The Bottom Line - Carbon dioxide (CO2) concentrations in the atmosphere now stand at 385 parts per million (ppm), 67% above the average of the past 800,000 years, and 29% above the maximum of the same period. - The rate of increase of CO2 is 2.2 ppm/year and getting faster. When atmospheric CO2 was above 450 ppm in the earth’s history, the planet was completely ice-free, and sea level was 75 meters higher than today. - The safe upper limit of atmospheric CO2 that we should aim to stay below is probably between 300-350 ppm. We have already overshot it and the longer we stay above that the more likely we will have set in motion long-term climate changes that will proceed beyond human control through powerful geophysical positive feedback mechanisms. - Sudden warmings of a few degrees in the past have triggered mass extinctions. Current species extinction rate is estimated at 100 to 1000 times the average extinction rates in the evolutionary time scale of planet Earth. A mass extinction is underway and we have to reverse course quickly enough to prevent the worst.