See title. With the rise to 455 ppm of CO2 (and likely higher for a while), I wonder what the lowest Earth’s had, was.
If I’m not mistaken, the highest was during the Permian-Triassic mass extinction, in where the Siberian Traps may have been responsible for making burrowing – and hence adaptations for stale air – necessary?
Not a scientist or geologist. But I can recommend the Wikipedia article about Carbon dioxide in the atmosphere. Or one of the TV documentaries about the ice core drillings in the Antarctic.