It was 300 ppm in 1920, 350 ppm in 1998, and now 400 ppm in 2015. The concentration is accelerating, 68 years to 350 ppm, and 15 years to 400. A simple projection suggests our hitting 450 ppm by 2020. 450 ppm is regarded by scientists as an irreversible limit:
If the world is to stay below 2C of warming, which scientists regard as the limit of safety, then emissions must be held to no more than 450 parts per million (ppm) of carbon dioxide in the atmosphereUnless we begin right now
it will become impossible to hold global warming to safe levels, and the last chance of combating dangerous climate change will be "lost for ever", according to the most thorough analysis yet of world energy infrastructure.This 400 ppm limit is a least case. All other uncertainties about the concentration of atmospheric greenhouse gases are worse cases.
I believe what is in store for us is dramatically increasing weather extremes over the next five years as we approach the 450 ppm level. In two or three years, the consequences of global warming won't be debated any more.