Editing IPCC:AR6/SROCC/Chapter-3 (section)

== CCB.8 Future Sea Level Changes and Marine Ice Sheet Instability ==

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Authors: Robert M. DeConto (USA), Alexey Ekaykin (Russian Federation), Andrew Mackintosh (Australia), Roderik van de Wal (Netherlands), Jeremy Bassis (USA)

Over the last century, glaciers were the main contributors to increasing ocean water mass (Section 4.2.1.2). However, most terrestrial frozen water is stored in Antarctic and Greenland ice sheets, and future changes in their dynamics and mass balance will cause sea level rise over the 21st century and beyond (Section 4.2.3).

About a third of the Antarctic Ice Sheet (AIS) is ‘marine ice sheet’, i.e., rests on bedrock below sea level (Figure 4.5), with most of the ice sheet margin terminating directly in the ocean. These features make the overlying ice sheet vulnerable to dynamical instabilities with the potential to cause rapid ice loss; so-called Marine Ice Sheet and Marine Ice Cliff instabilities, as discussed below.

In many places around the AIS margin, the seaward-flowing ice forms floating ice shelves (Figure CB8.1). Ice shelves in contact with bathymetric features on the sea floor or confined within embayments provide back stress (buttressing) that impedes the seaward flow of the upstream ice and thereby stabilises the ice sheet. The ice shelves are thus a key factor controlling AIS dynamics. Almost all Antarctic ice shelves provide substantial buttressing (Fürst et al., 2016 <sup>[[#fn:r1316|1316]]</sup> ) but some are currently thinning at an increasing rate (Khazendar et al., 2016 <sup>[[#fn:r1317|1317]]</sup> ). Today, thinning and retreat of ice shelves is associated primarily with ocean driven basal melt that, in turn, promotes iceberg calving (Section 3.3.1.2).

Accumulation and percolation of surface melt and rain water also impact ice shelves by lowering albedo, deepening surface crevasses, and causing flexural stresses that can lead to hydrofracturing and ice shelf collapse (Macayeal and Sergienko, 2013 <sup>[[#fn:r1319|1319]]</sup> ). In some cases supraglacial (i.e., flowing on the glacier surface) rivers might diminish destabilising impact of surface melt by removing meltwater before it ponds on the ice shelf surface (Bell et al., 2017 <sup>[[#fn:r1320|1320]]</sup> ). In summary, both ocean forcing and surface melt affect ice shelf mechanical stability ( ''high confidence'' ), but the precise importance of the different mechanisms remains poorly understood and observed.

The future dynamic response of the AIS to warming will largely be determined by changes in ice shelves, because their thinning or collapse will reduce their buttressing capacity, leading to an acceleration of the grounded ice and to thinning of the ice margin. In turn, this thinning can initiate grounding line retreat (Konrad et al., 2018 <sup>[[#fn:r1321|1321]]</sup> ). If the grounding line is located on bedrock sloping downwards toward the ice sheet interior (retrograde slope), initial retreat can trigger a positive feedback, due to non-linear response of the seaward ice flow to the grounding line thickness change. As a result, progressively more ice will flow into the ocean (Figure CB8.1a). This self-sustaining process is known as Marine Ice Sheet Instability (MISI). The onset and persistence of MISI is dependent on several factors in addition to overall bed slope, including the details of the bed geometry and conditions, ice shelf pinning points, lateral shear from the walls, self-gravitation effects on local sea level and isostatic adjustment. Hence, long-term retreat on every retrograde sloped bed is not necessarily unstoppable (Gomez et al., 2015 <sup>[[#fn:r1322|1322]]</sup> ).

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'''Figure CB8.1'''

<span id="schematic-representation-of-marine-ice-sheet-instability-misi-a-and-marine-ice-cliff-instability-mici-b-from-pattyn-2018.-a-thinning-of-the-buttressing-ice-shelf-leads-to-acceleration-of-the-ice-sheet-flow-and-thinning-of-the-marine-terminated-ice-margin.-because-bedrock-under-the-ice-sheet-is-sloping-towards-ice-sheet-interior-thinning-of"></span>
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'''Schematic representation of Marine Ice Sheet Instability (MISI, a) and Marine Ice Cliff Instability (MICI, b) from Pattyn (2018). (a) thinning of the buttressing ice shelf leads to acceleration of the ice sheet flow and thinning of the marine-terminated ice margin. Because bedrock under the ice sheet is sloping towards ice sheet interior, thinning of […]'''
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[[File:6ad5d3c4064bc3bacf6a9127be510c96 IPCC-SROCC-CB_8_1.jpg]]

Schematic representation of Marine Ice Sheet Instability (MISI, a) and Marine Ice Cliff Instability (MICI, b) from Pattyn (2018). (a) thinning of the buttressing ice shelf leads to acceleration of the ice sheet flow and thinning of the marine-terminated ice margin. Because bedrock under the ice sheet is sloping towards ice sheet interior, thinning of the ice causes retreat of the grounding line followed by an increase of the seaward ice flux, further thinning of the ice margin, and further retreat of the grounding line. (b) disintegration of the ice shelf due to bottom melting and/or hydro-fracturing produces an ice cliff. If the cliff is tall enough (at least ~800 m of total ice thickness, or about 100 m of ice above the water line), the stresses at the cliff face exceed the strength of the ice, and the cliff fails structurally in repeated calving events. Note that MISI requires a retrograde bed slope, while MICI can be realised on a flat or seaward-inclined bed. Like MISI, the persistence of MICI depends on the lack of ice shelf buttressing, which can stop or slow brittle ice failure at the grounding line by providing supportive backstress.

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The MISI process might be particularly important in West Antarctica, where most of the ice sheet is grounded on bedrock below sea level (Figure 4.5). Since AR5, there is growing observational and modelling evidence that accelerated retreat may be underway in several major Amundsen Sea outlets, including Thwaites, Pine Island, Smith, and Kohler glaciers (e.g., Rignot et al., 2014) supporting the MISI hypothesis, although observed grounding line retreat on retrograde slope is not definitive proof that MISI is underway.

It has been shown recently (Barletta et al., 2018 <sup>[[#fn:r1323|1323]]</sup> ) that the Amundsen Sea Embayment (ASE) experiences unexpectedly fast bedrock uplift (up to 41 mm yr -1 , due to mantle viscosity much lower than the global average) as an adjustment to reduced ice mass loading, which could help stabilise grounding line retreat.

One of the largest outlets of the East Antarctic Ice Sheet (EAIS), Totten glacier, has also been retreating and thinning in recent decades (Li et al., 2015b <sup>[[#fn:r1324|1324]]</sup> ). Totten’s current behaviour suggests that East Antarctica could become a substantial contributor to future sea level rise, as it has been in the previous warm periods (Aitken et al., 2016 <sup>[[#fn:r1325|1325]]</sup> ). It is not clear, however, if the changes observed recently are a linear response to increased ocean forcing (Section 3.3.1.2), or an indication that MISI has commenced (Roberts et al., 2018 <sup>[[#fn:r1326|1326]]</sup> ).

The disappearance of ice shelves may allow the formation of ice cliffs, which may be inherently unstable if they are tall enough (subaerial cliff height between 100 and 285 m) to generate stresses that exceed the strength of the ice (Parizek et al., 2019 <sup>[[#fn:r1327|1327]]</sup> ). This ice cliff failure can lead to ice sheet retreat via a process called marine ice cliff instability (MICI; Figure CB8.1b), that has been hypothesised to cause partial collapse of the West Antarctic Ice Sheet (WAIS) within a few centuries (Pollard et al., 2015 <sup>[[#fn:r1328|1328]]</sup> ; DeConto and Pollard, 2016 <sup>[[#fn:r1329|1329]]</sup> ).

''Limited evidence'' is available to confirm the importance of MICI. In Antarctica, marine-terminating ice margins with the grounding lines thick enough to produce unstable ice cliffs are currently buttressed by ice shelves, with a possible exception of Crane glacier on the Antarctic Peninsula (Section 4.2.3.1.2).

Overall, there is ''low agreement'' on the exact MICI mechanism and ''limited evidence'' of its occurrence in the present or the past. Thus the potential of MICI to impact the future sea level remains very uncertain (Edwards et al., 2019 <sup>[[#fn:r1330|1330]]</sup> ).

''Limited evidence'' from geological records and ice sheet modelling suggests that parts of AIS experienced rapid (i.e., on centennial time-scale) retreat ''likely'' due to ice sheet instability processes between 20,000 and 9,000 years ago (Golledge et al., 2014 <sup>[[#fn:r1331|1331]]</sup> ; Weber et al., 2014 <sup>[[#fn:r1332|1332]]</sup> ; Small et al., 2019 <sup>[[#fn:r1333|1333]]</sup> ). Both the WAIS (including Pine Island glacier) and EAIS also experienced rapid thinning and grounding line retreat during the early to mid-Holocene (Jones et al., 2015b <sup>[[#fn:r1334|1334]]</sup> ; Wise et al., 2017 <sup>[[#fn:r1335|1335]]</sup> ). In the Ross Sea, grounding lines may have retreated several hundred kilometers inland and then re-advanced to their present-day positions due to bedrock uplift after ice mass removal (Kingslake et al., 2018 <sup>[[#fn:r1336|1336]]</sup> ), thus supporting the stabilising role of glacial isostatic adjustment on ice sheets (Barletta et al., 2018 <sup>[[#fn:r1337|1337]]</sup> ). These past rapid changes have ''likely'' been driven by the incursion of Circumpolar Deep Water onto the Antarctic continental shelf (Section 3.3.1.5.1) (Golledge et al., 2014 <sup>[[#fn:r1338|1338]]</sup> ; Hillenbrand et al., 2017 <sup>[[#fn:r1339|1339]]</sup> ) and MISI (Jones et al., 2015b <sup>[[#fn:r1340|1340]]</sup> ). ''Limited evidence'' of past MICI in Antarctica is provided by deep iceberg plough marks on the sea-floor (Wise et al., 2017 <sup>[[#fn:r1341|1341]]</sup> ).

The ability of models to simulate the processes controlling MISI has improved since AR5 (Pattyn, 2018 <sup>[[#fn:r1342|1342]]</sup> ), but significant discrepancies in projections remain (Section 4.2.3.2) due to poor understanding of mechanisms and lack of observational data on bed topography, isostatic rebound rates, etc. to constrain the models. Inclusion of MICI in one ice sheet model has improved its ability to match (albeit uncertain) geological sea level targets in the Pliocene (Pollard et al., 2015 <sup>[[#fn:r1343|1343]]</sup> ) and Last Interglacial (DeConto and Pollard, 2016 <sup>[[#fn:r1344|1344]]</sup> ), although the MICI solution may not be unique (Aitken et al., 2016 <sup>[[#fn:r1345|1345]]</sup> ) (Section 4.2.3.1.2).

The Greenland Ice Sheet (GIS) has limited direct access to the ocean through relatively narrow subglacial troughs (Morlighem et al., 2017 <sup>[[#fn:r1346|1346]]</sup> ), and most of the bedrock at the ice sheet margin is above sea level (Figure 4.5). However, since AR5 it has been argued that several Greenland outlet glaciers (Petermann, Kangerdlugssuaq, Jakobshavn Isbræ, Helheim, Zachariæ Isstrøm) and North-East Greenland Ice Stream may contribute more than expected to future sea level rise (Mouginot et al., 2015 <sup>[[#fn:r1347|1347]]</sup> ). It has also been shown that Greenland was nearly ice free for extensive episodic periods during the Pleistocene, suggesting a sensitivity to deglaciation under climates similar to or slightly warmer than present (Schaefer et al., 2016 <sup>[[#fn:r1348|1348]]</sup> ).

A MICI-style behaviour is seen today in Greenland at the termini of Jakobshavn and Helheim glaciers (Parizek et al., 2019 <sup>[[#fn:r1349|1349]]</sup> ), but calving of these narrow outlets is controlled by a combination of ductile and brittle processes, which might not be representative examples of much wider Antarctic outlet glaciers, like Thwaites.

Overall, this assessment finds that unstable retreat and thinning of some Antarctic glaciers, and to a lesser extent Greenland outlet glaciers, may be underway. However, the timescale and future rate of these processes is not well known, casting deep uncertainty on projections of the sea level contributions from the AIS (Cross-Chapter Box 5 in Chapter 1, Section 4.2.3.1).

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