The easy way to explain Ice Age conditions is to posit an ON PERIOD where there's 2 mile deep ice on North America and an OFF PERIOD where there's an interglacial period like we have come to know and love.
The harder way is to note that it's not all ice all the time, nor is it warm all the time. The cycle continues throughout the period of performance ~ and sometimes we have a lot of ice, sometimes a moderate amount, and sometimes it all goes away, but even then there are times when it starts to come back.
Plus, we have TWO major controlling elements. One is North America and the other is Antarctica. As you will notice Antarctica will has thousands of feet of ice but it doesn't have a 2 mile deep ice sheet. Neither does North America. Both areas have sea ice, either in the Arctic Ocean basin or around Antarctica.
We need to worry when Antarctic sea ice increases its coverage AND when the ice gets deeper (which we'll know because the glaciers flowing to the sea will speed up).
I think that already happened.
Antarctica glaciation depth estimates range from 2,100-4,500 meters - or 2.1 to 4.5 kilometers respectively.
Major controlling elements are ocean temperatures and solar radiation - land areas are simply where the effects are manifest. Did you know that during the Ice Age that the bottom of South America also had enormous ice sheets?
However, the salient point about those long gone glaciers is that when they finished melting, sea levels world wide rose 400-700 feet. That's a LOT of fresh water. AGWer strike fear in the public’s heart crying that when the ice caps melt seal levels will rise an astounding 40 feet! Our ancestors should have been so lucky.