December 16, 2017
The world can no longer postulate a scenario that maintains global temperature rise at or below 4 degrees C, without significant removal of existing CO2 from the air. Based on the amount of CO2 already in the atmosphere (over 406 ppm) and the steady increase in CO2 emissions to the atmosphere, even the best possible efforts at reduction will fail to achieve a halt to warming at or below 4 degrees C.
The climate change crisis is so far advanced that even drastically cutting greenhouse gas emissions won’t prevent a convulsive future by itself — the amount of greenhouse gases already in the atmosphere ensures dire trouble ahead. A forward-looking calculation might postulate a need to return to 350 ppm and acknowledge that by 2040 we will be at 450 ppm.