The World At +3 Degrees C

Here is a nifty report predicting what will happen to some major cities if the average world temperature goes up by 3 degrees C:

https://www.theguardian.com/cities/ng-interactive/2017/nov/03/three-degree-world-cities-drowned-global-warming

I have a few dumb questions:

• The world's economy is moving away from ocean-related commerce, so why are so many people still living by the sea?
• If we already know this is going to happen, why aren't people moving?
• What will cost more, and what will be more effective - trying to stop the temperature rise, or adapting to it?

By the way, we're not all moving to Mars. According to this report, it will take quite a long time to terraform that planet to accommodate a human population:

http://www.users.globalnet.co.uk/~mfogg/zubrin.htm

And we don't have the means to get there anyway.

Jeez, are we screwed.