The Arctic and Antarctic sea ice expands and contracts with the seasons: melting occurs during summer and expansion occurs during winter. The Arctic reaches its minimum extent in September at the end of summer in the Northern Hemisphere. On the other hand, the Antarctic sea ice reaches its greatest extent in September at the end of winter in the Southern Hemisphere. The Arctic and Antarctic are very different geographically. The Arctic is surrounded almost entirely by land (North America, Greenland and Eurasia), which traps most of the sea ice in the Arctic Ocean and allows multiyear ice to develop. Antarctic sea ice is bounded to the south by the Antarctic continent and completely exposed to the Southern Ocean to the north, which allows sea ice to expand in the winter but does not offer much protection during the melt season. This means that there is little multiyear sea ice in the Antarctic and most of the sea ice melts during the summer season.
Arctic sea ice extent, August 2013.
Arctic sea ice has experienced an overall trend of 12 percent decline per decade since the late 1970s. During the last 60 years, the Arctic has warmed by more than 3.5 degrees Fahrenheit (2 degrees Celsius) and the remaining ice cover is 50 percent thinner than in previous decades. Arctic sea ice reached its minimum extent of 1.97 million square miles on September 13, 2013-the sixth lowest on satellite record. The lowest Arctic sea ice extent on record was 1.32 million square miles, reported on September 16, 2012. Thicker multiyear sea ice is being replaced by thinner seasonal sea ice; the latter melts faster, whereas multiyear sea ice can last up to two summers. Conversely, Antarctic sea ice extent has increased by approximately one percent per decade. Antarctic sea ice reached its maximum extent of 7.53 million square miles on September 18, 2013-tying last year's record. Sea ice cover in Antarctic is shaped by westerly winds, high waves and frequent storms. The moist maritime environment of the Southern Ocean produces large amounts of snow fall on Antarctic sea ice - the highest snowfall rates of any region in the world. Snow and ice increase the reflection of sunlight (albedo), protecting Antarctic ice from warming. Antarctic sea ice is in constant motion due to strong winds and storms, creating areas of exposed ocean water that freeze and expand the sea ice cover. A recent study also suggests Antarctic sea ice increased due to strengthening of the polar vortex, a large cyclone near the planet's geographical poles, creating thicker long-lasting ice and exposing ocean water and ice to cold winds that cause sea ice to grow.