If we were to continue CO2 emissions to an atmospheric concentration of 800 ppm of CO2, the IPCC formula translates into an ultimate average temperature change of 4.5 C (8.2 F) with a likely range between 3 C (5.4 F) and 6.8 C (12.3 F). The world has not seen this level of CO2 concentrations for some 50 million years, when crocodiles and palm trees thrived in the Arctic Circle, Greenland and Antarctica were ice-free, and sea levels were many thousands of feet higher than today.So, 600 ppm of CO2 looks a lot more worrisome than 400 ppm of CO2, and 800 ppm of CO2 looks a lot more worrisome than 600 ppm of CO2. The significance of just having blown past 400 ppm is that we seem to be on a business-as-usual growth trajectory that brings us to 800 ppm (or maybe even more) within a century from now.