Before areas become completely uninhabitable, we will see areas become increasingly stressed: heat waves, more extreme weather events, poorer crop yields, depleting aquifers.

Stress increases conflict risk. Fights for essential resources (land, water, food, shelter) will break out long before those essential resources are completely gone.

If we skip past the immense suffering and death part, we will probably end up on a planet where national borders have been redrawn by war and desperation, and a smaller population that lives in more northerly climes.