So far, sea level rise has been relatively steady but that may change soon. According to this new research up to a fifth of humanity may become refugees. This article references a plan for coastal cities to prepare, and it shows the cities that will be most directly affected by a 2 or 4 degree C rise in temperature. Remember, refugees won't all stick around, so even inland communities may be affected.
One fifth of the world's population could be a refugee by 2100 - The World Economic Forum