Quick questions:
If all the C02 content in oil(underground and above ground),
and coal, and current vegetation, and animals is added up,
what is the total? How much C02 was in the original
inorganic atmosphere since those sources of CO2 were
not around as a “carbon sink”? Could life exist under
those conditions?
I don't have any quick answers. However, you might find some here:
Environmental Effects of Increased Atmospheric Carbon Dioxide