"Continental glacier" redirects here. For the glacier located in Wyoming, see
Continental Glacier.
In
glaciology, 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
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]
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]
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]
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.[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
The icing of Antarctica began in the Late Palaeocene or middle
Eocene between 60[33] and 45.5 million years ago[34] and escalated during the
Eocene–Oligocene extinction event about 34 million years ago. CO2 levels were then about 760
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]
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
Pliocene, formed a lower planation surface at 500 to 1000 meters above sea level. A third stage of uplift created multiple
valleys and
fjords below the planation surfaces. This uplift intensified glaciation due to increased
orographic precipitation and
cooler surface temperatures, allowing ice to accumulate and persist.[42][43] As recently as 3 million years ago, during the Pliocene warm period, Greenland's ice was limited to the highest peaks in the east and the south.[44] Ice cover gradually expanded since then,[20] until the
atmospheric CO2 levels dropped to between 280 and 320
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]
^
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.
^Noël, B.; van Kampenhout, L.; Lenaerts, J. T. M.; van de Berg, W. J.; van den Broeke, M. R. (19 January 2021). "A 21st Century Warming Threshold for Sustained Greenland Ice Sheet Mass Loss". Geophysical Research Letters. 48 (5): e2020GL090471.
Bibcode:
2021GeoRL..4890471N.
doi:
10.1029/2020GL090471.
hdl:
2268/301943.
S2CID233632072.
^Höning, Dennis; Willeit, Matteo; Calov, Reinhard; Klemann, Volker; Bagge, Meike; Ganopolski, Andrey (27 March 2023). "Multistability and Transient Response of the Greenland Ice Sheet to Anthropogenic CO2 Emissions". Geophysical Research Letters. 50 (6): e2022GL101827.
doi:
10.1029/2022GL101827.
S2CID257774870.
^
abThiede, Jörn; Jessen, Catherine; Knutz, Paul; Kuijpers, Antoon; Mikkelsen, Naja; Nørgaard-Pedersen, Niels; Spielhagen, Robert F (2011). "Millions of Years of Greenland Ice Sheet History Recorded in Ocean Sediments". Polarforschung. 80 (3): 141–159.
hdl:
10013/epic.38391.
^Reyes, Alberto V.; Carlson, Anders E.; Beard, Brian L.; Hatfield, Robert G.; Stoner, Joseph S.; Winsor, Kelsey; Welke, Bethany; Ullman, David J. (25 June 2014). "South Greenland ice-sheet collapse during Marine Isotope Stage 11". Nature. 510 (7506): 525–528.
Bibcode:
2014Natur.510..525R.
doi:
10.1038/nature13456.
PMID24965655.
S2CID4468457.
^
abcCollins M., M. Sutherland, L. Bouwer, S.-M. Cheong, T. Frölicher, H. Jacot Des Combes, M. Koll Roxy, I. Losada, K. McInnes, B. Ratter, E. Rivera-Arriaga, R.D. Susanto, D. Swingedouw, and L. Tibig, 2019:
Chapter 6: Extremes, Abrupt Changes and Managing Risk. In:
IPCC Special Report on the Ocean and Cryosphere in a Changing Climate [H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA, pp. 589–655.
doi:
10.1017/9781009157964.008.
^
abJapsen, Peter; Green, Paul F.; Bonow, Johan M.; Nielsen, Troels F.D.; Chalmers, James A. (5 February 2014). "From volcanic plains to glaciated peaks: Burial, uplift and exhumation history of southern East Greenland after opening of the NE Atlantic". Global and Planetary Change. 116: 91–114.
Bibcode:
2014GPC...116...91J.
doi:
10.1016/j.gloplacha.2014.01.012.