One way we adjust is through war, but not the only one. Death of the less adapted for natural causes (temperature changes, floods, etc.) is another one, while the adoption of more adequate rules for living (surviving) in the new conditions is probably the best one. It is very unlikely that all people will adopt new rules, so Darwinian selection will probably settle part of the issue - and wars are to be expected in this case.

Since we are not sure of the depth and speed of change, I think the first step is to be ready to accept changes. With luck (time), nature will show us the types of change required.