I well remember the huffing and puffing over the release of James Hansen’s paper “Target Atmospheric CO2: Where Should Humanity Aim?” :-
“…Decreasing CO2 was the main cause of a cooling trend that began 50 million years ago, large scale glaciation occurring when CO2 fell to 425 +/- 75 ppm…”
The sceptic-deniers laughed and scoffed and said things to the effect that clearly there’s nothing to worry about that the current concentration of Carbon Dioxide in the air is over 390 parts per million – it won’t melt the polar ice caps.
What the sceptic-deniers haven’t understood, or pretend not to have understood, is that it is a combination of factors that caused major lasting glaciation on Earth. Yes, the level of Carbon Dioxide in the air is important. But the rate of change of Carbon Dioxide in the atmosphere is a significant component.
If the levels of Carbon Dioxide in the atmosphere change rapidly, the heating or cooling effect is amplified, in effect. You have to take account of the relative change in levels of Carbon Dioxide, not just its level at any particular point in time.