During each of the last four glacial advances, CO2 concentration fell below 190 ppm. At the end of the last glacial advance, it fell to 182 ppm, thought to be the lowest in the Earth’s history. Why is this alarming? Because below 150 ppm, most terrestrial plant life cannot exist. We came within about 30 ppm (30 molecules out of … Continue reading In the last four glacial advances, the CO2 level was dangerously low.