Editing IPCC:AR6/WGI/Chapter-9 (section)

==== 9.4.2.4 Ice-sheet Instabilities ====

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A major uncertainty in future Antarctic mass losses is the possibility of rapid and/or irreversible ice losses through instability of marine parts of the ice sheet, via the proposed mechanisms of marine ice sheet instability (MISI) and marine ice cliff instability (MICI), and whether these processes will lead to a collapse of the West Antarctic Ice Sheet (WAIS).

MISI is a proposed self-reinforcing mechanism within marine ice sheets that lie on a bed that slopes down towards the interior of the ice sheet, whereby, in the absence of ice-shelf buttressing, the position of the grounding line is inherently unstable until reaching an upward sloping bed. The SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ) noted advances in modelling MISI since AR5, but that ‘significant discrepancies’ remained in projections due to poor understanding of mechanisms, and lack of observational data to constrain the models. Since SROCC, modelling uncertainties have been more thoroughly explored, rather than constrained (compatibility of current observations in the Amundsen Sea Embayment with MISI is assessed in [[#9.4.2.1|Section 9.4.2.1]] ). Internal climate variability might either slow ( [[#Hoffman--2019|Hoffman et al., 2019]] ) or amplify ( [[#Robel--2019|Robel et al., 2019]] ) MISI, and stable grounding line positions can be reached on downward sloping beds if ice shelves provide buttressing ( [[#Sergienko--2019|Sergienko and Wingham, 2019]] ; [[#Cornford--2020|Cornford et al., 2020]] ). Ice-sheet model simulations that remove all Antarctic ice shelves (and prevent them from reforming) show 2–10 m SLE Antarctic mass loss after 500 years due to MISI, of which WAIS collapse contributes 2–5 m ( [[#Sun--2020|Sun et al., 2020]] ), with the majority of the mass loss in the first one to two centuries. Much of the multi-model variation is due to the sliding law ( [[#9.4.2.2|Section 9.4.2.2]] ). However, it is not expected that widespread ice-shelf loss will occur before the end of the 21st century ( [[#9.4.2.3|Section 9.4.2.3]] ; Box 9.4). A recent update of bed topography that unveiled large and overdeepened subglacial troughs in East Antarctica potentially vulnerable to MISI ( [[#Morlighem--2020|Morlighem et al., 2020]] ) has only been used by a few models ( [[#Seroussi--2020|Seroussi et al., 2020]] ; [[#Sun--2020|Sun et al., 2020]] ), so current projections could underestimate vulnerability in these regions. The sea level rise contribution of the AIS therefore crucially depends on the behaviour of individual ice shelves and outlet glacier systems and whether they enter MISI for a given level of warming (Box 9.4; [[#Pattyn--2020|Pattyn and Morlighem, 2020]] ). As for Antarctic simulations generally (Sections 9.4.2.2 and 9.4.2.3), there is ''medium confidence'' in simulating MISI but ''low confidence'' in projecting the sub-shelf melting and ice-shelf disintegration that drive it.

The SROCC noted ''limited evidence'' from geological records and ice-sheet modelling, suggesting that parts of the AIS experienced rapid (centennial) retreat ''likely'' due to MISI between 20,000 and 9,000 years ago, and also described more uncertain evidence for the Last Interglacial (LIG) and mid-Pliocene Warm Period (MPWP). Recent support for past MISI is provided by model simulations of the WAIS during the LIG ( [[#Clark--2020|Clark et al., 2020]] ), the British Ice Sheet during the last termination ( [[#Gandy--2018|Gandy et al., 2018]] ) and the Laurentide Ice Sheet during the Younger Dryas ( [[#Pico--2019|Pico et al., 2019]] ), which show progressive retreat despite declining temperatures, indicative of a true (ice dynamic) instability. Direct observational evidence of rapid paleo ice-sheet grounding line retreat is rare but, on the Larsen continental shelf, retreat rates of &gt;10 km yr <sup>–1</sup> during the deglaciation have been estimated ( [[#Dowdeswell--2020|Dowdeswell et al., 2020]] ). MISI has also been inferred from sedimentological evidence of ice loss from Wilkes Subglacial Basin, East Antarctica ( [[#Bertram--2018|Bertram et al., 2018]] ; [[#Wilson--2018|Wilson et al., 2018]] ; [[#Blackburn--2020|Blackburn et al., 2020]] ) but these reconstructions cannot unambiguously identify unstable from progressive retreat. Therefore, there is ''limited evidence'' to identify the operation of instability mechanisms such as MISI in paleo ice-sheet retreat.

The SROCC assessed that ice-sheet interactions with the solid Earth are not expected to substantially slow sea level rise from marine-based ice in Antarctica over the 21st century ( ''medium confidence'' ), but that these processes could become important on multi-century and longer time scales. More recent modelling of deglaciation of the Ross Embayment by [[#Lowry--2020|Lowry et al. (2020)]] is consistent with this assessment. However, new projections for Pine Island Glacier ( [[#Kachuck--2020|Kachuck et al., 2020]] ) support previous work ( [[#Barletta--2018|Barletta et al., 2018]] ) suggesting that lower mantle viscosity in this region leads to a negative feedback on decadal time scales. Grounding line stabilization by the solid Earth response may therefore occur over the 21st century in the Amundsen Sea Embayment, where most mass loss is occurring ( [[#9.4.2.1|Section 9.4.2.1]] ), but more generally occurs over multi-centennial to millennial time scales ( ''medium confidence'' ).

The MICI hypothesis describes rapid, unmitigated calving triggered by ice-shelf collapse ( [[#Pollard--2015|Pollard et al., 2015]] ). The SROCC noted that the MICI mechanism led one model ( [[#DeConto--2016|DeConto and Pollard, 2016]] ) to lose mass far more rapidly, but excluded the mechanism from its projections due to uncertainty in the timing of the ice-shelf disintegration ( [[#9.4.2.3|Section 9.4.2.3]] ). They stated that MICI could lead to sea level contributions beyond 2100 considerably higher than the ''likely'' range projected by other models. However, given the ''low agreement'' on the exact MICI mechanism and ''limited evidence'' of its occurrence in the present or the past ( [[#9.4.2.2|Section 9.4.2.2]] ), its potential to affect future sea level rise was very uncertain. Since SROCC, new simulations show later ice-shelf disintegration, in agreement with other models ( [[#9.4.2.3|Section 9.4.2.3]] ; [[#DeConto--2021|DeConto et al., 2021]] ), and therefore lower projections at 2100 ( [[#9.4.2.5|Section 9.4.2.5]] ). New theoretical evidence suggests that ice-cliff collapse may only occur after very rapid ice shelf disintegration caused by unusually high meltwater production ( [[#Clerc--2019|Clerc et al., 2019]] ; [[#Robel--2019|Robel and Banwell, 2019]] ), and that the subsequent rate of retreat depends on the terminus geometry ( [[#Bassis--2019|Bassis and Ultee, 2019]] ). As SROCC noted, only Crane Glacier on the Peninsula has shown retreat consistent with MICI, after the Larsen B ice shelf collapsed, and MICI-style behaviour at Jakobshavn and Helheim Glaciers in Greenland might not be representative of wider Antarctic glaciers. Observations from Greenland show that steep cliffs commonly evolve into short floating extensions, rather than collapsing catastrophically ( [[#Joughin--2020|Joughin et al., 2020]] ). As assessed in [[#9.4.2.2|Section 9.4.2.2]] and 9.4.2.3, there is therefore ''low confidence'' in simulating mechanisms that have the potential to cause widespread, sustained and very rapid ice loss from Antarctica this century through MICI, and ''low confidence'' in projecting the driver of ice-shelf disintegration.

In summary, poorly understood processes of instabilities, characterized by ''deep uncertainty'' , have the potential to strongly increase Antarctic mass loss under high greenhouse gas emissions on century-to-multicentury time scales (Box 9.4). These instabilities are therefore considered separately in assessments of the future contribution to global mean sea level (GMSL; Sections 9.4.2.5, 9.4.2.6, 9.6.3.2 and 9.6.3.5).

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