The time line. Look at a time line. These events did not occur simultaneously.
I was once at that Dome near Tucson, where those environmentalist nut jobs locked themselves in for a year or so. As I was looking at the museum, there was a display which showed the earth temperature over a period of time going back tens of thousands of years (tree rings and Glacial ice core studies help to estimate the temperatures).
The entire curve was almost sinusodial in its pattern, with the mid point being taken as zero, the peaks were +3 F and the valleys were -3 F. Thus a swing of average temperature of 6 degrees determined warm periods from glacial periods.
As I was observing this with my wife, and two women in birkenstocks were standing next to us, I commented: "So the earth has had a period of temperature fluctuations for thousands of years. How can we be sure now that this period is one of global warming and not simply a continuation of the historical patterns?"
The two turned and looked at me, puzzled at such a question, then quickly walked away.
I was labled as a "non believer." I love to piss off the left with reasonable questions.
Change in ocean currents, I would guess. Right now the North Atlantic is warmed by the Gulf Stream - but from what I have read about the Ice Ages, the circulation in the North Atlantic changed, blocking warm water from flowing northward. Hence the ice in Ohio and England. Likewise, a warm current could have kept Beringia ice-free.