In the ice ages, the concentration of CO2 in the atmosphere increased after increases in temperature, so CO2 can't be causing warming.
My short answer would be to say
Hey, how many coal-fired thermal power stations were running in the ice ages? Yeah, thats right, None!
But there is a Climate Crock of the week Youtube video explaining that the initial trigger (or forcing) for the ice age cycles has been the Earth's axis and that CO2 can be both a forcing and a feedback amplifying the impacts of the tilt and spin changes. And, as ever, James Hansen's work plays a key role.