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

==== 3.3.1.2 Components of Antarctic Ice Sheet Mass Change ====

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AIS mass changes are dominated by changes in snowfall and glacier flow. The WAIS and AP loss trends in recent decades are dominated by glacier flow acceleration (also known as dynamic thinning) ( ''very high confidence'' ) (Figure SM3.8). Dynamic thinning losses were –112 ± 12 Gt yr –1 for 2003–2013, largely from the ASE (Figure SM3.8) (Martín ‐ Español et al., 2016 <sup>[[#fn:r951|951]]</sup> ), which contributed –102 ± 10 Gt yr –1 from 2003 to 2011 (Sutterley et al., 2014 <sup>[[#fn:r952|952]]</sup> ). Total ASE ice discharge increased by 77% since the 1970s (Mouginot et al., 2014 <sup>[[#fn:r953|953]]</sup> ), primarily from acceleration of Pine Island Glacier that began around 1945, Smith, Pope and Kohler glaciers around 1980, and Thwaites Glacier around 2000 (Mouginot et al., 2014 <sup>[[#fn:r954|954]]</sup> ; Konrad et al., 2017 <sup>[[#fn:r955|955]]</sup> ; Smith et al., 2017c <sup>[[#fn:r956|956]]</sup> ). Dynamic thinning in the ASE and western AP accounted for 88% of the –36 ± 15 Gt yr –1 increase in AIS mass loss from 2008 to 2015 (Gardner et al., 2018 <sup>[[#fn:r957|957]]</sup> ). Glacier acceleration of up to 25% also affected the Getz Ice Shelf margin from 2007 to 2014 (Chuter et al., 2017 <sup>[[#fn:r958|958]]</sup> ).

Reduction or loss of ice shelf buttressing has dominated AIS dynamic thinning ( ''high confidence'' ). Ice shelves buttress 90% of AIS outflow (Depoorter et al., 2013 <sup>[[#fn:r959|959]]</sup> ; Rignot et al., 2014 <sup>[[#fn:r960|960]]</sup> ; Fürst et al., 2016 <sup>[[#fn:r961|961]]</sup> ; Reese et al., 2018 <sup>[[#fn:r962|962]]</sup> ), and ice shelf thinning increased in WAIS by 70% in the decade to 2012, averaged 8% thickness loss from 1994 to 2012 in the ASE (Paolo et al., 2015 <sup>[[#fn:r963|963]]</sup> ), and explains the post-2009 onset of rapid dynamic thinning on the southern-AP Bellingshausen Sea coast (Wouters et al., 2015 <sup>[[#fn:r964|964]]</sup> ; Hogg et al., 2017 <sup>[[#fn:r965|965]]</sup> ; Martin-Español et al., 2017 <sup>[[#fn:r966|966]]</sup> ) (Figure SM3.8). Grounding line retreat, an indicator of thinning, has been observed with ''high confidence'' (Rignot et al., 2014 <sup>[[#fn:r967|967]]</sup> ; Christie et al., 2016 <sup>[[#fn:r968|968]]</sup> ; Hogg et al., 2017 <sup>[[#fn:r969|969]]</sup> ; Konrad et al., 2018 <sup>[[#fn:r970|970]]</sup> ; Roberts et al., 2018 <sup>[[#fn:r971|971]]</sup> ). From 2010 to 2016, 22%, 3% and 10% of grounding lines in WAIS, EAIS and the AP respectively retreated at rates faster than 25 m yr −1 (the average pace since the Last Glacial Maximum; Konrad et al., 2018), with highest rates along the Amundsen and Bellingshausen Sea coasts, and around Totten Glacier, Wilkes Land, EAIS (Konrad et al., 2018), where dynamic thinning has occurred at least since 1979 (Roberts et al., 2018; Rignot et al., 2019 <sup>[[#fn:r972|972]]</sup> ). Ice shelf collapse has driven dynamic thinning in the northern AP over recent decades ( ''high confidence'' ) (Seehaus et al., 2015 <sup>[[#fn:r973|973]]</sup> ; Wuite et al., 2015 <sup>[[#fn:r974|974]]</sup> ; Friedl et al., 2018 <sup>[[#fn:r975|975]]</sup> ; Rott et al., 2018 <sup>[[#fn:r976|976]]</sup> ).

ASE ice shelf basal melting, grounding line retreat and dynamic thinning have varied with ocean forcing ( ''medium confidence'' ) (Dutrieux et al., 2014 <sup>[[#fn:r977|977]]</sup> ; Paolo et al., 2015 <sup>[[#fn:r978|978]]</sup> ; Christianson et al., 2016 <sup>[[#fn:r979|979]]</sup> ; Jenkins et al., 2018 <sup>[[#fn:r980|980]]</sup> ) but this variability is superimposed on sustained mass losses compatible with the onset of Marine Ice Sheet Instability (MISI) for several major glaciers ( ''medium confidence'' ) (Favier et al., 2014 <sup>[[#fn:r981|981]]</sup> ; Joughin et al., 2014 <sup>[[#fn:r982|982]]</sup> ; Mouginot et al., 2014 <sup>[[#fn:r983|983]]</sup> ; Rignot et al., 2014 <sup>[[#fn:r984|984]]</sup> ; Christianson et al., 2016 <sup>[[#fn:r985|985]]</sup> ). Whether unstable WAIS retreat has begun or is imminent remains a critical uncertainty (Cross-Chapter Box 8 in Chapter 3).

Mass gains due to increased snowfall have somewhat offset dynamic-thinning losses ( ''high confidence'' ). On the AP, snowfall began to increase in the 1930s, accelerated in the 1990s (Thomas et al., 2015 <sup>[[#fn:r986|986]]</sup> ; Goodwin et al., 2016 <sup>[[#fn:r987|987]]</sup> ), and now offsets sea-level rise by 6.2 ± 1.7 mm per century (Medley and Thomas, 2018 <sup>[[#fn:r988|988]]</sup> ). EAIS and WAIS snowfall increases offset 20th century sea-level rise by 7.7 ± 4.0 mm and 2.8 ± 1.7 mm respectively (Medley and Thomas, 2018 <sup>[[#fn:r989|989]]</sup> ) ( ''medium confidence'' ). AIS snowfall increased by 4 ± 1 then 14 ± 1 Gt per decade over the 19th and 20th centuries, of which EAIS contributed 10% (Thomas et al., 2017b <sup>[[#fn:r990|990]]</sup> ). Longer records suggest either an AIS snowfall decrease over the last 1000 years (Thomas et al., 2017a <sup>[[#fn:r991|991]]</sup> ) or a statistically negligible change over the last 800 years ( ''low confidence'' ) (Frezzotti et al., 2013 <sup>[[#fn:r992|992]]</sup> ).

Mass balance contributions from ice sheet basal melting were not described in AR5 (IPCC, 2013 <sup>[[#fn:r993|993]]</sup> ) and the sensitivity of the AIS subglacial hydrological system to climate change is poorly understood. Around half of the AIS bed melts (Siegert et al., 2017 <sup>[[#fn:r994|994]]</sup> ), producing ~65 Gt yr –1 of water (Pattyn, 2010 <sup>[[#fn:r995|995]]</sup> ) ( ''low confidence'' ), some of which refreezes (Bell, 2008 <sup>[[#fn:r996|996]]</sup> ) and some accumulates in subglacial lakes with a total volume of 10s of 1000s of km 3 (Popov and Masolov, 2007 <sup>[[#fn:r997|997]]</sup> ; Lipenkov et al., 2016 <sup>[[#fn:r998|998]]</sup> ; Siegert, 2017 <sup>[[#fn:r999|999]]</sup> ). This system contributes fresh water and nutrients to the ocean (Section 3.3.3.3) (Fricker et al., 2007 <sup>[[#fn:r1000|1000]]</sup> ; Siegert et al., 2007 <sup>[[#fn:r1001|1001]]</sup> ; Carter and Fricker, 2012 <sup>[[#fn:r1002|1002]]</sup> ; Horgan et al., 2013 <sup>[[#fn:r1003|1003]]</sup> ; Le Brocq, 2013 <sup>[[#fn:r1004|1004]]</sup> ; Flament et al., 2014 <sup>[[#fn:r1005|1005]]</sup> ; Siegert et al., 2016 <sup>[[#fn:r1006|1006]]</sup> ), and lubricates glacier sliding (e.g., Dow et al., 2018b). Changes in the ice sheet thickness can redistribute subglacial water, affecting drainage pathways and ice flow (Fricker et al., 2016 <sup>[[#fn:r1007|1007]]</sup> ), but hydrological observations are very scarce.

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