Antarctic ice sheet

Coordinates: 90°S 0°E / 90°S 0°E / -90; 0
Source: Wikipedia, the free encyclopedia.
Antarctic Ice Sheet
ice shelves in gray, and ice-free land in brown.
TypeIce sheet
LocationAntarctica
Area14×10^6 km2 (5.4×10^6 sq mi)[3]
Thickness2.2 km (1.4 mi) on average,[3] 4.9 km (3.0 mi) at maximum[2]
StatusOngoing net loss of ice, regionally variable[4][5]

The Antarctic ice sheet is a

ice flow, and glacier mass balance
between the three regions.

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 WAIS. In the 20th century, EAIS had been one of the only places on Earth which displayed limited cooling instead of warming, even as the WAIS warmed by over 0.1 °C/decade from 1950s to 2000, with an average warming trend of >0.05 °C/decade since 1957 across the whole continent. 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

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. This meltwater 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.

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

See also: Geography of Antarctica
The bedrock topography of Antarctica, critical to understand dynamic motion of the continental ice sheets.[1]

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.[6] A cubic kilometer of ice weighs approximately 0.92 metric gigatonnes, meaning that the ice sheet weighs about 24,380,000 gigatonnes. This ice is equivalent to around 61% of all fresh water on Earth.[7] The only other currently existing ice sheet on Earth is the Greenland ice sheet in the Arctic.[8]

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). Some glaciologists consider ice cover over the relatively small Antarctic Peninsula (also in West Antarctica) to be the third ice sheet in Antarctica,[9][10]: 2234  in part because its drainage basins are very distinct from the WAIS.[5] Collectively, these ice sheets have an average thickness of around 2 kilometres (1.2 mi),[3] Even the Transantarctic Mountains are largely covered by ice, with only some mountain summits and the McMurdo Dry Valleys being ice-free in the present. Some coastal areas also have exposed bedrock that is not covered by ice.[11] During the Late Cenozoic Ice Age, many of those areas had been covered by ice as well.[12][13]

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

outlet glaciers that drain into the Amundsen Sea.[15] Thwaites Glacier and Pine Island Glacier are the two most important outlet glaciers.[16]

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.[17]
This section is an excerpt from Climate change in Antarctica § Temperature and weather changes.[edit]
Parts of East Antarctica (marked in blue) are currently the only place on Earth to regularly experience negative greenhouse effect during certain months of the year. At greater warming levels, this effect is likely to disappear due to increasing concentrations of water vapor over Antarctica[18]

Antarctica is the coldest and driest continent on Earth, as well as the one with the highest average elevation.[19] Because Antarctica is so dry, there is little water vapor, so its air doesn't conduct heat well.[18] Further, it is surrounded by the Southern Ocean, which is far more effective at absorbing heat than any other ocean.[20] 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.[19] Antarctica is so cold that it is the only place on Earth where atmospheric temperature inversion occurs every winter.[19] 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 instead becomes cooler than middle layers of the atmosphere.[18] This means that 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.[18][19] Thus, even the early climate models predicted that temperature trends over Antarctica would emerge slower and be more subtle than they are elsewhere.[21]

Moreover, there were fewer than twenty permanent

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.[22] The only exception was the Antarctic Peninsula, where warming was both well-documented and strongly pronounced:[23] It was eventually found to have warmed by 3 °C (5.4 °F) since the mid-20th century.[24] 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).[25][26]

Antarctic surface temperature trends, in °C/decade. Red represents areas where temperatures have increased the most since the 1950s.[27]

A 2002 analysis led by

US Senate hearing in support of climate change denial,[40] and Peter Doran felt compelled to publish a statement in The New York Times decrying the misinterpretation of his work.[36] The British Antarctic Survey and NASA also issued statements affirming the strength of climate science after the hearing.[41][42]

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.[27][43] Subsequent research confirmed clear warming over West Antarctica in the 20th century with the only uncertainty being the magnitude.[44] 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,[45][46][47][48] although there has been some uncertainty about it.[49] 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.[50]

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).[51]

Local changes in atmospheric circulation patterns like 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.[52][53][54] 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,[55][56][57] and these changes were consistent with their predictions.[58] As these patterns reversed, the East Antarctica interior demonstrated clear warming over those two decades.[51][59] 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.[60][61] 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.[62]

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.[63] RCPs were developed in the late 2000s, and early 2020s research considers RCP8.5 much less likely[64] than the more "moderate" scenarios like RCP 4.5, which lies in between the worst-case and the Paris Agreement goals.[65][66]

Ice loss and accumulation

This section is an excerpt from Climate change in Antarctica § Observed changes in ice mass.[edit]
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.
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.[70]
: 1262 

Near-future sea level rise

This section is an excerpt from Climate change in Antarctica § 21st century ice loss and sea level rise.[edit]
An illustration of the theory behind marine ice sheet and marine ice cliff instabilities.[71]
By 2100, net ice loss from Antarctica alone is expected to add about 11 cm (5 in) to global
seafloor and the base of the ice sheet once it is no longer heavy enough to displace such flows,[72] 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)[73] 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.[70]: 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.[74][75][76] 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.[77]

Weakening Antarctic circulation

Normally, some seasonal meltwater from the Antarctic ice sheet helps to drive the lower-cell circulation.[78] However, climate change has greatly increased meltwater amounts, which threatens to destabilize it.[79]: 1240 

Ice loss from Antarctica also generates more fresh

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.[88][89]

Since the 1970s, the upper cell of the circulation has strengthened, while the lower cell weakened.[89]

While these effects weren't fully caused by climate change, with some role played by the natural cycle of

Interdecadal Pacific Oscillation,[90][91] 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.[79] 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.[92] One study suggests that the circulation would lose half its strength by 2050 under the worst climate change scenario,[82] with greater losses occurring afterwards.[93]

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

fisheries in the Southern Ocean with a potential collapse of certain marine ecosystems, are also expected to unfold over multiple centuries.[93]

Long-term future

This section is an excerpt from Climate change in Antarctica § Long-term sea level rise.[edit]
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).[97] If the emissions instead accelerate rapidly (top trace), sea levels could rise 5 m (16+1⁄2 ft) by the year 2300. Higher levels of sea level rise would involve substantial ice loss from Antarctica, including East Antarctica.[97]

Sea level rise will continue well after 2100, but potentially at very different rates. According to the most recent reports of the

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+1⁄2 ft)).[70]

Over even longer timescales,

Eemian period 125,000 years ago, when temperatures were similar to the early 21st century.[100][101][102][103][104] The Amundsen Sea also appears to be warming at rates which would make the ice sheet's collapse effectively inevitable.[105][106]

The only way to reverse 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.

isostatic rebound of land beneath the ice sheet.[112]

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.[113]
This section is an excerpt from East Antarctic Ice Sheet § Long-term future.[edit]

If global warming were to reach higher levels, then the EAIS would play an increasingly larger role in sea level rise occurring after 2100. According to the most recent reports of the

SROCC and the IPCC Sixth Assessment Report), the most intense climate change scenario, where the anthropogenic emissions increase continuously, RCP8.5, would result in Antarctica alone losing a median of 1.46 m (4 ft 9 in) (confidence interval between 60 cm (2.0 ft) and 2.89 m (9 ft 6 in)) by 2300, which would involve some loss from the EAIS in addition to the erosion of the WAIS. This Antarctica-only sea level rise would be in addition to ice losses from the Greenland ice sheet and mountain glaciers, as well as the thermal expansion of ocean water.[114] If the warming were to remain at elevated levels for a long time, then the East Antarctic Ice Sheet would eventually become the dominant contributor to sea level rise, simply because it contains the largest amount of ice.[114][115]

Sustained ice loss from the EAIS would begin with the significant erosion of the so-called subglacial basins, such as

Isostatic rebound of the newly ice-free land would also add 8 cm (3.1 in) and 57 cm (1 ft 10 in), respectively.[118]


The entire East Antarctic Ice Sheet holds enough ice to raise global sea levels by 53.3 m (175 ft).

Science Magazine concluded that the ice sheet would take a minimum of 10,000 years to fully 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).[115][117] Another estimate suggested that at least 6 °C (11 °F) would be needed to melt two thirds of its volume.[120]

If the entire 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.[115][117]

Situation during geologic time scales

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

ppm[123] 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.[124] 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.[125] The opening of the Drake Passage may have played a role as well[126] though models of the changes suggest declining CO2 levels to have been more important.[127]

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.[128] But there was no significant decline in the land-based Eastern Antarctic ice sheet.[129]

See also

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