The arctic and antarctic regions are literal ninth-circle hellscapes (the ninth circle of hell is the one that is frozen over). Significantly worse than the areas around Chernobyl which is apparently well on the way to becoming a national park.

I'm not sure that there is a 'risk' that the human brain can't process. Risk of what, exactly? Huge cost to the generation that has to move, massive risk if it hits farmland, sure. But even assuming that the 'millions of years' figure is factual - which I doubt - we are certainly going to run out of human race before we run out of land. The risks are all fairly short-term things.