Carbon Budget, and We're Halfway There
For the first time, the IPCC warned that the world needs to stick to a "carbon budget," and it specified the levels of future emissions that are allowable in order to have a reasonable chance of staying within 2 degrees Celsius (3.6 degrees Fahrenheit) of warming, the safe climate target.
In short, humans can only burn about 1 trillion tons of carbon before 3.6 degrees Fahrenheit of warming becomes inevitable. Nations have already consumed over half of that amount since the beginning of the Industrial Revolution.
By using the "carbon budget" concept, the IPCC endorsed the gist of the Potsdam Institute for Climate Impact study of 2009, popularized by climate activist Bill McKibben's Rolling Stone "Do the Math" article.
Credit: Señor Codo, Flickr