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

==== 3.3.1.1 Antarctic Ice Sheet Mass Change ====

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It is ''virtually certain'' that the Antarctic Peninsula (AP) and West Antarctic Ice Sheet (WAIS) combined have cumulatively lost mass since widespread measurements began in 1992, and that the rate of loss has increased since around the year 2006 and continued post-AR5 (Martín ‐ Español et al., 2016 <sup>[[#fn:r915|915]]</sup> ; Zwally et al., 2017 <sup>[[#fn:r916|916]]</sup> ; Bamber et al., 2018 <sup>[[#fn:r917|917]]</sup> ; Gardner et al., 2018 <sup>[[#fn:r918|918]]</sup> ; The IMBIE Team, 2018; Rignot et al., 2019 <sup>[[#fn:r919|919]]</sup> ), extending and reinforcing previous findings (IPCC, 2013 <sup>[[#fn:r920|920]]</sup> ) (Figure 3.7, Table 3.3, SM3.3.1.1). From ''medium evidence'' , there is ''high agreement'' in the sign and ''medium agreement'' in the magnitude of both WAIS and AP mass change between the complementary satellite methods (Mémin et al., 2015 <sup>[[#fn:r921|921]]</sup> ; The IMBIE Team, 2018).

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'''Table 3.3'''
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Mass balance (Gt yr -1 ) of the West Antarctic Ice Sheet (WAIS), Antarctic Peninsula (AP), East Antarctic Ice Sheet (EAIS), the combined Antarctic Ice Sheets (AIS) and the Greenland Ice Sheet (GIS) and the total sea level contribution (mm yr -1 ).
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[[File:92398abc51f11dac3260b63c795aa85c table3.3.png]]

WAIS mass loss and recent increases in loss were concentrated in the Amundsen Sea Embayment (ASE) ( ''high confidence'' ) with increases particularly in the late 2000s (Mouginot et al., 2014 <sup>[[#fn:r930|930]]</sup> ), accounting for most of the –112 ± 10 Gt yr –1 WAIS loss from 2003 to 2013 (Martín ‐ Español et al., 2016 <sup>[[#fn:r931|931]]</sup> ). The ice sheet margins of nearby Getz Ice Shelf also lost mass rapidly (–67 ± 27 Gt yr –1 , 2008–2015) (Gardner et al., 2018 <sup>[[#fn:r932|932]]</sup> ). This region also experienced losses during previous warm periods (Cross-Chapter Box 8 in Chapter 3).

On the AP, the Bellingshausen Sea ice sheet margin shifted from close to mass balance in the 2000s to rapid loss from 2009 (–56 ± 8 Gt yr -1 from 2010 to 2014) ( ''high confidence'' ) (Helm et al., 2014 <sup>[[#fn:r933|933]]</sup> ; McMillan et al., 2014b <sup>[[#fn:r934|934]]</sup> ; Wouters et al., 2015 <sup>[[#fn:r935|935]]</sup> ; Hogg et al., 2017 <sup>[[#fn:r936|936]]</sup> ). This shift accompanied ongoing mass loss ( ''high confidence'' ) from the smaller northeastern AP glaciers that fed the former Prince Gustav, Larsen A and B ice shelves, though now at a lower rate than immediately following shelf collapse in 1995 and 2002 (Seehaus et al., 2015 <sup>[[#fn:r937|937]]</sup> ; Wuite et al., 2015 <sup>[[#fn:r938|938]]</sup> ; Rott et al., 2018 <sup>[[#fn:r939|939]]</sup> ). Of 860 marine-terminating AP glaciers, 90% retreated from their 1940s positions (Cook et al., 2014 <sup>[[#fn:r940|940]]</sup> ), established in the early to mid-Holocene (Ó Cofaigh et al., 2014) ( ''medium confidence'' ). Early 21st century combined AP glacier (Fieber et al., 2018 <sup>[[#fn:r941|941]]</sup> ) and ice sheet loss was around –30 Gt yr –1 (Table 3.3).

The East Antarctic Ice Sheet (EAIS, covering 85% of the Antarctic Ice Sheet (AIS)) has remained close to balance, with large interannual variability and no clear mass trend over the satellite record ( ''medium confidence'' ) (Table 3.3, Figure 3.7, SM3.3.1.2), and relatively large observation uncertainties (SM3.3.1) (Velicogna et al., 2014 <sup>[[#fn:r942|942]]</sup> ; Martin-Español et al., 2017 <sup>[[#fn:r943|943]]</sup> ; Bamber et al., 2018 <sup>[[#fn:r944|944]]</sup> ). SMB trends are particularly ambiguous, leading to disagreement between one altimetry and one flux-based estimate of 136 ± 43 Gt yr–1 (spanning 1992–2008) (Zwally et al., 2017 <sup>[[#fn:r945|945]]</sup> ), and –41 ± 8 Gt yr–1 (1979–2017) (Rignot et al., 2019 <sup>[[#fn:r946|946]]</sup> ), respectively. Both differ from the multi-method averages reported here (Table 3.3).

EAIS mass gains on the Siple Coast and Dronning Maud Land (e.g., 63 ± 6 Gt yr –1 from 2003 to 2013 (Velicogna et al., 2014 <sup>[[#fn:r947|947]]</sup> )) contrast with Wilkes Land losses e.g., from –17 ± 4 Gt yr –1 from the Totten Glacier area, 2003–2013 (Velicogna et al., 2014) that drain a large area of deeply-grounded EAIS with potential for multi-metre sea level contributions (Zwally et al., 2017 <sup>[[#fn:r948|948]]</sup> ; Rignot et al., 2019 <sup>[[#fn:r949|949]]</sup> ). Limited palaeo ice sheet evidence suggests that this area has previously lost substantial mass in previous interglacials ( ''medium confidence'' ) (Aitken et al., 2016 <sup>[[#fn:r950|950]]</sup> ; Wilson et al., 2018).

Overall, 2012–2016 AIS mass losses were ''extremely likely'' greater than those from 2002 to 2011 and ''likely'' greater than from 1992 to 2001, and it is ''extremely likely'' that the negative 2012–2016 AIS mass balance was dominated by losses from WAIS (Table 3.3).

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'''Figure 3.7'''

<span id="a-cumulative-ice-sheet-mass-change-19922016-after-bamber-et-al.-2018-the-imbie-team-2018.-b-greenland-ice-sheet-gis-mass-change-components-from-surface-mass-balance-smb-orange-and-dynamic-thinning-blue-from-2000-to-2016-after-van-den-broeke-et-al.-2016-king-et-al.-2018.-uncertainties-are-1-standard-deviation."></span>
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'''(a) Cumulative Ice Sheet mass change, 1992–2016, (after Bamber et al., 2018; The IMBIE Team, 2018). (b) Greenland Ice Sheet (GIS) mass change components from surface mass balance (SMB) (orange) and dynamic thinning (blue) from 2000 to 2016, (after van den Broeke et al., 2016; King et al., 2018). Uncertainties are 1 standard deviation.'''
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[[File:ff677c1128be1370620c9dc35d602654 IPCC-SROCC-CH_3_7.jpg]]

(a) Cumulative Ice Sheet mass change, 1992–2016, (after Bamber et al., 2018; The IMBIE Team, 2018). (b) Greenland Ice Sheet (GIS) mass change components from surface mass balance (SMB) (orange) and dynamic thinning (blue) from 2000 to 2016, (after van den Broeke et al., 2016; King et al., 2018). Uncertainties are 1 standard deviation.

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