"The Day After Tomorrow"

I'm sure a lot of you have seen the movie The Day After Tomorrow, directed by Roland Emmerich with Dennis Quaid and Jake Gyllenhaal on the cast.  Normally I would never be one to bring fictional Hollywood rif-raf into a serious discussion, but what the movie outlines, a destabilization of the mid atlantic current, is actually an event that can happen if warming trends continue.  I'm not sure about this destabilization causing such huge, random weather so quickly, but it certainly would affect global climate. 
http://www.aip.org/history/climate/oceans.htm
This website explains in official terms what deep ocean currents are, and how they are driven by temperature and salinity.  Cold water sinks, so does dense, salty water.  Arctic waters are freezing cold, but Equatorial waters are very salty.  Currents are driven up and down by surprisingly subtle shifts and balances between salt and temperature.  
  I believe because of rising temperatures, and falling salinity, a double headed wrench is about to be thrown straight into the middle of our climate stabilizing currents.  As the earth warms up, ice melts.  This causes the salinity of the water to drop near arctic areas, making these cold waters even less salty, so they will push the current faster.  Also, as the earth warms up, more fresh water evaporates from equatorial areas, which in turn makes them more salty, and more dense.  This could produce a cycle that can spiral out of control, more heat means more salty areas, and less ice means more freshwater areas.  This could completely disrupt these currents, and I imagine the currents wouldn't transport heat nearly as effectively.  In fact, if we lose these stabilizing currents, the earth could warm up even more quickly, becoming worse and worse until the only places even close to being comfortable to live in would be the remnants of Greenland or Antarctica.