Photosynthesis caps CO2 – at the modern level of 2 parts per million.
In the little ice-age of the 18th century, it was at 4ppm. Just as man started burning coal. A global glaciation.
In the Jurassic photosynthesis was less evolved. There was 4ppm CO2 in the air, 65% more life on Earth, and global sea levels were 60m LOWER.
There were 3 natural ice-ages, with CO2 at 8ppm. So NATURAL CO2 levels rise in NATURAL GLOBAL COOLING. Man's extra CO2 released by burning the Fossil Fuels, has increased life on Earth – but global average CO2 in the air is capped at 2ppm.
It is at 4ppm, above the Arctics, at -50oC, in the winter.