Climate Central: The last time there was this much carbon dioxide (CO2) in the Earth's atmosphere, modern humans didn't exist. Megatoothed sharks prowled the oceans, the world's seas were up to 100 feet higher than they are today, and the global average surface temperature was up to 11°F warmer than it is now. As we near the record for the highest CO2 concentration in human history 400 parts per million climate scientists worry about where we were then, and where we're rapidly headed now. According to data...