How the Warming Arctic Affects Us All

The Arctic includes the Arctic Ocean and northern parts of Canada, Russia, Denmark (Greenland), Norway, the United States (Alaska), Sweden, Finland and Iceland. Arctic winters are cold and for a time, dark 24 hours a day. The ocean surface begins to freeze in mid-September and remains frozen until March, when it melts again through summer as the region gets sunlight 24 hours a day. The term “ice-free Arctic” refers to the Arctic Ocean in summer time.
During the last three decades, about 50 percent of Arctic sea ice cover, 80 percent of sea ice volume (thickness) and much of the old multiyear ice have been lost.
During the 1980s, half the winter ice remained for one or more seasons, but by 2012, only 25 percent of the ice that remained was more than a year old, with the rest thinner and more easily melted. Many climate models predicted that summer sea ice would remain through this century and into the next, so the speed at which sea ice loss is occurring has taken scientists by surprise.

http://blogs.ei.columbia.edu/2012/12/06/how-the-warming-arctic-affects-us-all/