6000 ppm Co2 was a thing at a prior time. That’s about fifteen times the current value. The Co2 thermal augmentation effect is tied to a logarithmic progression, so most of the heating happens with low percentage values of the gas in the mix. Water vapor is the real driver of heat retention.
Ocean circulation was different such that more warm surface flows similar to the Gulf Stream, carried oceanic warmth to the polar regions. The formation of the Isthmus of Panama made a large contribution to cooling.
I have read about the closing of the connection between the Atlantic and the Pacific by the Isthmus of Panama changing ocean currents and the weather.