Of course, possible 'extinction' is a scare tactic used to make a point, but not all environmentalists use language that strong.
It can happen to species who face sudden changes that they cannot adapt to, whether it be hunting, climate, food supply...etc. It is obvious that Polar bears are already under stress. ( I saw film of one desparate enough to attack a herd of walruses....and failing miserably).

   We cannot be sure what the ultimate adaptations might be, but as I say, to stifle discussion of the issue for political reasons is counterproductive.