While you're on that subject. The Paleo-Climatologists are having a discussion/debate over what the CO2 concentration was for paleo-Earth. The low estimate, so far, is 10%, or 100,000 ppm. The high estimate is 25%, or 250,000 ppm. It really doesn't matter what the actual number is, what we can say with a great degree of certainty is that the Earth did NOT go into runaway greenhouse mode. Our current CO2 concentration is ~ 400 ppm. One can surmise that we have a LOT of headroom and a considerable amount of time before we have a crisis on our hands.