"Continental glacier" redirects here. For the glacier located in Wyoming, see Continental Glacier.
One of Earth's two ice sheets: The Antarctic ice sheet covers about 98% of the Antarcticcontinent and is the largest single mass of ice on Earth, with an average thickness of over 2 kilometers.[1]
Inglaciology, an ice sheet, also known as a continental glacier,[2] is a mass of glacialice that covers surrounding terrain and is greater than 50,000 km2 (19,000 sq mi).[3] The only current ice sheets are the Antarctic ice sheet and the Greenland ice sheet. Ice sheets are bigger than ice shelves or alpine glaciers. Masses of ice covering less than 50,000 km2 are termed an ice cap. An ice cap will typically feed a series of glaciers around its periphery.
Although the surface is cold, the base of an ice sheet is generally warmer due to geothermal heat. In places, melting occurs and the melt-water lubricates the ice sheet so that it flows more rapidly. This process produces fast-flowing channels in the ice sheet — these are ice streams.
An ice sheet is "an ice body originating on land that covers an area of continental size, generally defined as covering >50,000 km2 , and that has formed over thousands of years through accumulation and compaction of snow".[4]: 2234
Common properties
Carbon stores and fluxes in present-day ice sheets (2019), and the predicted impact on carbon dioxide (where data exists). Estimated carbon fluxes are measured in Tg C a−1 (megatonnes of carbon per year) and estimated sizes of carbon stores are measured in Pg C (thousands of megatonnes of carbon). DOC = dissolved organic carbon, POC = particulate organic carbon.[5]
Ice sheets have the following properties: "An ice sheet flows outward from a high central ice plateau with a small average surface slope. The margins usually slope more steeply, and most ice is discharged through fast-flowing ice streams or outlet glaciers, often into the sea or into ice shelves floating on the sea."[4]: 2234
Ice movement is dominated by the motion of glaciers, whose activity is determined by a number of processes.[6] Their motion is the result of cyclic surges interspersed with longer periods of inactivity, on both hourly and centennial time scales.
Until recently, ice sheets were viewed as inert components of the carbon cycle and were largely disregarded in global models. Research in the past decade has transformed this view, demonstrating the existence of uniquely adapted microbial communities, high rates of biogeochemical/physical weathering in ice sheets and storage and cycling of organic carbon in excess of 100 billion tonnes, as well as nutrients (see diagram).[5]
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 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. 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.
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.
The Greenland ice sheet is an ice sheet which forms the second largest body of ice in the world. It is an average of 1.67 km (1.0 mi) thick, and over 3 km (1.9 mi) thick at its maximum.[7] It is almost 2,900 kilometres (1,800 mi) long in a north–south direction, with a maximum width of 1,100 kilometres (680 mi) at a latitude of 77°N, near its northern edge.[8] The ice sheet covers 1,710,000 square kilometres (660,000 sq mi), around 80% of the surface of Greenland, or about 12% of the area of the Antarctic ice sheet.[7] The term 'Greenland ice sheet' is often shortened to GIS or GrIS in scientific literature.[9][10][11][12]
Greenland has had major glaciers and ice caps for at least 18 million years,[13] but a single ice sheet first covered most of the island some 2.6 million years ago.[14] Since then, it has both grown[15][16] and contracted significantly.[17][18][19] The oldest known ice on Greenland is about 1 million years old.[20] Due to anthropogenic greenhouse gas emissions, the ice sheet is now the warmest it has been in the past 1000 years,[21] and is losing ice at the fastest rate in at least the past 12,000 years.[22]
Every summer, parts of the surface melt and ice cliffs calve into the sea. Normally the ice sheet would be replenished by winter snowfall,[10] but due to global warming the ice sheet is melting two to five times faster than before 1850,[23] and snowfall has not kept up since 1996.[24] If the Paris Agreement goal of staying below 2 °C (3.6 °F) is achieved, melting of Greenland ice alone would still add around 6 cm (2+1⁄2in) to global sea level rise by the end of the century. If there are no reductions in emissions, melting would add around 13 cm (5 in) by 2100,[25]: 1302 with a worst-case of about 33 cm (13 in).[26] For comparison, melting has so far contributed 1.4 cm (1⁄2in) since 1972,[27] while sea level rise from all sources was 15–25 cm (6–10 in)) between 1901 and 2018.[28]: 5
A narrated tour about Greenland's ice sheet.
If all 2,900,000 cubic kilometres (696,000 cu mi) of the ice sheet were to melt, it would increase global sea levels by ~7.4 m (24 ft).[7] Global warming between 1.7 °C (3.1 °F) and 2.3 °C (4.1 °F) would likely make this melting inevitable.[12] However, 1.5 °C (2.7 °F) would still cause ice loss equivalent to 1.4 m (4+1⁄2ft) of sea level rise,[29] and more ice will be lost if the temperatures exceed that level before declining.[12] If global temperatures continue to rise, the ice sheet will likely disappear within 10,000 years.[30][31] At very high warming, its future lifetime goes down to around 1,000 years.[26]
The melting of the Greenland and West Antarctic ice sheets will continue to contribute to sea level rise over long time-scales. The Greenland ice sheet loss is mainly driven by melt from the top. Antarctic ice loss is driven by warm ocean water melting the outlet glaciers.[32]: 1215
Future melt of the West Antarctic ice sheet is potentially abrupt under a high emission scenario, as a consequence of a partial collapse.[33]: 595–596 Part of the ice sheet is grounded on bedrock below sea level. This makes it possibly vulnerable to the self-enhancing process of marine ice sheet instability. Marine ice cliff instability could also contribute to a partial collapse. But there is limited evidence for its importance.[32]: 1269–1270 A partial collapse of the ice sheet would lead to rapid sea level rise and a local decrease in ocean salinity. It would be irreversible for decades and possibly even millennia.[33]: 595–596 The complete loss of the West Antarctic ice sheet would cause over 5 metres (16 ft) of sea level rise.[34]
In contrast to the West Antarctic ice sheet, melt of the Greenland ice sheet is projected to take place more gradually over millennia.[33]: 595–596 Sustained warming between 1 °C (1.8 °F) (low confidence) and 4 °C (7.2 °F) (medium confidence) would lead to a complete loss of the ice sheet. This would contribute 7 m (23 ft) to sea levels globally.[35]: 363 The ice loss could become irreversible due to a further self-enhancing feedback. This is called the elevation-surface mass balance feedback. When ice melts on top of the ice sheet, the elevation drops. Air temperature is higher at lower altitudes, so this promotes further melting.[35]: 362
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[36] and 45.5 million years ago[37] and escalated during the Eocene–Oligocene extinction event about 34 million years ago. CO2 levels were then about 760 ppm[38] 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.[39] 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.[40] The opening of the Drake Passage may have played a role as well[41] though models of the changes suggest declining CO2 levels to have been more important.[42]
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.[43] But there was no significant decline in the land-based Eastern Antarctic ice sheet.[44]
^ abIPCC, 2021: Annex VII: Glossary [Matthews, J.B.R., V. Möller, R. van Diemen, J.S. Fuglestvedt, V. Masson-Delmotte, C. Méndez, S. Semenov, A. Reisinger (eds.)]. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 2215–2256, doi:10.1017/9781009157896.022.
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^Fox-Kemper, B.; Hewitt, H.T.; Xiao, C.; Aðalgeirsdóttir, G.; Drijfhout, S.S.; Edwards, T.L.; Golledge, N.R.; Hemer, M.; Kopp, R.E.; Krinner, G.; Mix, A. (2021). Masson-Delmotte, V.; Zhai, P.; Pirani, A.; Connors, S.L.; Péan, C.; Berger, S.; Caud, N.; Chen, Y.; Goldfarb, L. (eds.). "Chapter 9: Ocean, Cryosphere and Sea Level Change"(PDF). Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, UK and New York, NY, US. Archived(PDF) from the original on 24 October 2022. Retrieved 22 October 2022.
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