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

==== 9.4.2.1 Recent Observed Changes ====

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As stated in [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.4|Section 2.3.2.4]] , satellite observations by Ice Sheet Mass Balance Intercomparison Exercise (IMBIE) combining multi-team estimates based on altimetry, gravity anomalies (GRACE) and the input-output method, already presented in SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ), are updated and extended to 2020 ( [[#The%20IMBIE%20Team--2021|The IMBIE Team, 2021]] ). The Antarctic Ice Sheet (AIS) lost 2670 [1800 to 3540] Gt mass over the period 1992–2020, equivalent to 7.4 [5.0 to 9.8] mm GMSL rise (for contribution to sea level budget, see Figures 9.16 and 9.18, and Table 9.5). Within uncertainties, this estimate agrees with a review of post-AR5 studies up to 2016 ( [[#Bamber--2018b|Bamber et al., 2018b]] ) and is consistent with recent single studies based on satellite laser altimetry ( [[#Smith--2020|Smith et al., 2020]] ), the input-output method ( [[#Rignot--2019|Rignot et al., 2019]] ) and gravimetry ( [[#Velicogna--2020|Velicogna et al., 2020]] ). The mass-loss rate was on average 49 [–2 to 100] Gt yr <sup>–1</sup> over the period 1992–1999, 70 [22 to 119] Gt yr <sup>–1</sup> over the period 2000–2009, and 148 [94 to 202] Gt yr <sup>–1</sup> over the period 2010–2016 (see Figures 9.16 and 9.18, and Table 9.SM.1). However, recent work suggests that the mass loss has not further increased since 2016 because of regional mass gains in Dronning Maud Land ( [[#Velicogna--2020|Velicogna et al., 2020]] ). Mass loss of the West Antarctic and Antarctic Peninsula ice sheets has increased since about 2000 ( ''very high confidence'' ), essentially due to increased ice discharge ( [[#Harig--2015|Harig and Simons, 2015]] ; [[#Paolo--2015|Paolo et al., 2015]] ; [[#Forsberg--2017|Forsberg et al., 2017]] ; [[#Bamber--2018b|Bamber et al., 2018b]] ; [[#Gardner--2018|Gardner et al., 2018]] ; [[#The%20IMBIE%20Team--2018|The IMBIE Team, 2018]] ; [[#Rignot--2019|Rignot et al., 2019]] ).

The SROCC reported with ''very high confidence'' that the acceleration, retreat and thinning of the principal West Antarctic outlet glaciers has dominated the observed Antarctic mass loss over the last decades, and stated with ''high confidence'' that these losses were driven by melting of ice shelves by warm ocean waters. The average West Antarctic Ice Sheet (WAIS) mass loss of 82 ± 9 Gt yr <sup>–1</sup> between 1992 and 2017 ( [[#The%20IMBIE%20Team--2021|The IMBIE Team, 2021]] ) leads to substantial observed surface lowering (e.g., [[#Schröder--2019|Schröder et al., 2019]] ; [[#Shepherd--2019|Shepherd et al., 2019]] ), particularly in coastal regions (Figure 9.18). Recent studies using satellite altimetry ( [[#Schröder--2019|Schröder et al., 2019]] ) and the input-output method ( [[#Rignot--2019|Rignot et al., 2019]] ) consistently show mass loss in these coastal regions since the late 1970s (Figure 9.16). Because of consistent multiple lines of evidence, there is ''high confidence'' in mass loss of the Totten Glacier in East Antarctica ( [[#Miles--2013|Miles et al., 2013]] ; X. [[#Li--2016|]] [[#Li--2016|Li et al., 2016]] ; [[#Mohajerani--2018|Mohajerani et al., 2018]] ; [[#Rignot--2019|Rignot et al., 2019]] ; [[#Schröder--2019|Schröder et al., 2019]] ; [[#Shepherd--2019|Shepherd et al., 2019]] ) since about 2000, dominated by changes in coastal ice dynamics (X. [[#Li--2016|]] [[#Li--2016|Li et al., 2016]] ). It is currently unclear whether mass loss of the EAIS over the last three decades has been significant ( [[#Rignot--2019|Rignot et al., 2019]] ) or, at 5 ± 46 Gt yr <sup>–1</sup> between 1992 and 2017, essentially zero within uncertainties ( [[#The%20IMBIE%20Team--2018|The IMBIE Team, 2018]] ). In summary, WAIS losses, through acceleration, retreat and thinning of the principal outlet glaciers, dominated the AIS mass losses over the last decades ( ''very high confidence'' ) and there is ''high confidence'' that this is the case since the late 1970s. Furthermore, parts of the EAIS have lost mass in the last two decades ( ''high confidence'' ).

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[[File:f9142cf45da1a832c41b4ffe2541a9e1 IPCC_AR6_WGI_Figure_9_18.png]]

'''Figure 9.18''' '''|''' '''Antarctic Ice Sheet cumulative mass change and equivalent sea level contribution. (a)''' A p-box ( [[#9.6.3.2|Section 9.6.3.2]] ) based estimate of the range of values of paleo Antarctic ice sheet mass and sea level equivalents relative to present day and the median over all central estimates ( [[#Bamber--2009|Bamber et al., 2009]] ; [[#Argus--2010|Argus and Peltier, 2010]] ; [[#Dolan--2011|Dolan et al., 2011]] ; [[#Mackintosh--2011|Mackintosh et al., 2011]] ; [[#Golledge--2012|Golledge et al., 2012]] , 2013, 2014, 2015, 2017b; K.G. [[#Miller--2012|]] [[#Miller--2012|Miller et al., 2012]] ; [[#Whitehouse--2012|Whitehouse et al., 2012]] ; [[#Ivins--2013|Ivins et al., 2013]] ; [[#Argus--2014|Argus et al., 2014]] ; [[#Briggs--2014|Briggs et al., 2014]] ; [[#Maris--2014|Maris et al., 2014]] ; [[#de%20Boer--2015|de Boer et al., 2015]] , 2017; [[#Dutton--2015|Dutton et al., 2015]] ; [[#Pollard--2015|Pollard et al., 2015]] ; [[#DeConto--2016|DeConto and Pollard, 2016]] ; [[#Gasson--2016|Gasson et al., 2016]] ; [[#Goelzer--2016|Goelzer et al., 2016]] ; [[#Yan--2016|Yan et al., 2016]] ; [[#Kopp--2017|Kopp et al., 2017]] ; [[#Simms--2019|Simms et al., 2019]] ); '''(b left)''' cumulative mass loss (and sea level equivalent) since 2015, with satellite observations shown from 1993 ( [[#Bamber--2018a|Bamber et al., 2018a]] ; [[#The%20IMBIE%20Team--2018|The IMBIE Team, 2018]] ; [[#WCRP%20Global%20Sea%20Level%20Budget%20Group--2018|WCRP Global Sea Level Budget Group, 2018]] ) and observations from 1979 ( [[#Rignot--2019|Rignot et al., 2019]] ), and projections from Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6) to 2100 under RCP8.5/SSP5-8.5 and RCP2.6/SSP1-2.6 scenarios (thin lines from [[#Seroussi--2020|Seroussi et al., 2020]] ; [[#Edwards--2021|Edwards et al., 2021]] ; [[#Payne--2021|Payne et al., 2021]] ) and ISMIP6 emulator under SSP5-8.5 and SSP1-2.6 to 2100 (shades and bold line; [[#Edwards--2021|Edwards et al., 2021]] ) ; (b, right) 17th–83rd, 5th–95th percentile ranges for ISMIP6, ISMIP6 emulator, and LARMIP-2 including surface mass balance (SMB) at 2100. (c – e) Schematic interpretations of individual reconstructions ( [[#Anderson--2002|Anderson et al., 2002]] ; [[#Bentley--2014|Bentley et al., 2014]] ; [[#de%20Boer--2015|de Boer et al., 2015]] ; [[#Goelzer--2016|Goelzer et al., 2016]] ) of the spatial extent of the Antarctic Ice Sheet are shown for the: '''(c)''' mid-Pliocene Warm Period, '''(d)''' Last Interglacial; and '''(e)''' Last Glacial Maximum ( [[#Fretwell--2013|Fretwell et al., 2013]] ): grey shading shows extent of grounded ice. (f – g) Maps of mean elevation changes '''(f)''' 1978–2017 derived from multi-mission satellite altimetry ( [[#Schröder--2019|Schröder et al., 2019]] ) and '''(g)''' ISMIP6: 2061–2100 projected changes for an ensemble using the Norwegian Climate Center’s Earth System Model (NorESM1-M) climate model under the RCP8.5 scenario ( [[#Seroussi--2020|Seroussi et al., 2020]] ). Further details on data sources and processing are available in the chapter data table (Table 9.SM.9).

As stated in SROCC, snowfall and glacier flow are the largest components determining AIS mass changes, with glacier flow acceleration (dynamic thinning) on the WAIS and the Antarctic Peninsula driving total loss trends in recent decades ( ''very high confidence'' ), and a partial offset of the dominating dynamic-thinning losses by increased snowfall ( ''high confidence'' ). The SROCC attributed ''medium confidence'' to estimates of 20th-century snowfall increases equivalent to a sea level change of –7.7 ± 4.0 mm on the EAIS, and –2.8 ± 1.7 mm on the WAIS, respectively ( [[#Medley--2019|Medley and Thomas, 2019]] ). Loss of buttressing, which can be caused by ice-shelf thinning, gradual ice-shelf front retreat or ice-shelf disintegration, has been linked to instantaneous ice velocity increases, and thus dynamic thinning, since the early 1990s. This link is clearly evident in the Amundsen and, to a lesser degree, Bellingshausen sectors ( [[#Gudmundsson--2019|Gudmundsson et al., 2019]] ), where passive shelf ice (ice that can be removed without major effects on the ice-shelf dynamics) is very limited or absent ( [[#Fürst--2016|Fürst et al., 2016]] ). Surface mass balance (SMB) changes, dominated by snowfall, exhibit strong regional and temporal variability, for example with multi-decadal increases in the Antarctic Peninsula inferred since the 1930s ( [[#Medley--2019|Medley and Thomas, 2019]] ), and dominate the interannual to decadal variability of the AIS mass balance ( [[#Rignot--2019|Rignot et al., 2019]] ). However, no significant continent-wide SMB trend is inferredsince 1979 ( [[#The%20IMBIE%20Team--2018|The IMBIE Team, 2018]] ; [[#Medley--2019|Medley and Thomas, 2019]] ; regional changes of Antarctic SMB are assessed further in [[IPCC:Wg1:Chapter:Atlas|Atlas]] [[IPCC:Wg1:Chapter:Chapter-11#11.1|Section 11.1]] ). In summary, there is ''very high confidence'' that the observed AIS mass loss since the early 1990s is primarily linked to ice-shelf changes.

The SROCC stated with ''high confidence'' that melting of ice shelves by warm ocean waters, leading to reduction of ice-shelf buttressing, has driven the observed ongoing thinning of major WAIS outlet glaciers. Since SROCC, digitized radar measurements have shown that the eastern ice shelf of Thwaites Glacier in the Amundsen Sea Embayment thinned between 10 and 33% during the three decades after 1978 ( [[#Schroeder--2019|Schroeder et al., 2019]] ), and the role of basal ice-shelf melting has been emphasized ( [[#Smith--2020|Smith et al., 2020]] ). Strong surface meltwater production has been noted as a precursor of ice-shelf disintegration in and since SROCC ( [[#Bell--2018|Bell et al., 2018]] ), and recent work placed strong meltwater production events ( [[#Lenaerts--2017|Lenaerts et al., 2017]] ; [[#Nicolas--2017|Nicolas et al., 2017]] ; [[#Wille--2019|Wille et al., 2019]] ) and seasons ( [[#Robel--2019|Robel and Banwell, 2019]] ) in this context. Antarctic ice-shelf basal meltwater flux varied between about 1100 ± 150 Gt yr <sup>–1</sup> in the mid-1990s and about 1570 ± 140 Gt yr <sup>–1</sup> in the late 2000s before decreasing to 1160 ± 150 Gt yr <sup>–1</sup> in 2018, and basal melt rates strongly vary with geographical position and depth, as a function of the surrounding water temperature ( [[#Adusumilli--2020|Adusumilli et al., 2020]] ). [[#9.2.2.3|Section 9.2.2.3]] assesses that the intrusion of warm Circumpolar Deep Water (CDW), which has warmed and shoaled since the 1980s, has been at least partially controlled by forcing with significant decadal variability ''. Limited evidence'' suggests that, beyond strong internal decadal wind variability, increased greenhouse gas forcing has slightly modified the mean local winds between 1920 and 2018, facilitating the intrusion of CDW heat on the Amundsen-Bellingshausen continental shelf, and increased ice shelf melt ( [[#9.2.2.3|Section 9.2.2.3]] ). However, theoretical understanding is still incomplete and in situ measurements within the ice–ocean boundary layer are sparse ( [[#Wåhlin--2020|Wåhlin et al., 2020]] ). Modelling, and therefore attribution of ice shelf basal melt, remains challenging because of insufficient process understanding, required spatial resolution, the paucity of in situ observations ( [[#Dinniman--2016|Dinniman et al., 2016]] ; [[#Asay-Davis--2017|Asay-Davis et al., 2017]] ; [[#Turner--2017|Turner et al., 2017]] ), and uncertainties of bathymetric datasets under ice-shelf cavities ( [[#Goldberg--2019|Goldberg et al., 2019]] , 2020; [[#Morlighem--2020|Morlighem et al., 2020]] ). In summary, ice-shelf thinning, mainly driven by basal melt, is widespread around the Antarctic coast and particularly strong around the WAIS ( ''high confidence'' ), although basal melt rates show substantial spatio-temporal variability.

Satellite observations suggest that changes in sea ice coverage and thickness can modulate iceberg calving, ice shelf flow and glacier terminus position around Antarctica ( [[#Miles--2013|Miles et al., 2013]] , 2016, 2017; [[#Massom--2015|Massom et al., 2015]] ; [[#Greene--2018|Greene et al., 2018]] ; [[#Bevan--2019|Bevan et al., 2019]] ), either through mechanical coupling or via changes to ocean stratification, influencing basal melting. A combined observational and modelling study ( [[#Massom--2018|Massom et al., 2018]] ) showed that regional loss of a protective sea ice buffer played a role in the rapid disintegration events of the Larsen A and B and Wilkins ice shelves in the Antarctic Peninsula between 1995 and 2009, by exposing damaged (rifted) outer ice shelf margins to enhanced flexure by storm-generated ocean swells. One observational study ( [[#Sun--2019|Sun et al., 2019]] ) suggests that the absence of sea ice in front of ice shelves, which leads to strengthened topographic waves, favours higher ice-shelf basal melt rates by increasing the baroclinic (depth varying) ocean heat flux which can enter the cavity ( [[#Wåhlin--2020|Wåhlin et al., 2020]] ). Paleo evidence for sea ice control on ice sheets is lacking, but geologic evidence shows a concordance between periods of ice-sheet growth and the expansion of sea ice ( [[#Patterson--2014|Patterson et al., 2014]] ; [[#Levy--2019|Levy et al., 2019]] ), both being favoured by reduced sea surface temperatures. Modelling confirms that sea ice controls the strength of ice mélange ( [[#Robel--2017|Robel, 2017]] ; [[#Schlemm--2021|Schlemm and Levermann, 2021]] ) and thus influences ice-shelf flexure and calving rates and stability of floating ice margins, but one model shows this had negligible effect on AIS retreat rates during past warm periods ( [[#Pollard--2018|Pollard et al., 2018]] ). Loss of ice-shelf-proximal sea ice is also associated with increased solar heating of surface waters and increased sub-shelf melting ( [[#Bendtsen--2017|Bendtsen et al., 2017]] ; [[#Stewart--2019|Stewart et al., 2019]] ). In summary, although in some cases sea ice decrease and glacier and ice-shelf flow and terminus position changes can have the same common cause, there is ''medium confidence'' that sea ice decrease ultimately favours the mass loss of nearby ice shelves through a variety of processes.

The SROCC stated with ''high confidence'' that ice-shelf disintegration has driven dynamic thinning in the northern Antarctic Peninsula over recent decades, and expressed ''high confidence'' in current ongoing mass loss from glaciers that fed now-disintegrated ice shelves. However, the mass loss rate has decreased in the 20 years since the immediate speed-up following ice-shelf disintegration in 1995 and 2002. Observed flow speed of these tributary glaciers is still 26% higher than before the ice shelf disintegration ( [[#Seehaus--2018|Seehaus et al., 2018]] ). Conversely, one study interpreted the increased flow speed of the Scar Inlet Ice Shelf’s tributary glaciers as a sign of evolving instability of the currently intact ice shelf ( [[#Qiao--2020|Qiao et al., 2020]] ).

Ongoing grounding line retreat, indicating dynamic thinning, is observed with ''high confidence'' in many areas of Antarctica, and particularly on the WAIS, with the highest rates being in the Amundsen and Bellingshausen Sea areas, and around Totten Glacier in East Antarctica, as stated in SROCC. Research published since SROCC has evidenced grounding line retreat of the West Antarctic Berry Glacier on the Getz Coast ( [[#Millan--2020|Millan et al., 2020]] ) and on the East Antarctic Denman Glacier ( [[#Brancato--2020|Brancato et al., 2020]] ), both since 1996. Furthermore observed grounding line retreat in excess of 1.5 km between 2003 and 2015 has been reported for parts of Marie Byrd Land ( [[#Christie--2018|Christie et al., 2018]] ). In summary, there is ''high confidence'' that grounding lines of marine-terminating glaciers are currently retreating in many areas around Antarctica, particularly around the WAIS, and additional areas of grounding line retreat have been evidenced since SROCC.

The SROCC stated with ''medium confidence'' that sustained mass losses of several major glaciers in the Amundsen Sea Embayment (ASE) are compatible with the onset of marine ice sheet instability (MISI). However, whether unstable WAIS retreat had begun, or was imminent, remained a critical uncertainty. New publications since SROCC have not substantially clarified this question. One study that combined satellite measurements with a numerical model and prescribed ice-shelf thinning ( [[#Gudmundsson--2019|Gudmundsson et al., 2019]] ) suggests that MISI is not required to explain the observed current mass loss rates of the WAIS, because they are consistent with external climate drivers. Furthermore, the fast grounding line retreat of the Pine Island Glacier in the ASE, which was triggered in the 1940s ( [[#Smith--2017|Smith et al., 2017]] ), observed after 1992 ( [[#Rignot--2014|Rignot et al., 2014]] ) and previously interpreted as a sign of MISI ( [[#Favier--2014|Favier et al., 2014]] ), seems to have stabilized recently ( [[#Milillo--2017|Milillo et al., 2017]] ; [[#Konrad--2018|Konrad et al., 2018]] ), and its current flow patterns do not suggest ongoing or imminent MISI ( [[#Bamber--2020|Bamber and Dawson, 2020]] ). However, sustained fast grounding line retreat has been observed for the Smith Glacier in the ASE ( [[#Scheuchl--2016|Scheuchl et al., 2016]] ), and an analysis of flow patterns and grounding line retreat of the ASE Thwaites Glacier between 1992 and 2017 ( [[#Milillo--2019|Milillo et al., 2019]] ) showed sustained, albeit spatially heterogeneous, grounding line retreat, highlighting ice–ocean interactions that lead to increased basal melt. In addition, Denman Glacier in East Antarctica was shown to hold potential for unstable retreat ( [[#Brancato--2020|Brancato et al., 2020]] ). In summary, the observed evolution of the ASE glaciers is compatible with, but not unequivocally indicating an ongoing MISI ( ''medium confidence'' ).

The SROCC reported ''limited evidence'' and ''medium agreement'' for anthropogenic forcing of the observed AIS mass balance changes. As stated in [[IPCC:Wg1:Chapter:Chapter-3#3.4.3.2|Section 3.4.3.2]] , there remains ''low confidence'' in attributing the causes of the observed mass of loss from the AIS since 1993, in spite of some additional process-based evidence to support attribution to anthropogenic forcing.

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