See my post and reply to that post about "families"
@builderofcastles/ecotrain-question-of-the-week-season-6-9-what-are-your-predictions-for-20-and-200-years

The great migration is everyone finding their family.
Migrants (from the Middle East, will start migrating home)
People in America will start forming small groups based on core values. Small self sufficient communities.

People in Canada will start migrating south. (any place where it is already cold)
People in America may start migrating to Mexico

So, it is more about the family groups forming. But, the ice-age will definitely kick some people's ass to get a move-on.