Throughout Earth history, sea levels have risen and fallen in response to higher and lower global temperatures. For most of that history, those sea levels have been higher, as the fossil record shows. The problem today is that global warming is slowly melting the polar ice caps, causing the sea levels to rise again. This wouldn't be a serious problem, except that humans have built entire cities on coastal regions which would be flooded out by those rising waters, and those people would have to be moved. Where can they go? Considering the vast environmental damage we have already done, transporting and replanting those displaced millions upland would only do MORE damage. It would be better to prevent the catastrophy. Scientists are even now exploring ways to do just that.
This is a perfect example of humans building a technological trap for themselves! We got a taste of what could happen when Hurricane Katrina hit New Orleans. That was a temporary flood, but what is being forecast would be PERMENANT and there would be NO chance for the city to be restored later!