[rant warning]

OK, so let's just suppose that this theory isn't totally WHACKED (which it is). Something like this is going to take a few lifetimes to actually make an impact. The rate of melt in the arctic is indeed increasing and the cold water will displace / reroute some of the warmer gulf stream water. If that is physically sound it is already happening. The resulting decrease in regional temperatures should quickly result in an increase in snowfall and a decrease in melting in the trouble spots long before there is a threatening impact on the climate of the broader region. As this response weaken (as it must for the scenario to be at all worth considering) average temperatures in the region will slowly drop say 1/2 degree per year (that would be extremely radical change for climate) Meaning that you might have a few decades of colder winters with more than usual snowfall but nothing really troublesome. For the situation to become truely dire you would have to see freezing of the channel in winter. If you got to that point there would be no melting from the greenland icecap or arctic cap since it would be cold enough to freeze the channel that far south. By that time you would be facing a climate similar to wisconsin and those folks muddle by without much more trouble than you tho with perhaps a bit more cheese. Climate change is something that we should worry about but its rate is slow and can be adapted to even on the scale of national infrastructure.

[/rant warning]