A good way of dealing with uncertainty is to calculate the worst possible outcome. In the case of global warming, that would be if all the ice in the world melted.
That would produce a rise in sea level of 66 meters spread over a period of between 2,000 and 20,000 years, and the loss of just 3% of our available land.
The study is here, hat tip Junk Science.com. It’s solid stuff – allowing for the fact that floating ice won’t raise levels when it melts, that many areas below the new sea level will stay dry (if there’s no way for the sea to reach them), and that, freed of their ice burden, Antarctica and Greenland would rise. The result is (my ellipsis):
Some larger areas now underwater (in a simulation of a 66 meter rise) are the southeastern United States, part of the Amazon River basin, northern Europe, Bangladesh, parts of Siberia along the Arctic Ocean, and portions of mainland China…
So why wouldn’t people drown? Again, a change in the Earth this dramatic would take thousands of years to effect from any realistic cause.
Over generations people would migrate as the coasts changed. Consider that virtually all of the settlements in the United States were established only in last 350 years.
Incidentally, the analysis points out that just 16,000 years ago, when men are thought to have first entered the Americas via the Bering land bridge, things were much worse:
During the last Ice Age the maximum extent of glaciation was around 16,000 B.C. At that time large ice sheets covered all of Canada, much of the American midwest and northeast, all of Scandinavia and some surrounding regions of Eurasia.
The total volume of ice then was perhaps 80,000,000 cubic kilometers, or between two and three times as much as today.