"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]
As a smaller part of Antarctica, WAIS is also more strongly affected by climate change. There has been warming over the ice sheet since the 1950s,[8][9] and a substantial retreat of its coastal glaciers since at least the 1990s.[10] Estimates suggest it added around 7.6 ± 3.9 mm (19⁄64 ± 5⁄32in) to the global sea level rise between 1992 and 2017,[11] and has been losing ice in the 2010s at a rate equivalent to 0.4 millimetres (0.016 inches) of annual sea level rise.[12] While some of its losses are offset by the growth of the East Antarctic ice sheet, Antarctica as a whole will most likely lose enough ice by 2100 to add 11 cm (4.3 in) to sea levels. Further, marine ice sheet instability may increase this amount by tens of centimeters, particularly under high warming.[13] Fresh meltwater from WAIS also contributes to ocean stratification and dilutes the formation of salty Antarctic bottom water, which destabilizes Southern Ocean overturning circulation.[13][14][15]
In the long term, the West Antarctic Ice Sheet is likely to disappear due to the warming which has already occurred.[16]Paleoclimate evidence suggests that this has already happened during the Eemian period, when the global temperatures were similar to the early 21st century.[17][18] It is believed that the loss of the ice sheet would take place between 2,000 and 13,000 years in the future,[19][20] although several centuries of high emissions may shorten this to 500 years.[21] 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. Total sea level rise from West Antarctica increases to 4.3 m (14 ft 1 in) if they melt as well,[22] but this would require a higher level of warming.[23]Isostatic rebound of ice-free land may also add around 1 m (3 ft 3 in) to the global sea levels over another 1,000 years.[21]
The preservation of WAIS may require a persistent reduction of global temperatures to 1 °C (1.8 °F) below the preindustrial level, or to 2 °C (3.6 °F) below the temperature of 2020.[24] Because the collapse of the ice sheet would be preceded by the loss of Thwaites Glacier and Pine Island Glacier, some have instead proposed interventions to preserve them. In theory, adding thousands of gigatonnes of artificially created snow could stabilize them,[25] but it would be extraordinarily difficult and may not account for the ongoing acceleration of ocean warming in the area.[16] Others suggest that building obstacles to warm water flows beneath glaciers would be able to delay the disappearance of the ice sheet by many centuries, but it would still require one of the largest civil engineering interventions in history.
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[28] results in the reported cold temperature records of nearly −100 °C (−148 °F).[29][30] 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.[28]
Due to these factors, East Antarctica had experienced slight cooling for decades while the rest of the world warmed as the result of climate change. Clear warming over East Antarctica only started to occur since the year 2000, and was not conclusively detected until the 2020s.[31][32] In the early 2000s, cooling over East Antarctica seemingly outweighing warming over the rest of the continent was frequently misinterpreted by the media and occasionally used as an argument for climate change denial.[33][34][35] After 2009, improvements in Antarctica's instrumental temperature record have proven that the warming over West Antarctica resulted in consistent net warming across the continent since the 1957.[36]
Because the East Antarctic ice sheet has barely warmed, it is still gaining ice on average.[37][38] for instance, GRACE satellite data indicated East Antarctica mass gain of 60 ± 13 billion tons per year between 2002 and 2010.[39] It is most likely to first see sustained losses of ice at its most vulnerable locations such as Totten Glacier and Wilkes Basin. Those areas are sometimes collectively described as East Antarctica's subglacial basins, and it is believed that once the warming reaches around 3 °C (5.4 °F), then they would start to collapse over a period of around 2,000 years,[40][41] This collapse 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.[42] The EAIS as a whole holds enough ice to raise global sea levels by 53.3 m (175 ft).[27] However, it would take global warming in a range between 5 °C (9.0 °F) and 10 °C (18 °F), and a minimum of 10,000 years for the entire ice sheet to be lost.[40][41]
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.[43] 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.[44] 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.[43] The term 'Greenland ice sheet' is often shortened to GIS or GrIS in scientific literature.[45][46][47][48]
Greenland has had major glaciers and ice caps for at least 18 million years,[49] but a single ice sheet first covered most of the island some 2.6 million years ago.[50] Since then, it has both grown[51][52] and contracted significantly.[53][54][55] The oldest known ice on Greenland is about 1 million years old.[56] Due to anthropogenic greenhouse gas emissions, the ice sheet is now the warmest it has been in the past 1000 years,[57] and is losing ice at the fastest rate in at least the past 12,000 years.[58]
Every summer, parts of the surface melt and ice cliffs calve into the sea. Normally the ice sheet would be replenished by winter snowfall,[46] but due to global warming the ice sheet is melting two to five times faster than before 1850,[59] and snowfall has not kept up since 1996.[60] 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,[61]: 1302 with a worst-case of about 33 cm (13 in).[62] For comparison, melting has so far contributed 1.4 cm (1⁄2in) since 1972,[63] while sea level rise from all sources was 15–25 cm (6–10 in)) between 1901 and 2018.[64]: 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).[43] Global warming between 1.7 °C (3.1 °F) and 2.3 °C (4.1 °F) would likely make this melting inevitable.[48] However, 1.5 °C (2.7 °F) would still cause ice loss equivalent to 1.4 m (4+1⁄2ft) of sea level rise,[65] and more ice will be lost if the temperatures exceed that level before declining.[48] If global temperatures continue to rise, the ice sheet will likely disappear within 10,000 years.[66][67] At very high warming, its future lifetime goes down to around 1,000 years.[62]
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.[68]: 1215
Future melt of the West Antarctic ice sheet is potentially abrupt under a high emission scenario, as a consequence of a partial collapse.[69]: 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.[68]: 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.[69]: 595–596 The complete loss of the West Antarctic ice sheet would cause over 5 metres (16 ft) of sea level rise.[70]
In contrast to the West Antarctic ice sheet, melt of the Greenland ice sheet is projected to take place more gradually over millennia.[69]: 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.[71]: 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.[71]: 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[72] and 45.5 million years ago[73] and escalated during the Eocene–Oligocene extinction event about 34 million years ago. CO2 levels were then about 760 ppm[74] 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.[75] 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.[76] The opening of the Drake Passage may have played a role as well[77] though models of the changes suggest declining CO2 levels to have been more important.[78]
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.[79] But there was no significant decline in the land-based Eastern Antarctic ice sheet.[80]
Timeline of the ice sheet's formation from 2.9 to 2.6 million years ago[45]
While there is evidence of large glaciersinGreenland for most of the past 18 million years,[49] 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.[81][82]
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.[81][82] 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.[83] Ice cover gradually expanded since then,[50] 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.[45]
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