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

===== 3.4.3.2.2 Antarctic Ice Sheet =====

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AR5 assessed that there was ''low confidence'' in attributing the causes of the observed mass loss from the Antarctic Ice Sheet since 1993 ( [[#Bindoff--2013|Bindoff et al., 2013]] ). The SROCC assessed that there is ''medium agreement'' but ''limited evidence'' of anthropogenic forcing of Antarctic mass balance through both surface mass balance and glacier dynamics. It further assessed that Antarctic ice loss is dominated by acceleration, retreat and rapid thinning of the major West Antarctic Ice Sheet outlet glaciers ( ''very high confidence'' ), driven by melting of ice shelves by warm ocean waters ( ''high confidence'' ). Based on updated observations, [[IPCC:Wg1:Chapter:Chapter-2|Chapter 2]] assesses that there is ''very high confidence'' that the Antarctic Ice Sheet lost mass between 1992 and 2017, and that there is ''medium confidence'' that this mass loss has accelerated. Models of Antarctic Ice Sheet evolution are evaluated in detail in Section 9.4.2.2, which assesses that there is ''medium confidence'' in many ice-sheet processes in Antarctic Ice Sheet models, but ''low confidence'' in the ocean forcing affecting basal melt rates. CMIP5 and CMIP6 models perform similarly in their simulation of Antarctic surface mass balance (Section 9.4.2.2, [[#Gorte--2020|Gorte et al., 2020]] ). Model evaluation of surface mass balance over the Antarctic Ice Sheet, including regional aspects, is also assessed in Atlas.11.1.3.

Ice discharge around the West Antarctic Ice Sheet is strongly influenced by variability in basal melt ( [[#Jenkins--2018|Jenkins et al., 2018]] ; [[#Hoffman--2019|Hoffman et al., 2019]] ), in particular at decadal and longer time scales ( [[#Snow--2017|Snow et al., 2017]] ). Basal melt rate variability can be induced by wind-driven ocean current changes, which may partly be of anthropogenic origin via greenhouse gas forcing ( [[#Holland--2019|Holland et al., 2019]] ). Moreover, ice discharge losses from the Antarctic Ice Sheet over the 2007–2017 period are close to the centre of the model-based range projected in AR5 ( [[#Slater--2020|Slater et al., 2020]] ). However, expert opinion differs as to whether recent Antarctic ice loss from the West Antarctic Ice Sheet has been driven primarily by external forcing or by internal variability, and there is no consensus ( [[#Bamber--2019|Bamber et al., 2019]] ). Anthropogenic influence on the Antarctic surface mass balance, which is expected to partially compensate for ice discharge losses through increases in snowfall, is currently masked by strong natural variability ( [[#Previdi--2016|Previdi and Polvani, 2016]] ; [[#Bodart--2019|Bodart and Bingham, 2019]] ), and observations suggest that it has been close to zero over recent years (see further discussion in Section 9.4.2.1; [[#Slater--2020|Slater et al., 2020]] ).

Overall, there is ''medium agreement'' but ''limited evidence'' of anthropogenic influence on Antarctic mass balance through changes in ice discharge.

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