Antarctic ice sheet

Antarctic Ice Sheet
South facing visualization of the Antarctic ice sheet from the Pacific sector of the Southern Ocean (West Antarctic ice sheet, foreground; Antarctic Peninsula, to the left; East Antarctic ice sheet, background).
Geographic map of Antarctica, with the grounded ice sheet in white, its floating ice shelves in gray, and ice-free land in brown.
TypeIce sheet
LocationAntarctica
Area14×10^6 km2 (5.4×10^6 sq mi)
Thickness2.2 km (1.4 mi) on average, 4.9 km (3.0 mi) at maximum
StatusOngoing net loss of ice, regionally variable

The Antarctic ice sheet is a continental glacier covering 98% of the Antarctic continent, with an area of 14 million square kilometres (5.4 million square miles) 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 (6,400,000 cubic miles) of ice, which is equivalent to 61% of all fresh water on Earth.

For the purposes of study, the ice sheet is often subdivided into three geographic regions based on topography, ice flow, and mass balance: The East Antarctic ice sheet (EAIS), West Antarctic ice sheet (WAIS), and Antarctic Peninsula (AP). The WAIS and EAIS are approximately divided along the Transantarctic Mountains, and the AP is defined by its more geographically distinct drainage basins. All three regions have negative mass balance trends and have lost significant volumes of ice over the observational record, accounting for about 10% of recent rates of global mean sea level rise (1993-2020).

Basin distinctions notwithstanding, the ice sheet surface is nearly continuous over the continent, disrupted only regionally by dry valleys, nunataks of the Antararctic mountain ranges, and sparse coastal bedrock—features which often show evidence of past ice cover over the Late Cenozoic Ice Age.

Because the East Antarctic ice sheet is over 10 times larger than the West Antarctic ice sheet and located at a higher elevation, it is less vulnerable to climate change than the West. In the 20th century, it had been one of the only places on Earth which displayed limited cooling instead of warming, even as the West Antarctic ice sheet warmed by over 0.1 °C/decade from 1950s to 2000, producing an average warming trend of >0.05 °C/decade since 1957 across the whole continent. EAIS also started to display a clear warming trend after 2000 (while the warming of the WAIS slowed), but as of early 2020s, there is still net mass gain over the EAIS (due to increased precipitation freezing on top of the ice sheet), yet the ice loss from the WAIS glaciers such as Thwaites and Pine Island Glacier is far greater.

By 2100, net ice loss from Antarctica alone would add around 11 cm (5 in) to the global sea level rise. Further, the way WAIS is located deep below the sea level leaves it vulnerable to marine ice sheet instability, which is difficult to simulate in ice sheet models. If instability is triggered before 2100, it has the potential to increase total sea level rise caused by Antarctica by tens of centimeters more, particularly with high overall warming. Ice loss from Antarctica also generates fresh meltwater, at a rate of 1100-1500 billion tons (GT) per year.: 1240  It dilutes the saline Antarctic bottom water, which weakens the lower cell of the Southern Ocean overturning circulation and may even contribute to its collapse, although this will likely take place over multiple centuries.

The two parts of the ice sheet also have a very different long-term outlook. Paleoclimate research and improved modelling show that the West Antarctic ice sheet is very likely to disappear even if the warming does not progress any further, and only reducing the warming to 2 °C (3.6 °F) below the temperature of 2020 may save it. It is believed that the loss of the ice sheet would take place between 2,000 and 13,000 years, although several centuries of high emissions may shorten this to 500 years. 3.3 m (10 ft 10 in) of sea level rise would occur if the ice sheet collapses but leaves ice caps on the mountains behind, and 4.3 m (14 ft 1 in) if those melt as well. Isostatic rebound may also add around 1 m (3 ft 3 in) to the global sea levels over another 1,000 years. On the other hand, the East Antarctic ice sheet is far more stable and may only cause 0.5 m (1 ft 8 in) - 0.9 m (2 ft 11 in) of sea level rise from the current level of warming, which is a small fraction of the 53.3 m (175 ft) contained in the full ice sheet. Around 3 °C (5.4 °F), vulnerable locations like Wilkes Basin and Aurora Basin may collapse over a period of around 2,000 years, which would add up to 6.4 m (21 ft 0 in) to sea levels. The loss of the entire ice sheet would require global warming in a range between 5 °C (9.0 °F) and 10 °C (18 °F), and a minimum of 10,000 years.

Geography

The bedrock topography of Antarctica, critical to understand dynamic motion of the continental ice sheets.

The Antarctic ice sheet covers an area of almost 14 million square kilometres (5.4 million square miles) and contains 26.5 million cubic kilometres (6,400,000 cubic miles) of ice. A cubic kilometer of ice weighs approximately 0.92 metric gigatonnes, meaning that the ice sheet weighs about 24,380,000 gigatonnes.

The Antarctic ice sheet is divided by the Transantarctic Mountains into two unequal sections called the East Antarctic Ice Sheet (EAIS) and the smaller West Antarctic Ice Sheet (WAIS). Other sources divide the Antarctic ice sheet into three sections: the East and West Antarctic Ice Sheets and the relatively small Antarctic Peninsula Ice Sheet (also in West Antarctica) as the third.: 2234  Collectively, they have an average thickness of around 2 kilometres (1.2 mi), though the smaller WAIS is around 1.05 km (0.7 mi) on average, and is predominantly grounded below sea level. The only other currently existing ice sheet on Earth is the Greenland ice sheet in the Arctic, which is about twice as large as the WAIS, but much smaller than the EAIS.

The EAIS rests on a major land mass, but the bed of the WAIS is, in places, more than 2,500 meters (8,200 feet) below sea level. It would be seabed if the ice sheet were not there. The WAIS is classified as a marine-based ice sheet, meaning that its bed lies below sea level and its edges flow into floating ice shelves. The WAIS is bounded by the Ross Ice Shelf, the Filchner-Ronne Ice Shelf, and outlet glaciers that drain into the Amundsen Sea.

Warming over the ice sheet

Antarctic Skin Temperature Trends between 1981 and 2007, based on thermal infrared observations made by a series of NOAA satellite sensors. Skin temperature trends do not necessarily reflect air temperature trends.
Antarctic surface temperature trends, in °C/decade. Red represents areas where temperatures have increased the most since the 1950s.

Antarctica is the coldest and driest continent on Earth, as well as the one with the highest average elevation. Further, it is surrounded by the Southern Ocean, which is far more effective at absorbing heat than any other ocean. It also has extensive year-around sea ice, which has a high albedo (reflectivity) and adds to the albedo of the ice's sheet own bright, white surface. Antarctica is so cold that it is the only place on Earth where atmospheric temperature inversion occurs every winter. Elsewhere, the atmosphere on Earth is at its warmest near the surface and it becomes cooler as elevation increases. During the Antarctic winter, the surface of central Antarctica becomes cooler than middle layers of the atmosphere. Thus, greenhouse gases trap heat in the middle atmosphere and reduce its flow towards the surface and towards space, instead of simply preventing the flow of heat from the lower atmosphere to the upper layers. This effect lasts until the end of the Antarctic winter. Thus, even the early climate models predicted that temperature trends over Antarctica would emerge slower and be more subtle than they are elsewhere.

Moreover, there were fewer than twenty permanent weather stations across the continent, with only two in the continent's interior, while automatic weather stations were deployed relatively late, and their observational record was brief for much of the 20th century. Likewise, satellite temperature measurements did not begin until 1981 and are typically limited to cloud-free conditions. Thus datasets representing the entire continent only began to appear by the very end of the 20th century. The only exception was the Antarctic Peninsula, where warming was both well-documented and strongly pronounced: It was eventually found to have warmed by 3 °C (5.4 °F) since the mid-20th century. Based on this limited data, several papers published in the early 2000s suggested that there had been an overall cooling over continental Antarctica (that is outside of the Peninsula).

A 2002 analysis led by Peter Doran received widespread media coverage after it also indicated stronger cooling than warming between 1966 and 2000, and found that McMurdo Dry Valleys in East Antarctica had experienced cooling of 0.7 °C per decade - a local trend confirmed by subsequent research at McMurdo. Multiple journalists suggested that these findings were "contradictory" to global warming, even though the paper itself noted the limited data, and still found warming over 42% of the continent. What became known as the "Antarctic Cooling Controversy" received further attention in 2004, when Michael Crichton wrote a novel State of Fear which alleged a conspiracy amongst climate scientists to make up global warming, and claimed that Doran's study definitively proved there was no warming in Antarctica outside of the Peninsula. Relatively few scientists responded to the book at the time, but it was subsequently brought up in a 2006 US Senate hearing in support of climate change denial, and Peter Doran felt compelled to publish a statement in The New York Times decrying the misinterpretation of his work. The British Antarctic Survey and NASA also issued statements affirming the strength of climate science after the hearing.

By 2009, research was finally able to combine historical weather station data with satellite measurements to create consistent temperature records going back to 1957, which demonstrated warming of >0.05 °C/decade since 1957 across the continent, with cooling in East Antractica offset by the average temperature increase of at least 0.176 ± 0.06 °C per decade in West Antarctica. Subsequent research confirmed clear warming over West Antarctica in the 20th century with the only uncertainty being the magnitude. Over 2012-2013, estimates based on WAIS Divide ice cores and the revised Byrd Station temperature record even suggested a much larger West Antarctica warming of 2.4 °C (4.3 °F) since 1958, or around 0.46 °C (0.83 °F) per decade, although there has been some uncertainty about it. In 2022, a study narrowed the warming of the Central area of the West Antarctic Ice Sheet between 1959 and 2000 to 0.31 °C (0.56 °F) per decade, and conclusively attributed it to increases in greenhouse gas concentrations caused by human activity.

East Antarctica cooled in the 1980s and 1990s, even as West Antarctica warmed (left-hand side). This trend largely reversed in 2000s and 2010s (right-hand side).

Local changes in atmospheric circulation patterns like the Interdecadal Pacific Oscillation or the Southern Annular Mode, slowed or even partially reversed the warming of West Antarctica between 2000 and 2020, with the Antarctic Peninsula experiencing cooling from 2002. While a variability in those patterns is natural, ozone depletion had also led the Southern Annular Mode (SAM) to be stronger than it had been in the past 600 years of observations. Studies predicted a reversal in the SAM once the ozone layer began to recover following the Montreal Protocol starting from 2002, and these changes were consistent with their predictions. As these patterns reversed, the East Antarctica interior demonstrated clear warming over those two decades. In particular, the South Pole warmed by 0.61 ± 0.34 °C per decade between 1990 and 2020, which is three times the global average. The Antarctica-wide warming trend also continued after 2000, and in February 2020, the continent recorded its highest temperature of 18.3 °C, which was a degree higher than the previous record of 17.5 °C in March 2015.

Models predict that under the most intense climate change scenario, known as RCP8.5, Antarctic temperatures will be up 4 °C (7.2 °F), on average, by 2100 and this will be accompanied by a 30% increase in precipitation and a 30% decrease in total sea ice. RCPs were developed in the late 2000s, and early 2020s research considers RCP8.5 much less likely than the more "moderate" scenarios like RCP 4.5, which lies in between the worst-case and the Paris Agreement goals.

Ice loss and accumulation

Mass change of ice in Antarctica between 2002–2020.
Contrasting temperature trends across parts of Antarctica, as well as its remoteness, mean that some locations lose mass, particularly at the coasts, while others that are more inland continue to gain it, and estimating an average trend can be difficult. In 2018, a systematic review of all previous studies and data by the Ice Sheet Mass Balance Inter-comparison Exercise (IMBIE) estimated an increase in West Antarctic ice sheet annual mass loss from 53 ± 29 Gt (gigatonnes) in 1992 to 159 ± 26 Gt in the final five years of the study. On the Antarctic Peninsula, the study estimated −20 ± 15 Gt per year with an increase in loss of roughly 15 Gt per year after year 2000, with a significant role played by the loss of ice shelves. The review's overall estimate was that Antarctica lost 2720 ± 1390 gigatons of ice from 1992 to 2017, averaging 109 ± 56 Gt per year. This would amount to 7.6 millimeters of sea level rise. Then, though, a 2021 analysis of data from four different research satellite systems (Envisat, European Remote-Sensing Satellite, GRACE and GRACE-FO and ICESat) indicated annual mass loss of only about 12 Gt from 2012-2016, due to much greater ice gain in East Antarctica than estimated earlier, which had offset most of the losses from West Antarctica. The East Antarctic ice sheet can still gain mass in spite of warming because effects of climate change on the water cycle increase precipitation over its surface, which then freezes and helps to build up more ice.: 1262 

Near-future sea level rise

An illustration of the theory behind marine ice sheet and marine ice cliff instabilities.
By 2100, net ice loss from Antarctica alone is expected to add about 11 cm (5 in) to global sea level rise.: 1270  However, processes such as marine ice sheet instability, which describes the potential for warm water currents to enter between the seafloor and the base of the ice sheet once it is no longer heavy enough to displace such flows, and marine ice cliff instability, when ice cliffs with heights greater than 100 m (330 ft) may collapse under their own weight once they are no longer buttressed by ice shelves (which has never been observed, and only occurs in some of the modelling) may cause West Antarctica have a much larger contribution. Such processes may increase sea level rise caused by Antarctica to 41 cm (16 in) by 2100 under the low-emission scenario and 57 cm (22 in) under the high-emission scenario.: 1270  Some scientists have even larger estimates, but all agree it would have a greater impact and become much more likely to occur under higher warming scenarios, where it may double the overall 21st century sea level rise to 2 meters or more. One study suggested that if the Paris Agreement is followed and global warming is limited to 2 °C (3.6 °F), the loss of ice in Antarctica will continue at the 2020 rate for the rest of the century, but if a trajectory leading to 3 °C (5.4 °F) is followed, Antarctica ice loss will accelerate after 2060 and start adding 0.5 cm to global sea levels per year by 2100.

Weakening Antarctic circulation

Normally, some seasonal meltwater from the Antarctic ice sheet helps to drive the lower-cell circulation. However, climate change has greatly increased meltwater amounts, which threatens to destabilize it.: 1240 

Ice loss from Antarctica also generates more fresh meltwater, at a rate of 1100-1500 billion tons (GT) per year.: 1240  This meltwater then mixes back into the Southern Ocean, which makes its water fresher. This freshening of the Southern Ocean results in increased stratification and stabilization of its layers,: 1240  and this has the single largest impact on the long-term properties of Southern Ocean circulation. These changes in the Southern Ocean cause the upper cell circulation to speed up, accelerating the flow of major currents, while the lower cell circulation slows down, as it is dependent on the highly saline Antarctic bottom water, which already appears to have been observably weakened by the freshening, in spite of the limited recovery during 2010s.: 1240  Since the 1970s, the upper cell has strengthened by 3-4 sverdrup (Sv; represents a flow of 1 million cubic meters per second), or 50-60% of its flow, while the lower cell has weakened by a similar amount, but because of its larger volume, these changes represent a 10-20% weakening.

Since the 1970s, the upper cell of the circulation has strengthened, while the lower cell weakened.

While these effects weren't fully caused by climate change, with some role played by the natural cycle of Interdecadal Pacific Oscillation, they are likely to worsen in the future. As of early 2020s, climate models' best, limited-confidence estimate is that the lower cell would continue to weaken, while the upper cell may strengthen by around 20% over the 21st century. A key reason for the uncertainty is limited certainty about future ice loss from Antarctica and the poor and inconsistent representation of ocean stratification in even the CMIP6 models - the most advanced generation available as of early 2020s. One study suggests that the circulation would lose half its strength by 2050 under the worst climate change scenario, with greater losses occurring afterwards.

It is possible that the South Ocean overturning circulation may not simply continue to weaken in response to increased warming and freshening, but will eventually collapse outright, in a way which would be difficult to reverse and constitute an example of tipping points in the climate system. This would be similar to some projections for Atlantic meridional overturning circulation (AMOC), which is also affected by the ocean warming and by meltwater flows from the declining Greenland ice sheet. However, Southern Hemisphere is only inhabited by 10% of the world's population, and the Southern Ocean overturning circulation has historically received much less attention than the AMOC. Some preliminary research suggests that such a collapse may become likely once global warming reaches levels between 1.7 °C (3.1 °F) and 3 °C (5.4 °F), but there is far less certainty than with the estimates for most other tipping points in the climate system. Even if initiated in the near future, the circulation's collapse is unlikely to be complete until close to 2300, Similarly, impacts such as the reduction in precipitation in the Southern Hemisphere, with a corresponding increase in the North, or a decline of fisheries in the Southern Ocean with a potential collapse of certain marine ecosystems, are also expected to unfold over multiple centuries.

Long-term future

If countries cut greenhouse gas emissions significantly (lowest trace), then sea level rise by 2100 can be limited to 0.3–0.6 m (1–2 ft). If the emissions instead accelerate rapidly (top trace), sea levels could rise 5 m (16+12 ft) by the year 2300. Higher levels of sea level rise would involve substantial ice loss from Antarctica, including East Antarctica.

Sea level rise will continue well after 2100, but potentially at very different rates. According to the most recent reports of the Intergovernmental Panel on Climate Change (SROCC and the IPCC Sixth Assessment Report), there will be a median rise of 16 cm (6.3 in) and maximum rise of 37 cm (15 in) under the low-emission scenario. On the other hand, the highest emission scenario results in a median rise of 1.46 m (5 ft) metres, with a minimum of 60 cm (2 ft) and a maximum of 2.89 m (9+12 ft)).

Over even longer timescales, West Antarctic ice sheet, which is much smaller than the East Antarctic ice sheet is and grounded deep below the sea level, is considered highly vulnerable. The melting of all the ice in West Antarctica would increase the total sea level rise to 4.3 m (14 ft 1 in). However, mountain ice caps not in contact with water are less vulnerable than the majority of the ice sheet, which is located below the sea level. Its collapse would cause ~3.3 m (10 ft 10 in) of sea level rise. This kind of collapse is now considered practically inevitable, because it appears to have already occurred during the Eemian period 125,000 years ago, when temperatures were similar to the early 21st century. The Amundsen Sea also appears to be warming at rates which would make the ice sheet's collapse effectively inevitable.

The only way to stop ice loss from West Antarctica once triggered is by lowering the global temperature to 1 °C (1.8 °F) below the preindustrial level. This would be 2 °C (3.6 °F) below the temperature of 2020. Other researchers suggested that a climate engineering intervention to stabilize the ice sheet's glaciers may delay its loss by centuries and give more time to adapt. However this is an uncertain proposal, and would end up as one of the most expensive projects ever attempted. Otherwise, the disappearance of the West Antarctic ice sheet would take an estimated 2000 years. The absolute minimum for the loss of West Antarctica ice is 500 years, and the potential maximum is 13,000 years. Once the ice sheet is lost, there would also be an additional 1 m (3 ft 3 in) of sea level rise over the next 1000 years, caused by isostatic rebound of land beneath the ice sheet.

Retreat of Cook Glacier - a key part of the Wilkes Basin - during the Eemian ~120,000 years ago and an earlier Pleistocene interglacial ~330,000 years ago. These retreats would have added about 0.5 m (1 ft 8 in) and 0.9 m (2 ft 11 in) to sea level rise.

If the warming were to remain at elevated levels for a long time, the East Antarctic Ice Sheet would eventually become the dominant contributor to sea level rise, simply because it contains far more ice than any other large ice mass. First, though, it would see sustained erosion at the so-called subglacial basins, such as Totten Glacier and Wilkes Basin, which are located in vulnerable locations below the sea level. Estimates suggest that they would be committed to disappearance once the global warming reaches 3 °C (5.4 °F), although the plausible temperature range is between 2 °C (3.6 °F) and 6 °C (11 °F). Once it becomes too warm for these subglacial basins, their collapse would unfold over a period of around 2,000 years, although it may be as fast as 500 years or as slow as 10,000 years. The loss of all this ice would ultimately add between 1.4 m (4 ft 7 in) and 6.4 m (21 ft 0 in) to sea levels, depending on the ice sheet model used. Isostatic rebound of the newly ice-free land would also add 8 cm (3.1 in) and 57 cm (1 ft 10 in), respectively. Evidence from the Pleistocene shows that partial loss can also occur at lower warming levels: Wilkes Basin is estimated to have lost enough ice to add 0.5 m (1 ft 8 in) to sea levels between 115,000 and 129,000 years ago, during the Eemian, and about 0.9 m (2 ft 11 in) between 318,000 and 339,000 years ago, during the Marine Isotope Stage 9.

The entire East Antarctic Ice Sheet holds enough ice to raise global sea levels by 53.3 m (175 ft). Its complete melting is also possible, but it would require very high warming and a lot of time:n 2022, an extensive assessment of tipping points in the climate system published in the Science Magazine concluded that the ice sheet would take a minimum of 10,000 years to fuly melt. It would most likely be committed to complete disappearance only once the global warming reaches about 7.5 °C (13.5 °F), with the minimum and the maximum range between 5 °C (9.0 °F) and 10 °C (18 °F). Another estimate suggested that at least 6 °C (11 °F) would be needed to melt two thirds of its volume. It would also for the entire ice sheet to be lost.

If the ice sheet were to disappear, then the change in ice-albedo feedback would increase the global temperature by 0.6 °C (1.1 °F), while the regional temperatures would increase by around 2 °C (3.6 °F). The loss of the subglacial basins alone would only add about 0.05 °C (0.090 °F) to global temperatures due to their relatively limited area, and a correspondingly low impact on global albedo.

Situation during geologic time scales

Polar climatic temperature changes throughout the Cenozoic, showing glaciation of Antarctica toward the end of the Eocene, thawing near the end of the Oligocene and subsequent Miocene re-glaciation.

The icing of Antarctica began in the Late Palaeocene or middle Eocene between 60 and 45.5 million years ago and escalated during the Eocene–Oligocene extinction event about 34 million years ago. CO2 levels were then about 760 ppm and had been decreasing from earlier levels in the thousands of ppm. Carbon dioxide decrease, with a tipping point of 600 ppm, was the primary agent forcing Antarctic glaciation. The glaciation was favored by an interval when the Earth's orbit favored cool summers but oxygen isotope ratio cycle marker changes were too large to be explained by Antarctic ice-sheet growth alone indicating an ice age of some size. The opening of the Drake Passage may have played a role as well though models of the changes suggest declining CO2 levels to have been more important.

The Western Antarctic ice sheet declined somewhat during the warm early Pliocene epoch, approximately five to three million years ago; during this time the Ross Sea opened up. But there was no significant decline in the land-based Eastern Antarctic ice sheet.

See also


This page was last updated at 2024-02-13 23:49 UTC. Update now. View original page.

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