Ice sheet

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One of Earth's two ice sheets: The Antarctic ice sheet covers about 98% of the Antarctic continent and is the largest single mass of ice on Earth, with an average thickness of over 2 kilometers.[1]

In glaciology, an ice sheet, also known as a continental glacier,[2] is a mass of glacial ice 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.

In previous geologic time spans (

Weichselian ice sheet covered Northern Europe and the Patagonian Ice Sheet covered southern South America
.

Definition

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

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]

Earth's current two ice sheets

Antarctic ice sheet

The
Ronne Ice Shelf, and outlet glaciers that drain into the Amundsen Sea.[7]

The

South Magnetic Pole and the Amundsen–Scott South Pole Station
.

The surface of the EAIS is the driest, windiest, and coldest place on Earth. Lack of moisture in the air, high
albedo from the snow as well as the surface's consistently high elevation[10] results in the reported cold temperature records of nearly −100 °C (−148 °F).[11][12] It is the only place on Earth cold enough for atmospheric temperature inversion to occur consistently. That is, while the atmosphere is typically warmest near the surface and becomes cooler at greater elevation, atmosphere during the Antarctic winter is cooler at the surface than in its middle layers. Consequently, greenhouse gases actually trap heat in the middle atmosphere and reduce its flow towards the surface while the temperature inversion lasts.[10]

Greenland ice sheet

Greenland ice sheet as seen from space

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.[13] 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.[14] 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.[13] The term 'Greenland ice sheet' is often shortened to GIS or GrIS in the scientific literature.[15][16][17][18]

Greenland has had major glaciers and ice caps for at least 18 million years,[19] but a single ice sheet first covered most of the island some 2.6 million years ago.[20] Since then, it has both grown[21][22] and contracted significantly.[23][24][25] The oldest known ice on Greenland is about 1 million years old.[26] Due to anthropogenic greenhouse gas emissions, the ice sheet is now the warmest it has been in the past 1000 years,[27] and is losing ice at the fastest rate in at least the past 12,000 years.[28]

A narrated tour about Greenland's ice sheet.

Melting due to climate change

The melting of the

outlet glaciers.[29]
: 1215 

Future melt of the West Antarctic ice sheet is potentially abrupt under a high emission scenario, as a consequence of a partial collapse.[30]: 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.[29]: 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.[30]: 595–596  The complete loss of the West Antarctic ice sheet would cause over 5 metres (16 ft) of sea level rise.[31]

In contrast to the West Antarctic ice sheet, melt of the Greenland ice sheet is projected to take place more gradually over millennia.[30]: 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.[32]: 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.[32]: 362 

In geologic timescales

Antarctic ice sheet during geologic timescales

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[35] 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.[36] 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.[37] The opening of the Drake Passage may have played a role as well[38] though models of the changes suggest declining CO2 levels to have been more important.[39]

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.[40] But there was no significant decline in the land-based Eastern Antarctic ice sheet.[41]

Greenland ice sheet during geologic timescales

Timeline of the ice sheet's formation from 2.9 to 2.6 million years ago[15]

While there is evidence of large glaciers in Greenland for most of the past 18 million years,[19] these ice bodies were probably similar to various smaller modern examples, such as Maniitsoq and Flade Isblink, which cover 76,000 and 100,000 square kilometres (29,000 and 39,000 sq mi) around the periphery. Conditions in Greenland were not initially suitable for a single coherent ice sheet to develop, but this began to change around 10 million years ago, during the middle Miocene, when the two passive continental margins which now form the uplands of West and East Greenland experienced uplift, and ultimately formed the upper planation surface at a height of 2000 to 3000 meter above sea level.[42][43]

Later uplift, during the
ppm 2.7–2.6 million years ago, by which time temperatures had dropped sufficiently for the disparate ice caps to connect and cover most of the island.[15]

See also

References

  1. ^ "Ice Sheets". National Science Foundation.
  2. ^ American Meteorological Society, Glossary of Meteorology Archived 2012-06-23 at the Wayback Machine
  3. ^ "Glossary of Important Terms in Glacial Geology". Archived from the original on 2006-08-29. Retrieved 2006-08-22.
  4. ^ a b IPCC, 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.
  5. ^
    doi:10.1038/s41467-019-11394-4. Material was copied from this source, which is available under a Creative Commons Attribution 4.0 International License
    .
  6. .
  7. ^ Davies, Bethan (21 October 2020). "West Antarctic Ice Sheet". AntarcticGlaciers.org.
  8. .
  9. .
  10. ^ .
  11. .
  12. ^ Vizcarra, Natasha (25 June 2018). "New study explains Antarctica's coldest temperatures". National Snow and Ice Data Center. Retrieved 10 January 2024.
  13. ^ a b "How Greenland would look without its ice sheet". BBC News. 14 December 2017. Archived from the original on 7 December 2023. Retrieved 7 December 2023.
  14. ^ Greenland Ice Sheet. 24 October 2023. Archived from the original on 30 October 2017. Retrieved 26 May 2022.
  15. ^
    PMID 30420596
    .
  16. .
  17. .
  18. .
  19. ^ .
  20. ^ (PDF) from the original on 8 November 2020. Retrieved 7 December 2023.
  21. (PDF) from the original on 20 December 2023. Retrieved 7 December 2023.
  22. from the original on 7 December 2023. Retrieved 7 December 2023.
  23. .
  24. .
  25. ^ Gautier, Agnieszka (29 March 2023). "How and when did the Greenland Ice Sheet form?". National Snow and Ice Data Center. Archived from the original on 28 May 2023. Retrieved 5 December 2023.
  26. .
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  28. .
  29. ^
  30. ^ .
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  32. ^ .
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  34. ^ Sedimentological evidence for the formation of an East Antarctic ice sheet in Eocene/Oligocene time Archived 2012-06-16 at the Wayback Machine Palaeogeography, palaeoclimatology, & palaeoecology ISSN 0031-0182, 1992, vol. 93, no1-2, pp. 85–112 (3 p.)
  35. ^ "New CO2 data helps unlock the secrets of Antarctic formation". phys.org. September 13, 2009. Retrieved 2023-06-06.
  36. S2CID 206533232
    . Retrieved 2014-01-28.
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External links