Well, if the glaciers were well covering northern states, they couldn’t have been used for farming very well. That wasn’t that long ago in geological terms. The weather couldn’t have been too great in southern states either.
So far, warming temperatures are an improvement. It also tends to imply that the weather now is “normal” and that anything else isn’t. Pretty tall order.
For the last couple of million years “normal” has been ice in the North. It’s pretty much like a desert South of that until you get to the subtropics where it’s Temperate. The tropical zone is much smaller too.