Human societies are tough and resilient, study how countries like Germany or Japan managed to not only survive, but fight back while absorbing devastating air bombardment. Today, look at places like Syria, Lebanon and Libya. Long before terrestrial life is threatened by the loss of water and oxygen, people will be more concerned with things like unemployment, inflation, and loss of sewer, water and electricity.

But sooner or later, the economy goes south, and civic and social services will become intermittent, and then fail. Still, the last people to be seriously inconvenienced are those with money. That's why they never seem to keen to do anything about the problem.

Eventually though, money will become useless, and then even a gun will do you little good unless you have lots of friends with guns.

But even then, you can still drink rainwater and breath the air, if you don't mind the smoke...