"Continental glacier" redirects here. For the glacier located in Wyoming, seeContinental Glacier.
One of Earth's two ice sheets: TheAntarctic ice sheet covers about 98% of theAntarcticcontinent and is the largest single mass ofice on Earth. It has an average thickness of over 2 kilometers.[1]
Inglaciology, anice sheet, also known as acontinental glacier,[2] is a mass ofglacialice that covers surrounding terrain and is greater than 50,000 km2 (19,000 sq mi).[3] The only current ice sheets are theAntarctic ice sheet and theGreenland ice sheet. Ice sheets are bigger thanice shelves or alpineglaciers. Masses of ice covering less than 50,000 km2 are termed anice 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 togeothermal 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 areice streams.
Even stable ice sheets are continually in motion as the ice gradually flows outward from the central plateau, which is the tallest point of the ice sheet, and towards the margins. The ice sheet slope is low around the plateau but increases steeply at the margins.[4]
Increasing global air temperatures due to climate change take around 10,000 years to directly propagate through the ice before they influence bed temperatures, but may have an effect through increased surface melting, producing moresupraglacial lakes. These lakes may feed warm water to glacial bases and facilitate glacial motion.[5]
An ice sheet is a body of ice which covers a land area of continental size - meaning that it exceeds 50,000 km2.[4] The currently existing two ice sheets inGreenland andAntarctica have a much greater area than this minimum definition, measuring at 1.7 million km2 and 14 million km2, respectively. Both ice sheets are also very thick, as they consist of a continuous ice layer with an average thickness of 2 km (1 mi).[1][6] This ice layer forms because most of the snow which falls onto the ice sheet never melts, and is instead compressed by the mass of newer snow layers.[4]
This process of ice sheet growth is still occurring nowadays, as can be clearly seen in an example that occurred inWorld War II. ALockheed P-38 Lightning fighter plane crashed in Greenland in 1942. It was only recovered 50 years later. By then, it had been buried under 81 m (268 feet) of ice which had formed over that time period.[7]
Glacial flow rate in the Antarctic ice sheet.The motion of ice in Antarctica
Even stable ice sheets are continually in motion as the ice gradually flows outward from the central plateau, which is the tallest point of the ice sheet, and towards the margins. The ice sheet slope is low around the plateau but increases steeply at the margins.[4] This difference in slope occurs due to an imbalance between high ice accumulation in the central plateau and lower accumulation, as well as higherablation, at the margins. This imbalance increases theshear stress on a glacier until it begins to flow. The flow velocity and deformation will increase as the equilibrium line between these two processes is approached.[8][9] This motion is driven bygravity but is controlled by temperature and the strength of individual glacier bases. A number of processes alter these two factors, resulting in cyclic surges of activity interspersed with longer periods of inactivity, on time scales ranging from hourly (i.e. tidal flows) to thecentennial (Milankovich cycles).[9]
On an unrelated hour-to-hour basis, surges of ice motion can be modulated by tidal activity. The influence of a 1 m tidal oscillation can be felt as much as 100 km from the sea.[10] During largerspring tides, an ice stream will remain almost stationary for hours at a time, before a surge of around a foot in under an hour, just after the peak high tide; a stationary period then takes hold until another surge towards the middle or end of the falling tide.[11][12] At neap tides, this interaction is less pronounced, and surges instead occur approximately every 12 hours.[11]
Increasing global air temperatures due to climate change take around 10,000 years to directly propagate through the ice before they influence bed temperatures, but may have an effect through increased surface melting, producing moresupraglacial lakes. These lakes may feed warm water to glacial bases and facilitate glacial motion.[5] Lakes of a diameter greater than ~300 m are capable of creating a fluid-filled crevasse to the glacier/bed interface. When these crevasses form, the entirety of the lake's (relatively warm) contents can reach the base of the glacier in as little as 2–18 hours – lubricating the bed and causing the glacier tosurge.[13] Water that reaches the bed of a glacier may freeze there, increasing the thickness of the glacier by pushing it up from below.[14]
The collapse of theLarsen B ice shelf had profound effects on the velocities of its feeder glaciers.Accelerated ice flows after the break-up of an ice shelf
As the margins end at the marine boundary, excess ice is discharged through ice streams oroutlet glaciers. Then, it either falls directly into the sea or is accumulated atop the floatingice shelves.[4]: 2234 Those ice shelves thencalve icebergs at their periphery if they experience excess of ice. Ice shelves would also experience accelerated calving due to basal melting. In Antarctica, this is driven by heat fed to the shelf by thecircumpolar deep water current, which is 3 °C above the ice's melting point.[15]
The presence of ice shelves has a stabilizing influence on the glacier behind them, while an absence of an ice shelf becomes destabilizing. For instance, whenLarsen B ice shelf in theAntarctic Peninsula had collapsed over three weeks in February 2002, the four glaciers behind it -Crane Glacier,Green Glacier,Hektoria Glacier andJorum Glacier - all started to flow at a much faster rate, while the two glaciers (Flask and Leppard) stabilized by the remnants of the ice shelf did not accelerate.[16]
The collapse of the Larsen B shelf was preceded by thinning of just 1 metre per year, while some other Antarctic ice shelves have displayed thinning of tens of metres per year.[5] Further, increased ocean temperatures of 1 °C may lead to up to 10 metres per year of basal melting.[5] Ice shelves are always stable under mean annual temperatures of −9 °C, but never stable above −5 °C; this places regional warming of 1.5 °C, as preceded the collapse of Larsen B, in context.[5]
In the 1970s,Johannes Weertman proposed that becauseseawater is denser than ice, then any ice sheets grounded belowsea level inherently become less stable as they melt due toArchimedes' principle.[17] Effectively, these marine ice sheets must have enough mass to exceed the mass of the seawater displaced by the ice, which requires excess thickness. As the ice sheet melts and becomes thinner, the weight of the overlying ice decreases. At a certain point, sea water could force itself into the gaps which form at the base of the ice sheet, andmarine ice sheet instability (MISI) would occur.[17][18]
Even if the ice sheet is grounded below the sea level, MISI cannot occur as long as there is a stable ice shelf in front of it.[19] The boundary between the ice sheet and the ice shelf, known as thegrounding line, is particularly stable if it is constrained in anembayment.[19] In that case, the ice sheet may not be thinning at all, as the amount of ice flowing over the grounding line would be likely to match the annual accumulation of ice from snow upstream.[18] Otherwise, ocean warming at the base of an ice shelf tends to thin it through basal melting. As the ice shelf becomes thinner, it exerts less of a buttressing effect on the ice sheet, the so-called back stress increases and the grounding line is pushed backwards.[18] The ice sheet is likely to start losing more ice from the new location of the grounding line and so become lighter and less capable of displacing seawater. This eventually pushes the grounding line back even further, creating aself-reinforcing mechanism.[18][20]
Distribution of meltwater hotspots caused by ice losses inPine Island Bay, the location of both Thwaites (TEIS refers to Thwaites Eastern Ice Shelf) and Pine Island Glaciers.[21]
Because the entire West Antarctic Ice Sheet is grounded below the sea level, it would be vulnerable to geologically rapid ice loss in this scenario.[22][23] In particular, theThwaites andPine Island glaciers are most likely to be prone to MISI, and both glaciers have been rapidly thinning and accelerating in recent decades.[24][25][26][27] As a result, sea level rise from the ice sheet could be accelerated by tens of centimeters within the 21st century alone.[28]
The majority of the East Antarctic Ice Sheet would not be affected.Totten Glacier is the largest glacier there which is known to be subject to MISI - yet, its potential contribution to sea level rise is comparable to that of the entire West Antarctic Ice Sheet.[29] Totten Glacier has been losing mass nearly monotonically in recent decades,[30] suggesting rapid retreat is possible in the near future, although the dynamic behavior of Totten Ice Shelf is known to vary on seasonal to interannual timescales.[31][32][33] The Wilkes Basin is the only major submarine basin in Antarctica that is not thought to be sensitive to warming.[26] Ultimately, even geologically rapid sea level rise would still most likely require several millennia for the entirety of these ice masses (WAIS and the subglacial basins) to be lost.[34][35]
A collage of footage and animation to explain the changes that are occurring on the West Antarctic Ice Sheet, narrated by glaciologistEric Rignot
A related process known asMarine Ice Cliff Instability (MICI) posits that ice cliffs which exceed ~90 m (295+1⁄2 ft) in above-ground height and are ~800 m (2,624+1⁄2 ft) in basal (underground) height are likely to collapse under their own weight once the peripheral ice stabilizing them is gone.[36] Their collapse then exposes the ice masses following them to the same instability, potentially resulting in a self-sustaining cycle of cliff collapse and rapid ice sheet retreat - i.e. sea level rise of a meter or more by 2100 from Antarctica alone.[18][37][19][38] This theory had been highly influential - in a 2020 survey of 106 experts, the paper which had advanced this theory was considered more important than even the year 2014IPCC Fifth Assessment Report.[39] Sea level rise projections which involve MICI are much larger than the others, particularly under high warming rate.[40]
At the same time, this theory has also been highly controversial.[36] It was originally proposed in order to describe how the large sea level rise during thePliocene and theLast Interglacial could have occurred[36][37] - yet more recent research found that these sea level rise episodes can be explained without any ice cliff instability taking place.[41][36][42] Research inPine Island Bay inWest Antarctica (the location ofThwaites andPine Island Glacier) had foundseabed gouging by ice from theYounger Dryas period which appears consistent with MICI.[43][41] However, it indicates "relatively rapid" yet still prolonged ice sheet retreat, with a movement of >200 km (120 mi) inland taking place over an estimated 1100 years (from ~12,300 yearsBefore Present to ~11,200 B.P.)[43]
If MICI can occur, the structure of the glacierembayment (viewed from the top) would do a lot to determine how quickly it may proceed. Bays which are deep or narrow towards the exit would experience much less rapid retreat than the opposite[44]
In recent years, 2002-2004 fast retreat ofCrane Glacier immediately after the collapse of theLarsen B ice shelf (before it reached a shallowfjord and stabilized) could have involved MICI, but there weren't enough observations to confirm or refute this theory.[45] The retreat ofGreenland ice sheet's three largest glaciers -Jakobshavn,Helheim, andKangerdlugssuaq Glacier - did not resemble predictions from ice cliff collapse at least up until the end of 2013,[41][46] but an event observed at Helheim Glacier in August 2014 may fit the definition.[41][47] Further, modelling done after the initial hypothesis indicates that ice-cliff instability would require implausibly fast ice shelf collapse (i.e. within an hour for ~90 m (295+1⁄2 ft)-tall cliffs),[48] unless the ice had already been substantially damaged beforehand.[45] Further, ice cliff breakdown would produce a large number of debris in the coastal waters - known asice mélange - and multiple studies indicate their build-up would slow or even outright stop the instability soon after it started.[49][50][51][44]
Some scientists - including the originators of the hypothesis, Robert DeConto and David Pollard - have suggested that the best way to resolve the question would be to precisely determine sea level rise during theLast Interglacial.[41] MICI can be effectively ruled out if SLR at the time was lower than 4 m (13 ft), while it is very likely if the SLR was greater than6 m (19+1⁄2 ft).[41] As of 2023, the most recent analysis indicates that the Last Interglacial SLR is unlikely to have been higher than 2.7 m (9 ft),[52] as higher values in other research, such as5.7 m (18+1⁄2 ft),[53] appear inconsistent with the newpaleoclimate data fromThe Bahamas and the known history of the Greenland Ice Sheet.[52]
As a smaller part of Antarctica, WAIS is also more strongly affected byclimate change. There has been warming over the ice sheet since the 1950s,[57][58] and a substantial retreat of its coastal glaciers since at least the 1990s.[59] Estimates suggest it added around7.6 ± 3.9 mm (19⁄64 ± 5⁄32 in) to the globalsea level rise between 1992 and 2017,[60] and has been losing ice in the 2010s at a rate equivalent to 0.4 millimetres (0.016 inches) of annual sea level rise.[61] While some of its losses are offset by the growth of theEast 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.[62] Freshmeltwater from WAIS also contributes toocean stratification and dilutes the formation of saltyAntarctic bottom water, which destabilizesSouthern Ocean overturning circulation.[62][63][64]
In the long term, the West Antarctic Ice Sheet is likely to disappear due to the warming which has already occurred.[65]Paleoclimate evidence suggests that this has already happened during theEemian period, when the global temperatures were similar to the early 21st century.[66][67] It is believed that the loss of the ice sheet would take place between 2,000 and 13,000 years in the future,[68][69] although several centuries of high emissions may shorten this to 500 years.[70] 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,[71] but this would require a higher level of warming.[72]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.[70]
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.[73] Because the collapse of the ice sheet would be preceded by the loss ofThwaites Glacier andPine Island Glacier, some have instead proposedinterventions to preserve them. In theory, adding thousands of gigatonnes of artificially createdsnow could stabilize them,[74] but it would be extraordinarily difficult and may not account for the ongoing acceleration of ocean warming in the area.[65] 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 largestcivil engineering interventions in history.
The surface of the EAIS is the driest, windiest, and coldest place on Earth. Lack of moisture in the air, highalbedo from the snow as well as the surface's consistently high elevation[79] results in the reported cold temperature records of nearly −100 °C (−148 °F).[80][81] It is the only place on Earth cold enough for atmospheric temperature inversion to occur consistently. That is, while theatmosphere 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.[79]
Due to these factors, East Antarctica had experienced slight cooling for decades while the rest of the world warmed as the result ofclimate change. Clear warming over East Antarctica only started to occur since the year 2000, and was not conclusively detected until the 2020s.[82][83] In the early 2000s, cooling over East Antarctica seemingly outweighing warming over the rest of the continent was frequentlymisinterpreted by the media and occasionally used as an argument forclimate change denial.[84][85][86] After 2009, improvements in Antarctica'sinstrumental temperature record have proven that the warming overWest Antarctica resulted in consistent net warming across the continent since the 1957.[87]
Because the East Antarctic ice sheet has barely warmed, it is still gaining ice on average.[88][89] for instance,GRACE satellite data indicated East Antarctica mass gain of60 ± 13 billion tons per year between 2002 and 2010.[90] It is most likely to first see sustained losses of ice at its most vulnerable locations such asTotten Glacier andWilkes 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,[91][92] 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 theice sheet model used.[93] The EAIS as a whole holds enough ice to raise global sea levels by 53.3 m (175 ft).[78] 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.[91][92]
TheGreenland 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.[96] 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 of77°N, near its northern edge.[97] The ice sheet covers 1,710,000 square kilometres (660,000 sq mi), around 80% of the surface ofGreenland, or about 12% of the area of theAntarctic ice sheet.[96] The term 'Greenland ice sheet' is often shortened to GIS or GrIS inscientific literature.[98][99][100][101]
Greenland has had majorglaciers andice caps for at least 18 million years,[102] but a single ice sheet first covered most of the island some 2.6 million years ago.[103] Since then, it has both grown[104][105] and contracted significantly.[106][107][108] The oldest known ice on Greenland is about 1 million years old.[109] Due to anthropogenicgreenhouse gas emissions, the ice sheet is now the warmest it has been in the past 1000 years,[110] and is losing ice at the fastest rate in at least the past 12,000 years.[111]
Every summer, parts of the surface melt andice cliffs calve into the sea. Normally the ice sheet would be replenished by winter snowfall,[99] but due toglobal warming the ice sheet is melting two to five times faster than before 1850,[112] and snowfall has not kept up since 1996.[113] If theParis Agreement goal of staying below 2 °C (3.6 °F) is achieved, melting of Greenland ice alone would still add around6 cm (2+1⁄2 in) to globalsea 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,[114]: 1302 with a worst-case of about 33 cm (13 in).[115] For comparison, melting has so far contributed1.4 cm (1⁄2 in) since 1972,[116] while sea level rise from all sources was 15–25 cm (6–10 in) between 1901 and 2018.[117]: 5
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).[96] Global warming between 1.7 °C (3.1 °F) and 2.3 °C (4.1 °F) would likely make this melting inevitable.[101] However, 1.5 °C (2.7 °F) would still cause ice loss equivalent to1.4 m (4+1⁄2 ft) of sea level rise,[118] and more ice will be lost if the temperatures exceed that level before declining.[101] If global temperatures continue to rise, the ice sheet will likely disappear within 10,000 years.[119][120] At very high warming, its future lifetime goes down to around 1,000 years.[115]
Beneath the Greenland ice sheet are mountains and lake basins.
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.[121]
Historically, ice sheets were viewed as inert components of thecarbon cycle and were largely disregarded in global models. In 2010s, research had demonstrated the existence of uniquely adaptedmicrobial communities, high rates ofbiogeochemical and physical weathering in ice sheets, and storage and cycling of organic carbon in excess of 100 billion tonnes.[121]
There is a massive contrast in carbon storage between the two ice sheets. While only about 0.5-27 billion tonnes of pure carbon are present underneath the Greenland ice sheet, 6000-21,000 billion tonnes of pure carbon are thought to be located underneath Antarctica.[121] This carbon can act as aclimate change feedback if it is gradually released through meltwater, thus increasing overallcarbon dioxide emissions.[122]
For comparison, 1400–1650 billion tonnes are contained within the Arcticpermafrost.[123] Also for comparison, the annual human caused carbon dioxide emissions amount to around 40 billion tonnes of CO2.[28]: 1237
In Greenland, there is one known area, atRussell Glacier, where meltwater carbon is released into the atmosphere asmethane, which has a much largerglobal warming potential than carbon dioxide.[124] However, it also harbours large numbers ofmethanotrophic bacteria, which limit those emissions.[125][126]
A reconstruction of how Heinrich events would have likely proceeded, with the Laurentide ice sheet first growing to an unsustainable position, where the base of its periphery becomes too warm, and then rapidly losing ice until it is reduced to sustainable size[127]
Normally, the transitions between glacial and interglacial states are governed byMilankovitch cycles, which are patterns ininsolation (the amount of sunlight reaching the Earth). These patterns are caused by the variations in shape of the Earth's orbit and its angle relative to the Sun, caused by the gravitational pull of other planets as they go through their own orbits.[128][129]
For instance, during at least the last 100,000 years, portions of the ice sheet covering much of North America, theLaurentide Ice Sheet broke apart sending large flotillas of icebergs into the North Atlantic. When these icebergs melted they dropped the boulders and other continental rocks they carried, leaving layers known asice rafted debris. These so-calledHeinrich events, named after their discovererHartmut Heinrich, appear to have a 7,000–10,000-yearperiodicity, and occur during cold periods within the last interglacial.[130]
Internal ice sheet "binge-purge" cycles may be responsible for the observed effects, where the ice builds to unstable levels, then a portion of the ice sheet collapses. External factors might also play a role in forcing ice sheets.Dansgaard–Oeschger events are abrupt warmings of the northern hemisphere occurring over the space of perhaps 40 years. While these D–O events occur directly after each Heinrich event, they also occur more frequently – around every 1500 years; from this evidence, paleoclimatologists surmise that the same forcings may drive both Heinrich and D–O events.[131]
Hemispheric asynchrony in ice sheet behavior has been observed by linking short-term spikes of methane in Greenland ice cores and Antarctic ice cores. DuringDansgaard–Oeschger events, the northern hemisphere warmed considerably, dramatically increasing the release of methane from wetlands, that were otherwise tundra during glacial times. This methane quickly distributes evenly across the globe, becoming incorporated in Antarctic and Greenland ice. With this tie, paleoclimatologists have been able to say that the ice sheets on Greenland only began to warm after the Antarctic ice sheet had been warming for several thousand years. Why this pattern occurs is still open for debate.[132][133]
Polar climatic temperature changes throughout theCenozoic, showingglaciation of Antarctica toward the end of theEocene, thawing near the end of theOligocene and subsequentMiocene re-glaciation.
The icing of Antarctica began in the Late Palaeocene or middleEocene between 60[134] and 45.5 million years ago[135] and escalated during theEocene–Oligocene extinction event about 34 million years ago. CO2 levels were then about 760ppm[136] and had been decreasing from earlier levels in the thousands of ppm. Carbon dioxide decrease, with atipping point of 600 ppm, was the primary agent forcing Antarctic glaciation.[137] The glaciation was favored by an interval when the Earth's orbit favored cool summers butoxygen isotope ratio cycle marker changes were too large to be explained by Antarctic ice-sheet growth alone indicating anice age of some size.[138] The opening of theDrake Passage may have played a role as well[139] though models of the changes suggest declining CO2 levels to have been more important.[140]
The Western Antarctic ice sheet declined somewhat during the warm earlyPliocene epoch, approximately five to three million years ago; during this time theRoss Sea opened up.[141] But there was no significant decline in the land-based Eastern Antarctic ice sheet.[142]
Timeline of the ice sheet's formation from 2.9 to 2.6 million years ago[98]
While there is evidence of largeglaciers inGreenland for most of the past 18 million years,[102] these ice bodies were probably similar to various smaller modern examples, such asManiitsoq andFlade 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 10million years ago, during the middleMiocene, when the twopassive continental margins which now form the uplands of West and East Greenland experienceduplift, and ultimately formed the upper planation surface at a height of 2000 to 3000 meterabove sea level.[143][144]
Later uplift, during thePliocene, formed a lower planation surface at 500 to 1000 meters above sea level. A third stage of uplift created multiplevalleys andfjords below the planation surfaces. This uplift intensified glaciation due to increasedorographic precipitation andcooler surface temperatures, allowing ice to accumulate and persist.[143][144] 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.[145] Ice cover gradually expanded since then,[103] until theatmospheric CO2 levels dropped to between 280 and 320ppm 2.7–2.6 million years ago, by which time temperatures had dropped sufficiently for the disparateice caps to connect and cover most of the island.[98]
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