I'm guessing not since global warming is a relatively recent issue and it would take a long time for something like that to happen.
You would be incorrect on that. The Pacific island nation of Kiribati is being completely evacuated and dissolved, as it is swallowed by the sea.
As far as I'm aware, this hasn't happened for a whole nation, but several islands in the Maldives were made uninhabitable after the Boxing Day Tsunami, and so the population had to be relocated to neighbouring, previously uninhabited islands.
Also I suggest you look up the after-effects of the Grote Mandrenke: http://en.wikipedia.org/wiki/Grote_Mandrenke
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