The Arctic ice cap grows each winter as the sun sets for several months and intense cold ensues. In the summer, wind and ocean currents cause some of the ice naturally to flow out of the Arctic, while much of it melts in place. But not all of the Arctic ice melts each summer; the thicker, older ice is more likely to survive. Seasonal sea ice usually reaches about 6 feet in thickness, while multi-year ice averages 9 feet. In recent years, the amount of ice replaced in the winter has not been sufficient to offset summer ice losses. The result is more open water in summer, which then absorbs more heat, warming the ocean and further melting the ice. Between 2004 and 2008, multi-year ice cover shrank 595,000 square miles -- nearly the size of Alaska's land area.
http://www.nasa.gov/home/hqnews/2009/jul/HQ_09-155_Thin_Sea_Ice.html
