Antarctic sea ice

Last updated
Earth image on September 21, 2005, with the full Antarctic region visible Antarktyda i Antarktyka.jpg
Earth image on September 21, 2005, with the full Antarctic region visible

Antarctic sea ice is the sea ice of the Southern Ocean. It extends from the far north in the winter and retreats to almost the coastline every summer. [1] Sea ice is frozen seawater that is usually less than a few meters thick. This is the opposite of ice shelves, which are formed by glaciers; they float in the sea, and are up to a kilometre thick. There are two subdivisions of sea ice: fast ice, which are attached to land; and ice floes, which are not.

Contents

Sea ice that comes from the Southern Ocean melts from the bottom instead of the surface like Arctic ice because it is covered in snow on top. 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. [2] Another difference between the two ice packs, is that while there is clear Arctic sea ice decline, the trend in Antarctica is roughly flat. [1] Antarctic 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, expanding to an area roughly equal to that of Antarctica in winter. It peaks (~18 × 10^6 km2) during September (comparable to the surface area of Pluto), which marks the end of austral winter, and retreats to a minimum (~3 × 10^6 km2) in February. [2] [3] 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 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. [4]

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. [5] 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.png

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.

Sea ice extent in Antarctica varies a lot year by year. This makes it difficult determine a trend, and record highs and record lows have been observed between 2013 and 2023. The general trend since 1979, the start of the satellite measurements, has been roughly flat. Between 2015 and 2023, there has been a decline in sea ice, but due to the high variability, this does not correspond to a significant trend. [1] The flat trend is in contrast with Arctic sea ice, which has seen a declining trend. [1] [6]

1978- Antarctic sea ice extent - Purich and Doddridge.png
Reporting reducing Antarctic sea ice extent in mid 2023, researchers concluded that a "regime shift" may be taking place "in which previously important relationships no longer dominate sea ice variability". [7]
Antarctic Grows.jpg
The (then-record) 2012 Antarctic sea ice extent; compare with the yellow outline, which shows the median September extent from 1979 to 2000.
ClimateDashboard-Antarctic-sea-ice-summer-minimum-graph-20230307-1400px.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.
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. [8] :7 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. [9] 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. [10] 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. [11]

Recent changes in wind patterns, which are connected to regional changes in the number of extratropical cyclones and anticyclones, [12] around Antarctica have advected the sea ice farther north in some areas and not as far north in others.

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. [13] 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. [14]

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. [12] Consequently, the near-surface winds steered around weather systems are thought to explain large parts of the inhomogeneous Antarctica sea-ice trends.

The 2021 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. [15] 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.

In early January 2023, the National Snow and Ice Data Center reported that Antarctic sea ice extent stood at the lowest in the 45-year satellite record—more than 500,000 square kilometers (193,000 square miles) below the previous record (2018), with four of the five lowest years for the last half of December having occurred since 2016. [16]

Implications

Monitoring changes in sea ice is important as this impacts the psychrophiles that live here. [17]

Changes in Antarctic sea ice are also important because of implications for atmospheric and oceanic circulation. [18] 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

[T]he greatest part of this southern continent (supposing there is one), must lie within the polar circle, where the sea is so pestered with ice, that the land is thereby inaccessible.

Captain James Cook. A Voyage Towards the South Pole and Round the World, etc. [19]

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.

See also

Related Research Articles

<span class="mw-page-title-main">Causes of climate change</span> Effort to scientifically ascertain mechanisms responsible for recent global warming

The scientific community has been investigating the causes of climate change for decades. After thousands of studies, it came to a consensus, where it is "unequivocal that human influence has warmed the atmosphere, ocean and land since pre-industrial times." This consensus is supported by around 200 scientific organizations worldwide, The dominant role in this climate change has been played by the direct emissions of carbon dioxide from the burning of fossil fuels. Indirect CO2 emissions from land use change, and the emissions of methane, nitrous oxide and other greenhouse gases play major supporting roles.

<span class="mw-page-title-main">Cryosphere</span> Earths surface where water is frozen

The cryosphere is an all-encompassing term for the portions of Earth's surface where water is in solid form, including sea ice, lake ice, river ice, snow cover, glaciers, ice caps, ice sheets, and frozen ground. Thus, there is a wide overlap with the hydrosphere. The cryosphere is an integral part of the global climate system. It also has important feedbacks on the climate system. These feedbacks come from the cryosphere's influence on surface energy and moisture fluxes, clouds, the water cycle, atmospheric and oceanic circulation.

<span class="mw-page-title-main">Climate of Antarctica</span> Overview of climactic conditions in Antarctica

The climate of Antarctica is the coldest on Earth. The continent is also extremely dry, averaging 166 mm (6.5 in) of precipitation per year. Snow rarely melts on most parts of the continent, and, after being compressed, becomes the glacier ice that makes up the ice sheet. Weather fronts rarely penetrate far into the continent, because of the katabatic winds. Most of Antarctica has an ice-cap climate with extremely cold and dry weather.

<span class="mw-page-title-main">Sea ice</span> Outcome of seawater as it freezes

Sea ice arises as seawater freezes. Because ice is less dense than water, it floats on the ocean's surface. Sea ice covers about 7% of the Earth's surface and about 12% of the world's oceans. Much of the world's sea ice is enclosed within the polar ice packs in the Earth's polar regions: the Arctic ice pack of the Arctic Ocean and the Antarctic ice pack of the Southern Ocean. Polar packs undergo a significant yearly cycling in surface extent, a natural process upon which depends the Arctic ecology, including the ocean's ecosystems. Due to the action of winds, currents and temperature fluctuations, sea ice is very dynamic, leading to a wide variety of ice types and features. Sea ice may be contrasted with icebergs, which are chunks of ice shelves or glaciers that calve into the ocean. Depending on location, sea ice expanses may also incorporate icebergs.

<span class="mw-page-title-main">Ice shelf</span> Large floating platform of ice caused by glacier flowing onto ocean surface

An ice shelf is a large platform of glacial ice floating on the ocean, fed by one or multiple tributary glaciers. Ice shelves form along coastlines where the ice thickness is insufficient to displace the more dense surrounding ocean water. The boundary between the ice shelf (floating) and grounded ice is referred to as the grounding line; the boundary between the ice shelf and the open ocean is the ice front or calving front.

<span class="mw-page-title-main">West Antarctic Ice Sheet</span> Segment of the continental ice sheet covering West (or Lesser) Antarctica

The West Antarctic Ice Sheet (WAIS) is the segment of the continental ice sheet that covers West Antarctica, the portion of Antarctica on the side of the Transantarctic Mountains that lies in the Western Hemisphere. It is classified as a marine-based ice sheet, meaning that its bed lies well below sea level and its edges flow into floating ice shelves. The WAIS is bounded by the Ross Ice Shelf, the Ronne Ice Shelf, and outlet glaciers that drain into the Amundsen Sea.

<span class="mw-page-title-main">Polar ice cap</span> High-latitude region of an astronomical body with major parts covered in ice

A polar ice cap or polar cap is a high-latitude region of a planet, dwarf planet, or natural satellite that is covered in ice.

<span class="mw-page-title-main">Antarctic ice sheet</span> Earths southern polar ice cap

The Antarctic ice sheet is a continental glacier covering 98% of the Antarctic continent, with an area of 14 million square kilometres and an average thickness of over 2 kilometres (1.2 mi). It is the largest of Earth's two current ice sheets, containing 26.5 million cubic kilometres of ice, which is equivalent to 61% of all fresh water on Earth. Its surface is nearly continuous, and the only ice-free areas on the continent are the dry valleys, nunataks of the Antarctic mountain ranges, and sparse coastal bedrock. However, it is often subdivided into East Antarctic ice sheet (EAIS), West Antarctic ice sheet (WAIS), and Antarctic Peninsula (AP), due to the large differences in topography, ice flow, and glacier mass balance between the three regions.

<span class="mw-page-title-main">Climate change in the Arctic</span> Impacts of climate change on the Arctic

Due to climate change in the Arctic, this polar region is expected to become "profoundly different" by 2050. The speed of change is "among the highest in the world", with the rate of warming being 3-4 times faster than the global average. This warming has already resulted in the profound Arctic sea ice decline, the accelerating melting of the Greenland ice sheet and the thawing of the permafrost landscape. These ongoing transformations are expected to be irreversible for centuries or even millennia.

<span class="mw-page-title-main">Climate of the Arctic</span> Climate types in the Arctic region

The climate of the Arctic is characterized by long, cold winters and short, cool summers. There is a large amount of variability in climate across the Arctic, but all regions experience extremes of solar radiation in both summer and winter. Some parts of the Arctic are covered by ice year-round, and nearly all parts of the Arctic experience long periods with some form of ice on the surface.

<span class="mw-page-title-main">Tipping points in the climate system</span> Large and possibly irreversible changes in the climate system

In climate science, a tipping point is a critical threshold that, when crossed, leads to large, accelerating and often irreversible changes in the climate system. If tipping points are crossed, they are likely to have severe impacts on human society and may accelerate global warming. Tipping behavior is found across the climate system, for example in ice sheets, mountain glaciers, circulation patterns in the ocean, in ecosystems, and the atmosphere. Examples of tipping points include thawing permafrost, which will release methane, a powerful greenhouse gas, or melting ice sheets and glaciers reducing Earth's albedo, which would warm the planet faster. Thawing permafrost is a threat multiplier because it holds roughly twice as much carbon as the amount currently circulating in the atmosphere.

<span class="mw-page-title-main">Ross Gyre</span> Circulating system of ocean currents in the Ross Sea

The Ross Gyre is one of three gyres that exists within the Southern Ocean around Antarctica, the others being the Weddell Gyre and Balleny Gyre. The Ross Gyre is located north of the Ross Sea, and rotates clockwise. The gyre is formed by interactions between the Antarctic Circumpolar Current and the Antarctic Continental Shelf. The Ross Gyre is bounded by the Polar Front of the Antarctic Circumpolar Current to the north, the Antarctic Slope Current to the south, the Balleny Gyre to the west, and a variable boundary to the east from semiannual changes in sea surface height (SSH) in the Amundsen Sea. Circulation in the Ross Gyre has been estimated to be 20 ± 5 Sverdrup (Sv) and plays a large role in heat exchange in this region.

<span class="mw-page-title-main">IPCC Fifth Assessment Report</span> Intergovernmental report on climate change

The Fifth Assessment Report (AR5) of the United Nations Intergovernmental Panel on Climate Change (IPCC) is the fifth in a series of such reports and was completed in 2014. As had been the case in the past, the outline of the AR5 was developed through a scoping process which involved climate change experts from all relevant disciplines and users of IPCC reports, in particular representatives from governments. Governments and organizations involved in the Fourth Report were asked to submit comments and observations in writing with the submissions analysed by the panel. Projections in AR5 are based on "Representative Concentration Pathways" (RCPs). The RCPs are consistent with a wide range of possible changes in future anthropogenic greenhouse gas emissions. Projected changes in global mean surface temperature and sea level are given in the main RCP article.

<span class="mw-page-title-main">Polar seas</span> Collective term for the Arctic Ocean and the southern part of the Southern Ocean

Polar seas is a collective term for the Arctic Ocean and the southern part of the Southern Ocean. In the coldest years, sea ice can cover around 13 percent of the Earth's total surface at its maximum, but out of phase in the two hemispheres. The polar seas contain a huge biome with many organisms.

<span class="mw-page-title-main">Ice–albedo feedback</span> Positive feedback climate process

Ice–albedo feedback is a climate change feedback, where a change in the area of ice caps, glaciers, and sea ice alters the albedo and surface temperature of a planet. Because ice is very reflective, it reflects far more solar energy back to space than open water or any other land cover. It occurs on Earth, and can also occur on exoplanets.

<span class="mw-page-title-main">Beaufort Gyre</span> Wind-driven ocean current in the Arctic Ocean polar region

The Beaufort Gyre is one of the two major ocean currents in the Arctic Ocean. It is roughly located north of the Alaskan and Canadian coast. In the past, Arctic sea-ice would circulate in the Beaufort gyre up to several years, leading to the formation of very thick multi-year sea-ice. Due to warming temperatures in the Arctic, the gyre has lost an extensive amount of ice, practically turning what used to be a nursery for sea-ice to mature and grow into the thickest and oldest ice of the Arctic Ocean into a "graveyard" for older ice.

<span class="mw-page-title-main">Measurement of sea ice</span> Records made for navigational safety and environmental monitoring

Measurement of sea ice is important for safety of navigation and for monitoring the environment, particularly the climate. Sea ice extent interacts with large climate patterns such as the North Atlantic oscillation and Atlantic Multidecadal Oscillation, to name just two, and influences climate in the rest of the globe.

<span class="mw-page-title-main">Arctic sea ice decline</span> Sea ice loss observed in recent decades in the Arctic Ocean

Sea ice in the Arctic region has declined in recent decades in area and volume due to climate change. It has been melting more in summer than it refreezes in winter. Global warming, caused by greenhouse gas forcing is responsible for the decline in Arctic sea ice. The decline of sea ice in the Arctic has been accelerating during the early twenty‐first century, with a decline rate of 4.7% per decade. Summertime sea ice will likely cease to exist sometime during the 21st century.

<span class="mw-page-title-main">Arctic ice pack</span> The sea ice cover of the Arctic Ocean and its vicinity

The Arctic ice pack is the sea ice cover of the Arctic Ocean and its vicinity. The Arctic ice pack undergoes a regular seasonal cycle in which ice melts in spring and summer, reaches a minimum around mid-September, then increases during fall and winter. Summer ice cover in the Arctic is about 50% of winter cover. Some of the ice survives from one year to the next. Currently, 28% of Arctic basin sea ice is multi-year ice, thicker than seasonal ice: up to 3–4 m (9.8–13.1 ft) thick over large areas, with ridges up to 20 m (65.6 ft) thick. Besides the regular seasonal cycle there has been an underlying trend of declining sea ice in the Arctic in recent decades as well.

<span class="mw-page-title-main">Climate change in Antarctica</span> Impacts of climate change on Antarctica

Climate change caused by greenhouse gas emissions from human activities occurs everywhere on Earth, and while Antarctica is less vulnerable to it than any other continent, climate change in Antarctica has already been observed. There has been an average temperature increase of >0.05 °C/decade since 1957 across the continent, although it had been uneven. While West Antarctica warmed by over 0.1 °C/decade from the 1950s to the 2000s and the exposed Antarctic Peninsula has warmed by 3 °C (5.4 °F) since the mid-20th century, the colder and more stable East Antarctica had been experiencing cooling until the 2000s. Around Antarctica, the Southern Ocean has absorbed more heat than any other ocean, with particularly strong warming at depths below 2,000 m (6,600 ft) and around the West Antarctic, which has warmed by 1 °C (1.8 °F) since 1955.

References

  1. 1 2 3 4 "Understanding climate: Antarctic sea ice extent". NOAA Climate.gov. 14 March 2023. Retrieved 2023-03-26.
  2. 1 2 Vaughan, D. G.; Comiso, J. C.; Allison, I.; Carrasco, J.; et al. (2013). "Chapter 4: Observations: Cryosphere" (PDF). IPCC AR5 WG1 2013. pp. 317–382.
  3. NASA (2009-05-22). "Antarctic Sea Ice".
  4. AP staff. (19 March 2022). "Both of the planet's poles experience extreme heat, and Antarctica breaks records". NPR website Retrieved 27 March 2022.
  5. Fountain, Henry (22 February 2022). "Sea Ice Around Antarctica Reaches a Record Low". The New York Times . New York City. Retrieved 24 February 2022. A complex group of factors is at play when it come to Antarctic sea ice. Large-scale atmospheric patterns, often occurring far from the continent, as well as local ocean currents and winds can all increase or reduce sea-ice coverage.
  6. "Arctic Sea Ice News and Analysis". National Snow & Ice Data Centre. 15 March 2023. Retrieved 26 March 2023.
  7. Purich, Ariaan; Doddridge, Edward W. (13 September 2023). "Record low Antarctic sea ice coverage indicates a new sea ice state". Communications Earth & Environment. 4 (1): 314. Bibcode:2023ComEE...4..314P. doi: 10.1038/s43247-023-00961-9 .
  8. IPCC (2013). "Summary for Policymakers" (PDF). IPCC AR5 WG1 2013. pp. 3–29.
  9. Lynch, Patrick (Oct 7, 2014). "Q&A with NASA's Joey Comiso: What is Happening with Antarctic Sea Ice?". NASA. Archived from the original on 2014-10-11.
  10. Sun, Shantong; Eisenman, Ian (2021). "Observed Antarctic sea ice expansion reproduced in a climate model after correcting biases in sea ice drift velocity". Nature Communications. 12 (1): 1060. Bibcode:2021NatCo..12.1060S. doi:10.1038/s41467-021-21412-z. PMC   7887216 . PMID   33594079.
  11. Bintanja, R.; van Oldenborgh, G. J.; Katsman, C. A. (2015). "The effect of increased fresh water from Antarctic ice shelves on future trends in Antarctic sea ice". Annals of Glaciology. 56 (69): 120–126. Bibcode:2015AnGla..56..120B. doi: 10.3189/2015AoG69A001 . S2CID   54759501.
  12. 1 2 Schemm, Sebastian (25 June 2018). "Regional Trends in Weather Systems Help Explain Antarctic Sea Ice Trends". Geophysical Research Letters. 45 (14): 7165–7175. Bibcode:2018GeoRL..45.7165S. doi:10.1029/2018GL079109. hdl: 20.500.11850/286394 . S2CID   134805912.
  13. Zhang, Jinlun (2007). "Increasing Antarctic Sea Ice under Warming Atmospheric and Oceanic Conditions" (PDF). Journal of Climate. 20 (11): 2515–2529. Bibcode:2007JCli...20.2515Z. doi:10.1175/JCLI4136.1.
  14. Holland, Paul R.; Bruneau, Nicolas; Enright, Clare; Losch, Martin; Kurtz, Nathan T.; Kwok, Ron (January 17, 2014). "Modeled Trends in Antarctic Sea Ice Thickness" (PDF). Journal of Climate. 27 (10): 3784–3801. Bibcode:2014JCli...27.3784H. doi:10.1175/JCLI-D-13-00301.1. S2CID   53678373.
  15. IPCC (2021). "Chapter 9" (PDF). IPCC AR6 WG1 2021. pp. 1251–1254.
  16. "Ice down under". 3 January 2023. Archived from the original on 6 January 2023. Fig. 5a
  17. Andrew Martin; Andrew McMinn (2018). "Sea ice, extremophiles and life on extra-terrestrial ocean worlds". International Journal of Astrobiology. 17 (1): 1–16. Bibcode:2018IJAsB..17....1M. doi: 10.1017/S1473550416000483 .
  18. "Sea Ice and Global Climate". NSIDC. Retrieved 11 Jul 2018. NSIDC
  19. 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)