Antarctic sea ice

Antarctic sea ice is the sea ice of the Southern Ocean. It extends far north in winter and retreats almost to the coastline each summer. Sea ice is frozen seawater that is usually less than a few metres thick. This is in contrast to ice shelves, which are formed by glaciers, float in the sea, and are up to a kilometer thick. There are two subdivisions of sea ice: fast ice, which is attached to land; and ice floes, which are not.Antarctic sea ice cover grows in autumn and winter, and shrinks again each spring and summer. In 2013 (black line) and 2012 (red line), the ice reached the highest extents ever recorded, but it was only slightly above the historical average (blue line). Light blue regions show the range of natural variability.The (then-record) 2012 Antarctic sea ice extent; compare with the yellow outline, which shows the median sea ice extent in September from 1979 to 2000. Sea ice coverage in the Arctic has shrunk at a much faster rate than it has expanded in the Southern Ocean.Antarctic sea ice cover shrinks to its minimum extent each year in February or March; the ice cover then grows until reaching its maximum extent in September or October. The graph above shows the maximum extent for each September since 1979, in millions of square kilometers. There is variability from year to year, though the overall trend shows growth of about 1.5 percent per decade.An animation of the Antarctic sea ice growing from its seasonal minimum to seasonal maximum extent during southern hemisphere autumn and winter (between March 21 and September 19, 2014; note labels on animation). Spring melting in not shown.Captain James Cook. A Voyage Towards the South Pole and Round the World, etc. Antarctic sea ice is the sea ice of the Southern Ocean. It extends far north in winter and retreats almost to the coastline each summer. Sea ice is frozen seawater that is usually less than a few metres thick. This is in contrast to ice shelves, which are formed by glaciers, float in the sea, and are up to a kilometer thick. There are two subdivisions of sea ice: fast ice, which is attached to land; and ice floes, which are not. Sea ice in the Southern Ocean melts from the bottom instead of from the surface like Arctic ice because it is covered in snow. As a result, melt ponds are rarely observed. On average, Antarctic sea ice is younger, thinner, warmer, saltier, and more mobile than Arctic sea ice. Due to its inaccessibility, it is not as well-studied as Arctic ice. The Antarctic sea ice cover is highly seasonal, with very little ice in the austral summer, expanding to an area roughly equal to that of Antarctica in winter. It peaks (~18 × 10^6 km^2) during September, which marks the end of austral winter, and retreats to a minimum (~3 × 10^6 km^2) in February. Consequently, most Antarctic sea ice is first year ice, a few metres thick, but the exact thickness is not known. The area of 18 million km^2 of ice is 18 trillion square metres, so for each metre of thickness, given that the density of ice is about 0.88, the mass of the top metre of Antarctic sea ice is roughly 16 teratonnes (trillion metric tons) in late winter. Since the ocean off the Antarctic coast usually is much warmer than the air over it, the extent of the sea ice is largely controlled by the winds and currents that push it northwards. If it is pushed quickly, the ice can travel much further north before it melts. Most ice is formed along the coast, as the northward-moving ice leaves areas of open water (coastal latent heat polynyas), which rapidly freeze.