Hysteresis.

The 'catastrophe point' at which a change becomes no longer easily reversible at the original energy level

That's what triggered the rapid changes in temperature testified to by frozen mammoths.

The recent movie got it partly right, just left out a lot of science, some of which is still being discovered. And there is no way that goddam statue would still be standing!

If you add up ALL of the individual causations, insignificant in each of themselves, you get a SYNERGISTIC response, where the total effect is much greater that the simple um of all the individual effects - a sort of multiplication, rather than an addition.

At least in the movie, the USA got thinned out, I suppose...