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.
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.
Measurements of sea 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 teratonnes/million km^3, 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.
Because Antarctic ice is mainly first-year ice, which is not as thick as multiyear ice, it is generally less than a few meters thick. Snowfall and flooding of the ice can thicken it substantially, and the layer structure of Antarctic ice is often quite complex.
Recent trends and climate change
Recent changes in wind patterns, which are connected to regional changes in the number of extratropical cyclones and anticyclones, around Antarctica have advected the sea ice farther north in some areas and not as far north in others (see images). The net change is a slight increase in the area of sea ice in the Antarctic seas (unlike the Arctic Ocean, which is showing a much stronger decrease in the area of sea ice). Increased sea ice extent does not indicate that the Southern Ocean is cooling, since the Southern Ocean is warming.
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.
The IPCC AR5 report concluded that "it is very likely" that annual mean Antarctic sea ice extent increased 1.2 to 1.8% per decade, which is 0.13 to 0.20 million km2 per decade, during the period 1979 to 2012. IPCC AR5 also concluded that due to the lack of data it is not possible to determine the trend in total volume or mass of the sea ice. The increase in sea ice area probably has a number of causes. These are tied to changes in the southern hemispheric westerly winds, which are a combination of natural variability and forced change from greenhouse gases and the ozone hole. Another possible driver is ice-shelves melting, which increases freshwater input to the ocean; this increases the weakly stratified ocean surface layer and so reduces the ability of warm subsurface water to reach the surface. A 2015 study found this effect in climate models run to simulate future climate change, resulting in an increase of sea ice in the winter months.
Atmospheric and oceanic drivers likely have contributed to the formation of regionally varying trends in Antarctic sea-ice extent. For example, temperatures in the atmosphere and Southern Ocean have increased during the period 1979–2004. However, sea ice grows faster than it melts, due to a weakly stratified Ocean. Thus, this oceanic mechanism is, among others, contributing to an increase in the net ice production, potentially resulting in more sea ice. Although thickness observations are limited, modelling suggests that observed ice-drift toward the coastal regions makes an additional contribution for dynamical sea-ice thickening during autumn and winter. Observed autumn and spring trends in the number of extratropical cyclones, anticyclones and blocks, which have a strong thermodynamic control through temperature advection, and a strong dynamic control through ice-drift, on sea-ice extent during the same and also during following seasons are almost everywhere around Antarctica in agreement with the observed, regionally varying, trends in sea-ice extent. Consequently, the near-surface winds steered around weather systems are thought to explain large parts of the inhomogeneous Antarctica sea-ice trends.
After gradual increases in sea ice as referenced above, southern hemisphere spring (i.e. September, October and November) 2016 saw a rapid decline in Antarctic sea ice. 
Changes in Antarctic sea ice are also important because of implications for atmospheric and oceanic circulation. When sea ice forms, it rejects salt (ocean water is saline but sea ice is largely fresh) so dense salty water is formed which sinks and plays a key role in formation of Antarctic Bottom Water.
The force of moving ice is considerable; it can crush ships that are caught in the ice pack, and severely limits the areas where ships can reach the land, even in summer. Icebreakers, iceports and ice piers are used to land supplies.
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- Holland, Paul R., Nicolas Bruneau, Clare Enright, Martin Losch, Nathan T. Kurtz, Ron Kwok (January 17, 2014). "Modeled Trends in Antarctic Sea Ice Thickness" (PDF). Journal of Climate. 27 (10): 3784–3801. doi:10.1175/JCLI-D-13-00301.1.CS1 maint: uses authors parameter (link)
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