Well the scenario about an ice age spurred by global warming is certainly a possibility as ice core records have found but the movie really blew it by speeding the process up so. I did watch a Discovery Channel program on the phenomena though and was amazed at how quickly the changes can occur - decades instead of thousands of years one would expect.

Guess the last time something like this happened where an influx of unsalted water stopped the underwater convection train was when a giant inland lake in North America let loose and changed the salinity of the Northern Atlantic. If you ever see that program on again I highly encourage it. Also liked the one about the flipping of the poles.