The last thing you might expect from global warming is colder winters, but that's exactly what some recent research suggests could happen in some highly populated areas like eastern North America and western Europe.

The key to this apparent paradox is a disruption of ocean circulation in the Atlantic. Normally, the warm Gulf Stream moves up the East Coast of North America and then angles toward Europe, providing a moderating influence to winters on both sides of the Atlantic. This water cools as it moves north, and some of this chilly, heavier water sinks to great depths, helping to propel a 3-D conveyor belt of ocean currents. Now if some of the northern ice sheets were to melt due to global warming, that would add fresh water to the North Atlantic. This would make the water less salty, and less salty water isn't as heavy. So there wouldn't be as much sinking. This would slow or even redirect the warm waters of the Gulf Stream, which could lead to colder winters in the North Atlantic region, perhaps by as much as five degrees.

This isn't as far-fetched as it sounds. There's evidence that it happened about 12,000 years ago and then again about 500 years ago. Although we don't know exactly how much melting would be needed to trigger this change in global ocean circulation, it is possible that global warming could actually lead to a big chill.