One of the problems (besides Climate changes) is that ONLY about 1 billion people are sustainable for 1000 years or more here on earth

This is IF we are trying to ONLY live on the outside of the planet and not begin to live and grow food underground more.

So, when we look at when we reached 1 billion people on earth this was around the early 1800s. So, ever since then human populations couldn't be sustained ongoing at these levels.

Since this is true (about keeping humans alive on the surface of the planet) I also realize that if everyone lived underground and the surface of the earth was ONLY for farming and recreation then I see that about 14 billion people might be able to be sustained long term if people lived underground.

However, I think this is coming anyway simply because when enough people die from storms and winds people will naturally begin to live under the ground and under the sea just to survive on a daily basis.

So, though it might be a traumatic shift I think this will begin to happen this century where people start to spend more time underground as all the wooden houses on earth are either blown away or washed away one of the two or both. 

One of the problems of living underground is that you cannot really live underground in a flood plain unless you are in some kind of submarine kind of situation where water cannot get inside your underground home.

So, this problem needs to be addressed too.

Many people try to solve this problem by tying up their boat or yacht to their home. But, this doesn't work for everyone partly because everyone isn't a sailor and also winds over 100 mph will smash almost any ship under about 100 feet long and most people cannot afford a ship this large and people who do often sail this size of ship around storms so the ship isn't destroyed.