In the last four ice ages, the CO2 level was dangerously low.
During each of the last four ice ages, CO2 concentration fell below 190 ppm. At the end of the last ice age, 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 every one million) to the extinction of most plant life on land, and with it the extinction of all higher terrestrial life-forms that depend on it. Bear in mind that, before we began adding CO2 to the atmosphere, we weren’t sure that we wouldn’t cross that critical 150-ppm threshold during the next glacial period.