Ice sheet
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In glaciology, an ice sheet, also known as a continental glacier,<ref>American Meteorological Society, Glossary of Meteorology Template:Webarchive</ref> is a mass of glacial ice that covers surrounding terrain and is greater than Template:Convert.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.
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.<ref name="IPCC_AR6_AnnexVII" />
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.<ref name="IPCCc4" />
In previous geologic time spans (glacial periods) there were other ice sheets. During the Last Glacial Period at Last Glacial Maximum, the Laurentide Ice Sheet covered much of North America. In the same period, the Weichselian ice sheet covered Northern Europe and the Patagonian Ice Sheet covered southern South America.
OverviewEdit
An ice sheet is a body of ice which covers a land area of continental size - meaning that it exceeds 50,000 km2.<ref name="IPCC_AR6_AnnexVII" /> 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 Template:Cvt.<ref name="NSFfactsheet" /><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.<ref name="IPCC_AR6_AnnexVII" />
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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
DynamicsEdit
Glacial flowsEdit
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.<ref name="IPCC_AR6_AnnexVII" /> 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.<ref name="Easterbrook">Easterbrook, Don J., Surface Processes and Landforms, 2nd Edition, Prentice-Hall Inc., 1999Template:Page needed</ref><ref name="GreveBlatter2009">Template:Cite bookTemplate:Page needed</ref> 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).<ref name="GreveBlatter2009" />
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.<ref name=Clarke2005>Template:Cite journal</ref> During larger 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.<ref name=Bindschalder2003>Template:Cite journal</ref><ref name=Anandakrishnan2003>Template:Cite journal</ref> At neap tides, this interaction is less pronounced, and surges instead occur approximately every 12 hours.<ref name=Bindschalder2003/>
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.<ref name=IPCCc4>Sections 4.5 and 4.6 of Template:IPCC4/wg1/4</ref> 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.<ref name=Krawczynski2007>Template:Cite conference</ref> Water that reaches the bed of a glacier may freeze there, increasing the thickness of the glacier by pushing it up from below.<ref name="Bell2011">Template:Cite journal</ref>
Boundary conditionsEdit
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.<ref name="IPCC_AR6_AnnexVII">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.</ref>Template:Rp 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.<ref name=Walker2007>Template:Cite journal</ref>
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.<ref name=Scambos2004>Template:Cite journal</ref>
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.<ref name="IPCCc4" /> Further, increased ocean temperatures of 1 °C may lead to up to 10 metres per year of basal melting.<ref name="IPCCc4" /> 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.<ref name="IPCCc4" />
Marine ice sheet instabilityEdit
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.<ref name="Weertman1974" /> 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.<ref name="Weertman1974">Template:Cite journal</ref><ref name="Pollard2015" />
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.<ref name="Pattyn 2018" /> 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.<ref name="Pattyn 2018" /> 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.<ref name="Pollard2015" /> 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.<ref name="Pollard2015" /> 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.<ref name="Pollard2015">Template:Cite journal</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Vulnerable locationsEdit
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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> In particular, the Thwaites and Pine Island glaciers are most likely to be prone to MISI, and both glaciers have been rapidly thinning and accelerating in recent decades.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="Gardner 2018">Template:Cite journal</ref><ref>Template:Cite journal</ref> As a result, sea level rise from the ice sheet could be accelerated by tens of centimeters within the 21st century alone.<ref name="IPCC AR6 WG1 Ch.9">Template:Cite journal</ref>
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.<ref>Template:Cite journal</ref> Totten Glacier has been losing mass nearly monotonically in recent decades,<ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> The Wilkes Basin is the only major submarine basin in Antarctica that is not thought to be sensitive to warming.<ref name="Gardner 2018" /> 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.<ref name="ArmstrongMcKay2022">Template:Cite journal</ref><ref name="Explainer">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Marine ice cliff instabilityEdit
A related process known as Marine Ice Cliff Instability (MICI) posits that ice cliffs which exceed ~Template:Cvt in above-ground height and are ~Template:Cvt in basal (underground) height are likely to collapse under their own weight once the peripheral ice stabilizing them is gone.<ref name="Zhang2022" /> 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.<ref name="Pollard2015" /><ref name="DeConto2016" /><ref name="Pattyn 2018">Template:Cite journal</ref><ref>Template:Cite journal</ref> 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 2014 IPCC Fifth Assessment Report.<ref name="Horton2020">Template:Cite journal</ref> Sea level rise projections which involve MICI are much larger than the others, particularly under high warming rate.<ref name="Slangen2022">Template:Cite journal</ref>
At the same time, this theory has also been highly controversial.<ref name="Zhang2022">Template:Cite conference</ref> It was originally proposed in order to describe how the large sea level rise during the Pliocene and the Last Interglacial could have occurred<ref name="Zhang2022" /><ref name="DeConto2016">Template:Cite journal</ref> - yet more recent research found that these sea level rise episodes can be explained without any ice cliff instability taking place.<ref name="Gilford2020" /><ref name="Zhang2022" /><ref>Template:Cite journal</ref> Research in Pine Island Bay in West Antarctica (the location of Thwaites and Pine Island Glacier) had found seabed gouging by ice from the Younger Dryas period which appears consistent with MICI.<ref name="Wise2017" /><ref name="Gilford2020" /> However, it indicates "relatively rapid" yet still prolonged ice sheet retreat, with a movement of >Template:Cvt inland taking place over an estimated 1100 years (from ~12,300 years Before Present to ~11,200 B.P.)<ref name="Wise2017">Template:Cite journal</ref>
In recent years, 2002-2004 fast retreat of Crane Glacier immediately after the collapse of the Larsen B ice shelf (before it reached a shallow fjord and stabilized) could have involved MICI, but there weren't enough observations to confirm or refute this theory.<ref name="Needell2023">Template:Cite journal</ref> The retreat of Greenland ice sheet's three largest glaciers - Jakobshavn, Helheim, and Kangerdlugssuaq Glacier - did not resemble predictions from ice cliff collapse at least up until the end of 2013,<ref name="Gilford2020" /><ref>Template:Cite journal</ref> but an event observed at Helheim Glacier in August 2014 may fit the definition.<ref name="Gilford2020" /><ref>Template:Cite journal</ref> 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 ~Template:Cvt-tall cliffs),<ref>Template:Cite journal</ref> unless the ice had already been substantially damaged beforehand.<ref name="Needell2023" /> Further, ice cliff breakdown would produce a large number of debris in the coastal waters - known as ice mélange - and multiple studies indicate their build-up would slow or even outright stop the instability soon after it started.<ref>Template:Cite news</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref name="Schlemm2022">Template:Cite journal</ref>
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 the Last Interglacial.<ref name="Gilford2020" /> MICI can be effectively ruled out if SLR at the time was lower than Template:Cvt, while it is very likely if the SLR was greater than Template:Cvt.<ref name="Gilford2020">Template:Cite journal</ref> As of 2023, the most recent analysis indicates that the Last Interglacial SLR is unlikely to have been higher than Template:Cvt,<ref name="Dumitru2023" /> as higher values in other research, such as Template:Cvt,<ref>Template:Cite journal</ref> appear inconsistent with the new paleoclimate data from The Bahamas and the known history of the Greenland Ice Sheet.<ref name="Dumitru2023">Template:Cite journal</ref>
Earth's current two ice sheetsEdit
Antarctic ice sheetEdit
West Antarctic ice sheetEdit
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East Antarctic ice sheetEdit
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Greenland ice sheetEdit
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Role in carbon cycleEdit
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.<ref name="Wadham2019" />
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 and physical weathering in ice sheets, and storage and cycling of organic carbon in excess of 100 billion tonnes.<ref name="Wadham2019" />
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.<ref name="Wadham2019">Template:Cite journal</ref> This carbon can act as a climate change feedback if it is gradually released through meltwater, thus increasing overall carbon dioxide emissions.<ref>Template:Cite journal</ref>
For comparison, 1400–1650 billion tonnes are contained within the Arctic permafrost.<ref>Template:Cite journal</ref> Also for comparison, the annual human caused carbon dioxide emissions amount to around 40 billion tonnes of Template:CO2.<ref name="IPCC AR6 WG1 Ch.9" />Template:Rp
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.<ref name="Christiansen2018">Template:Cite journal</ref> However, it also harbours large numbers of methanotrophic bacteria, which limit those emissions.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
In geologic timescalesEdit
Normally, the transitions between glacial and interglacial states are governed by Milankovitch cycles, which are patterns in 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.<ref>Template:Cite journal</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
For instance, during at least the last 100,000 years, portions of the ice sheet covering much of North America, the Laurentide 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 as ice rafted debris. These so-called 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.<ref>Template:Cite journal</ref>
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.<ref>Template:Cite book</ref>
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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Antarctic ice sheet during geologic timescalesEdit
Greenland ice sheet during geologic timescalesEdit
See alsoEdit
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ReferencesEdit
External linksEdit
- United Nations Environment Programme: Global Outlook for Ice and Snow
- Marine Ice Sheet Instability "For Dummies"
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