Prior to the use of Fossil Fuels, it is thought that CO2 increased as an effect of warming. It is happening today in the same way.
During warmer periods, plants grow and die as is the natural order of things. In Northern areas like Siberia, these dead plants and the carbon they contain fall on the ground and are subsequently trapped in the permafrost terrain.
When warming thaws the Permafrost, CO2 outgasses from the Permafrost. The warming causes the rise in CO2 in nature.
Conversely, when the ice age cooling begins, it causes atmospheric CO2 concentrations to drop.
All of this is published by scientists. Why do you deny science?
When permafrost melts, what happens to all that stored carbon?