Antarctic sea ice is the

sea ice Sea ice arises as seawater freezes. Because ice is less dense than water, it floats on the ocean's surface (as does fresh water ice, which has an even lower density). Sea ice covers about 7% of the Earth's surface and about 12% of the world's oce ...

of the

Southern Ocean The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the World Ocean, generally taken to be south of 60° S latitude and encircling Antarctica. With a size of , it is regarded as the second-small ...

. It extends from the far north in the winter and retreats to almost the coastline every summer, getting closer and closer to the coastline every year due to sea ice melting. Sea ice is frozen seawater that is usually less than a few meters thick. This is the opposite of

ice shelves An ice shelf is a large floating platform of ice that forms where a glacier or ice sheet flows down to a coastline and onto the ocean surface. Ice shelves are only found in Antarctica, Greenland, Northern Canada, and the Russian Arctic. The b ...

, which are formed by

glaciers A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such as ...

, they float in the sea, and are up to a kilometre thick. There are two subdivisions of sea ice:

fast ice Fast ice (also called ''land-fast ice'', ''landfast ice'', and ''shore-fast ice'') is sea ice that is "fastened" to the coastline, to the sea floor along shoals or to grounded icebergs.Leppäranta, M. 2011. The Drift of Sea Ice. Berlin: Springer- ...

, which are attached to land; and

ice floes An ice floe () is a large pack ice, pack of Drift ice, floating ice often defined as a flat piece at least 20 m across at its widest point, and up to more than 10 km across. Drift ice is a floating field of sea ice composed of several ice fl ...

, which are not. Sea ice that comes from the Southern Ocean melts from the bottom instead of the surface like

Arctic ice The Arctic ( or ) is a polar region located at the northernmost part of Earth. The Arctic consists of the Arctic Ocean, adjacent seas, and parts of Canada (Yukon, Northwest Territories, Nunavut), Danish Realm (Greenland), Finland, Iceland, N ...

because it is covered in snow on top. As a result,

melt ponds Melt ponds are pools of open water that form on sea ice in the warmer months of spring and summer. The ponds are also found on glacial ice and ice shelves. Ponds of melted water can also develop under the ice. Melt ponds are usually darker tha ...

are rarely observed. On average, Antarctic sea ice is younger, thinner, warmer, saltier, and more mobile than Arctic sea ice. Sea ice is not studied very well in comparison to Arctic ice since it is less accessible.

Measurements of sea ice

Extent

The Antarctic sea ice cover is highly seasonal, with very little ice in the

austral summer Summer is the hottest of the four temperate seasons, occurring after spring and before autumn. At or centred on the summer solstice, the earliest sunrise and latest sunset occurs, daylight hours are longest and dark hours are shortest, ...

, 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 area of 18 million km^2 of ice is 18 trillion square meters, so for each meter of thickness, given that the

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...

of ice is about 0.88 teratonnes/million km^3, the mass of the top meter of Antarctic sea ice is roughly 16 teratonnes (trillion metric tons) in late winter. Record low summer sea ice was measured in February 2022 at 741,000 square miles (1.9 million square kilometers) by the National Snow and Ice Data Center. Since the ocean off the Antarctic coast usually is much warmer than the air above 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. Katabatic-wind hg

Thickness

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 cyclone Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are low-pressure areas which, along with the anticyclones of high-pressure areas, drive the weather over much of the Earth. Extratropical cyclones are capable of ...

s and

anticyclone An anticyclone is a weather phenomenon defined as a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere as viewed from abov ...

s, around Antarctica have advected the sea ice farther north in some areas and not as far north in others. 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. File:Spseaice extent 2013 chart.jpg, Antarctic sea ice cover grows in autumn and winter, and shrinks again each spring and

summer Summer is the hottest of the four temperate seasons, occurring after spring and before autumn. At or centred on the summer solstice, the earliest sunrise and latest sunset occurs, daylight hours are longest and dark hours are shortest, wit ...

. 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. File:Antarctic Grows.jpg, 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 ha
shrunk at a much faster rate
than it has expanded in the Southern Ocean. File:Spseaice sss 2012170 chart.jpg, 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. File:Maximum Antarctic Sea Ice 2014.ogg, 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 the lack of data precludes determining 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. The winds drive sea ice drift, and modelling research suggests that the observed sea ice expansion was driven by changes in the sea ice drift velocity. 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, because of 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

s 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. The IPCC AR6 report confirms the observed increasing trend in the mean Antarctic sea ice area over the period from 1979 to 2014 but assesses that there was a decline after 2014, with the least extent reached in 2017, and a following growth. The report then concludes that there is “high confidence” that there is no significant trend in the satellite observed Antarctic sea ice area from 1979 to 2020 in both winter and summer.

Implications

Monitoring changes in sea ice is important as this impacts the

psychrophiles Psychrophiles or cryophiles (adj. ''psychrophilic'' or ''cryophilic'') are extremophilic organisms that are capable of growth and reproduction in low temperatures, ranging from to . They have an optimal growth temperature at . They are found in ...

that live here. 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.

Effects on Navigation

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.

Icebreaker An icebreaker is a special-purpose ship or boat designed to move and navigate through ice-covered waters, and provide safe waterways for other boats and ships. Although the term usually refers to ice-breaking ships, it may also refer to smaller ...

iceport An iceport is a more-or-less permanent indentation in the front of an ice shelf, that can serve as a natural ice harbour. Though useful, they are not always reliable, as calving of surrounding ice shelves can render an iceport temporarily unstabl ...

s and

ice pier An ice pier or ice wharf is a man-made structure used to assist the unloading of ships in Antarctica. It is constructed by pumping seawater into a contained area and allowing the water to freeze. By repeating this procedure several times, additio ...

s are used to land supplies.

References

External links

NSIDC , Why is there so much Antarctic sea ice?
{{Antarctica , expanded Forms of water Hydrology Sea ice

Sea ice Sea ice arises as seawater freezes. Because ice is less dense than water, it floats on the ocean's surface (as does fresh water ice, which has an even lower density). Sea ice covers about 7% of the Earth's surface and about 12% of the world's oce ...

Oceans surrounding Antarctica Articles containing video clips Climate change and the environment