evidence: Accurate: It is possible that the concentration of atmospheric CO2 was this high a little less than 3 million years ago, but it is certainly true that this was before the appearance of humans. [...] 1. Estimates for CO2 in pre-ice-core time (>~0.8 Ma) are much less certain. So 415 might be as high as the Earth has been in the last 3 million years, but it’s not out of the range of estimates that hasn’t been this high in 15-18 million years. Even for the warm middle-late Eocene (34-56 million years ago) there are CO2 estimates as low as 400-500 ppm (though also many more near 1000 ppm). So it is possible that today’s levels (and especially those of the next few decades) might be unprecedented in the last 45-50 Ma. As with so much news about climate, we try to be "reasonable" from the start, but the upper bounds of our estimates don’t exclude much more extreme scenarios. The eventual equilibrium climate with 500 ppm of CO2 could mean no polar ice and hundreds of feet of sea level rise, whereas it seems extremely unlikely it would be cooler than we think.