Ice sheet

Source: Wikipedia, the free encyclopedia.
(Redirected from
Marine ice sheet instability
)
"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 Antarctic continent and is the largest single mass of ice on Earth. It has 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
.

Overview

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 in Greenland and Antarctica 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][5][1] 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 in World War II. A Lockheed 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.[6]

Dynamics

Glacial flows

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 higher ablation, at the margins. This imbalance increases the shear 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.[7][8] This motion is driven by gravity 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 the centennial (Milankovich cycles).[8]

On an 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.

spring 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.[10][11] At neap tides, this interaction is less pronounced, and surges instead occur approximately every 12 hours.[10]

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 more supraglacial lakes. These lakes may feed warm water to glacial bases and facilitate glacial motion.[12] 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 to surge.[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]

Boundary conditions

Larsen 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 or

outlet glaciers. Then, it either falls directly into the sea or is accumulated atop the floating ice shelves.[4]: 2234  Those ice shelves then calve 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 the circumpolar 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, when

Larsen B ice shelf in the Antarctic Peninsula had collapsed over three weeks in February 2002, the four glaciers behind it - Crane Glacier, Green Glacier, Hektoria Glacier and Jorum 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.[12] Further, increased ocean temperatures of 1 °C may lead to up to 10 metres per year of basal melting.[12] 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.[12]

Marine ice sheet instability

In the 1970s, Johannes Weertman proposed that because seawater is denser than ice, then any ice sheets grounded below sea level inherently become less stable as they melt due to Archimedes' 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, and marine 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 the grounding line, is particularly stable if it is constrained in an embayment.[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 an 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 a self-reinforcing mechanism.[18][20]

Vulnerable locations

Pine 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.

Pine 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 the 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]

Marine Ice Cliff Instability

A collage of footage and animation to explain the changes that are occurring on the West Antarctic Ice Sheet, narrated by glaciologist Eric Rignot

A related process known as Marine Ice Cliff Instability (MICI) posits that due to the physical characteristics of ice, subaerial ice cliffs exceeding ~90 meters in height are likely to collapse under their own weight, and this could lead to self-sustaining ice sheet retreat.[18] It is thought to occur when an ice sheet grounded below sea level with an inland-sloping bed has exposed ice cliffs after the removal of peripheral ice. The tall cliffs, exposed to hydrofracturing forces and without buttressing, are structurally unstable due to their mass, and their collapse is thought to then expose the ice behind it to the same instability, resulting in a cycle of cliff collapse. Surface melt can further enhance MICI through ponding and hydrofracture.[19][36] However, this theory is controversial, and has never been directly observed in the present, only in geological records.[37] Recent research has highlighted the importance of glacial geometry in causing or preventing marine ice cliff instability, suggesting that buttressing ice cliffs might be a way to prevent their collapse.[38][39]

Earth's current two ice sheets

Antarctic ice sheet

This section is an excerpt from Antarctic ice sheet.[edit]
The
ice flow, and glacier mass balance
between the three regions.

West Antarctic ice sheet

West Antarctic ice sheet
TypeIce sheet
Area<1,970,000 km2 (760,000 sq mi)[40]
Thickness~1.05 km (0.7 mi) (average),[41] ~2 km (1.2 mi) (maximum)[40]
StatusReceding
This section is an excerpt from West Antarctic Ice Sheet.[edit]

The

Ronne Ice Shelf, and outlet glaciers that drain into the Amundsen Sea.[42]

As a smaller part of Antarctica, WAIS is also more strongly affected by

marine ice sheet instability may increase this amount by tens of centimeters, particularly under high warming.[48] Fresh meltwater from WAIS also contributes to ocean stratification and dilutes the formation of salty Antarctic bottom water, which destabilizes Southern Ocean overturning circulation.[48][49][50]

In the long term, the West Antarctic Ice Sheet is likely to disappear due to the warming which has already occurred.

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.[56]

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.[59] 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,[60] but it would be extraordinarily difficult and may not account for the ongoing acceleration of ocean warming in the area.[51] 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.

East Antarctic ice sheet

East Antarctic ice sheet
TypeIce sheet
Thickness~2.2 km (1.4 mi) (average),[61] ~4.9 km (3.0 mi) (maximum) [62]
This section is an excerpt from East Antarctic Ice Sheet.[edit]

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[65] results in the reported cold temperature records of nearly −100 °C (−148 °F).[66][67] 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.[65]

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.[68][69] 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.[70][71][72] 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.[73]

Because the East Antarctic ice sheet has barely warmed, it is still gaining ice on average.
ice sheet model used.[79] The EAIS as a whole holds enough ice to raise global sea levels by 53.3 m (175 ft).[64] 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.[77][78]

Greenland ice sheet

This section is an excerpt from Greenland ice sheet.[edit]
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.[80] 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.[81] 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.[80] The term 'Greenland ice sheet' is often shortened to GIS or GrIS in scientific literature.[82][83][84][85]

Greenland has had major glaciers and ice caps for at least 18 million years,[86] but a single ice sheet first covered most of the island some 2.6 million years ago.[87] Since then, it has both grown[88][89] and contracted significantly.[90][91][92] The oldest known ice on Greenland is about 1 million years old.[93] Due to anthropogenic greenhouse gas emissions, the ice sheet is now the warmest it has been in the past 1000 years,[94] and is losing ice at the fastest rate in at least the past 12,000 years.[95]

Every summer, parts of the surface melt and ice cliffs calve into the sea. Normally the ice sheet would be replenished by winter snowfall,[83] but due to global warming the ice sheet is melting two to five times faster than before 1850,[96] and snowfall has not kept up since 1996.[97] 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+1⁄2 in) 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,[98]: 1302  with a worst-case of about 33 cm (13 in).[99] For comparison, melting has so far contributed 1.4 cm (1⁄2 in) since 1972,[100] while sea level rise from all sources was 15–25 cm (6–10 in)) between 1901 and 2018.[101]: 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).[80] Global warming between 1.7 °C (3.1 °F) and 2.3 °C (4.1 °F) would likely make this melting inevitable.[85] However, 1.5 °C (2.7 °F) would still cause ice loss equivalent to 1.4 m (4+1⁄2 ft) of sea level rise,[102] and more ice will be lost if the temperatures exceed that level before declining.[85] If global temperatures continue to rise, the ice sheet will likely disappear within 10,000 years.[103][104] At very high warming, its future lifetime goes down to around 1,000 years.[99]

Carbon cycle

particulate organic carbon.[105]

Historically, ice sheets were viewed as inert components of the

carbon cycle and were largely disregarded in global models. In 2010s, research had demonstrated 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.[105] There is a massive hemispheric 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 are thought to be located underneath Antarctica.[105]

For comparison, 1400–1650 billion tonnes are contained within the Arctic

climate change feedback if it is gradually released through meltwater, thus increasing overall carbon dioxide emissions.[107]

In Greenland, there is one known area, at Russell Glacier, where meltwater carbon is released into the atmosphere as methane, which has a much larger global warming potential than carbon dioxide:[108] however, it also harbours large numbers of methanotrophic bacteria, which limit those emissions.[109][110]

In geologic timescales

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[111]

Normally, the transitions between glacial and interglacial states are governed by

insolation (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.[112][113]

For instance, during at least the last 100,000 years, portions of the ice sheet covering much of North America, the

Heinrich events, named after their discoverer Hartmut Heinrich, appear to have a 7,000–10,000-year periodicity, and occur during cold periods within the last interglacial.[114]

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.[115]

Hemispheric asynchrony in ice sheet behavior has been observed by linking short-term spikes of methane in Greenland ice cores and Antarctic ice cores. During Dansgaard–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.[116][117]

Antarctic ice sheet during geologic timescales

This section is an excerpt from Antarctic ice sheet § Situation during geologic time scales.[edit]
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[120] 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.[121] 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.[122] The opening of the Drake Passage may have played a role as well[123] though models of the changes suggest declining CO2 levels to have been more important.[124]

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.[125] But there was no significant decline in the land-based Eastern Antarctic ice sheet.[126]

Greenland ice sheet during geologic timescales

This section is an excerpt from Greenland ice sheet § Geological history.[edit]
Timeline of the ice sheet's formation from 2.9 to 2.6 million years ago[82]

While there is evidence of large glaciers in Greenland for most of the past 18 million years,[86] 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.[127][128]

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.[82]

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External links


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