The minimum necessary CO2 level for plant life is about 100 ppm. That is a quarter of current levels. No plant life, no animal life, no life on the continents.

The minimum necessary CO2 level for plant life is about 100 ppm. That is a quarter of current levels. No plant life, no animal life, no life on the continents. Only a few chemeotroph bacteria survive. The long term geobiological trend is for C02 to decrease through absorption by weathering rocks and burial of biogenic limestone in subduction zones. C02 was thought to be as much 50% early Earth. Then a percent or two in early Phanerozoic 400 million years ago. And natural about .025% in the current ice age.