Even a modest rise in sea levels (1.5m-3m) by 2100 would mean massive problems for coastal regions across the world, causing an increase in the frequency of severe flooding events and making some places effectively uninhabitable. It's not that people cannot move (as FK seems to recommend), it's the cost of doing so that needs to be considered.
As for the longer term impacts, the truth is that not enough people care. The science could be utterly convincing and settled (in favour of a warming scenario) and yet some people would simply dismiss it as 'not their problem'. The problem for everyone else is, they're right - it most likely will not be their problem, or their children's, or their grandchildren's... but that is not to say that the potential impacts should be ignored. Just as today's generation have a responsibility not to contaminate the planet forever with nuclear waste, we also share a responsibility not to set into motion physical processes that may be irreversible, or at least do irreversible damage, either to the planet itself or to the cities and infrastructure that mankind has laboured to produce.
Of course, whatever mankind can (and cannot) do to the climate system, Mother Nature can out-do by orders of magnitude... the hospitable/habitable regions of Earth today will not be in the future, regardless of whatever we have done/will do. The question is, however, are we absolutely certain that all past, current and near-future human activity is completely benign, and is it a safe assumption that there's nothing we can do to endanger our hospitable zones (which are predominately near water for obvious reasons) within a timescale that is too short for us to adequately mitigate against or affordably relocate? If anything, I think the current evidence points in the opposite direction - that there is good reason to not assume that our actions are irrelevant. But, no matter what the evidence turns out to say, the only responsible thing to do is to at least study this question, because (IMO) we cannot afford not to, hence why I cannot understand the attitude that we should stop funding climate research and concentrate on other things.
