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 meters 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 meters thick, but the exact thickness is not known. The mass of 18 million km^2 of ice, for each meter of thickness, is 18,000 km^3 and roughly 16 gigatonnes (billion metric tons).
Since the ocean off the Antarctic coast is almost always 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 changes in wind patterns 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. The effect probably has more than one cause.
The IPCC fifth Assessment report (WG1) 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.
Implications for sub-surface temperatures and coastal currents have been identified, mainly related to changes in the southern hemispheric westerly winds, among possible implications is rapid Glacier melting, thus affecting calving rates and subsequently sea ice formation.
Temperatures in the atmosphere and Southern Ocean have increased during the period 1979–2004. However, sea ice grows faster than it melts, because of a weakly stratified Ocean. Thus, this mechanism is responsible for an increase in the net ice production, contributing to more sea ice. The sea ice volume increase presents about half the size of the increased freshwater supply from the Antarctic ice sheets. Modelling suggests that observed ice-drift toward the coastal regions are responsible for dynamical thickening during autumn and winter.
A study published in 2015 found that the subsurface ocean warming of ice-shelves increased freshwater runoff simulated by models, due to basal melt, and was responsible for an increase of sea ice in the winter months.
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.
- NASA. "Antarctic Sea Ice".
- One cubic meter of water weighs a metric ton, and there are 18 trillion cubic meters in 18 million square kilometers, one meter thick (18 million x 1 million x 1). The weight of ice is 88% that of water.
- IPCC AR5 WG1 (2013). "The Physical Science Basis" (PDF): 7. Archived from the original (PDF) on March 8, 2014.
- "Q&A with NASA's Joey Comiso: What is Happening with Antarctic Sea Ice?". Oct 7, 2014.
- Paul Spence; Stephen M. Griffies; Matthew H. England; Andrew Mc C. Hogg; Oleg A. Saenko; Nicolas C. Jourdain (2014). "Rapid subsurface warming and circulation changes of Antarctic coastal waters by poleward shifting winds". Geophysical Research Letters. 41 (13): 4601. doi:10.1002/2014GL060613.
- Matthew H. England; Shayne McGregor; Paul Spence; Gerald A. Meehl; Axel Timmermann; Wenju Cai; Alex Sen Gupta; Michael J. McPhaden; Ariaan Purich; Agus Santoso (February 9, 2014). "Recent intensification of wind-driven circulation in the Pacific and the ongoing warming hiatus". Nature Climate Change. 4 (3): 222–227. doi:10.1038/nclimate2106.
- Zhang, Jinlun (2007). "Increasing Antarctic Sea Ice under Warming Atmospheric and Oceanic Conditions" (PDF). Climate. 20 (11): 2515–2529. doi:10.1175/JCLI4136.1.Abstract
- Holland, Paul R., Nicolas Bruneau, Clare Enright, Martin Losch, Nathan T. Kurtz, Ron Kwok (January 17, 2014). "Modeled Trends in Antarctic Sea Ice Thickness". Climate. 27 (10): 3784–3801. doi:10.1175/JCLI-D-13-00301.1.
- R. Bintanja, G. J. Van Oldenborgh, C. A. Katsman (2015). "The effect of increased fresh water from Antarctic ice shelves on future trends in Antarctic sea ice" (PDF). Annals of Glaciology. 56 (69): 120–126. doi:10.3189/2015AoG69A001.
- Andrew Martin, Andrew McMinn (2017). "Sea ice, extremophiles and life on extra-terrestrial ocean worlds". International Journal of Astrobiology: 1–16. doi:10.1017/S1473550416000483.
- Cook, James. (1777). A Voyage Towards the South Pole, and Round the World. Performed in His Majesty's Ships the Resolution and Adventure, In the Years 1772, 1773, 1774, and 1775. In which is included, Captain Furneaux's Narrative of his Proceedings in the Adventure during the Separation of the Ships. Volume II. London: Printed for W. Strahan and T. Cadell. (Relevant fragment)