As any carbonated beverage warms, the solubility of Carbon Dioxide in the water in the beverage declines (warm beer goes flat quicker).
As the planet warmed, CO2 dissolved in fresh water and the oceans came out of solution and went into the atmosphere, which is why CO2 is a trailing indicator, not a forcing component. When it got colder, more CO2 would remain in solution, because it is more soluble at lower temperatures.
This whole thing about CO2 being a pollutant has been bogus from day one. Unless you are working with it in an enclosed space--or a Hell of a lot of it in the open, it isn't a problem, and not driving the climate either way.