Excerpts from Books and Wikipedia
In the December 2015 Paris Agreement, the world’s nations unanimously agreed to keep ratcheting down carbon pollution to keep total warming 'well below 2°C.' . . . Roughly speaking, to have a significant chance—greater than 50%—of keeping total warming below 2°C, we need to be on a path that cuts emissions of carbon dioxide and other major GHG pollutants by more than 50% by mid-century, which in turn means that global GHG emissions must peak in the mid-2020s or so and start a rapid decline. That decline must continue through century’s end so that by 2100, the world’s total net emissions of GHGs needs to be zero, and preferably below zero, especially if we delay serious action much longer. The goal is to stabilize atmospheric concentrations of CO2 below 450 parts per million." [Romm, p. 159]