I recall a Discovery or History Channel show that stated the influx of freshwater from the melted polar caps would have dramatic (and difficult to model) effects on the undersea 'rivers' and their very existence. Halting the flow of water around the Earth would be far worse than the flooding that would come from the increased volume of water.

It was an interesting show. Sorry I don't have a better grasp of the implications or functional factors for you.

Another thing I recall was a google-maps based webpage that would 'flood' areas in view based on % melt. That was also interesting.