&gt;In such a case it would seem that a warm world, even with a higher sea level and the human movement that would require, would be less disruptive than much of current civilization being under 200 feet of ice.<p>How about the fact that many human settlements are supposed to be near bodies of water (both the sea coasts and inland lakes and rivers)? At least for the former, the higher sea level could affect things a lot. Though I agree being under a lot of ice would be worse - with higher sea level at least there is the opportunity to move to higher levels of land. But there could be several factors that could still affect things or cost a huge, possibly impossible amount - one example being the amount of land used for agricultural and other use land that might be submerged. Also, people need to be reasonably near the land they work on (even if it is an office), and again, suitable replacement land for whatever use (farms, industry, offices) may not be easy to find at the higher levels that people move too.