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

==== 9.3.2.1 Antarctic Sea Ice Coverage ====

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The SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ) assessed that there was no significant trend in annual mean Antarctic sea ice area over the period of reliable satellite retrievals starting in 1979 ( ''high confidence'' ). The updated time series is consistent with this assessment. It includes a maximum sea ice area in 2014, then a substantial decline until the minimum sea ice area in 2017, and an increase in sea ice area since 2017 (Figures 2.20 and 9.15; [[#Schlosser--2018|Schlosser et al., 2018]] ; [[#Maksym--2019|Maksym, 2019]] ; [[#Parkinson--2019|Parkinson, 2019]] ). As assessed in [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.1.2|Section 2.3.2.1.2]] , the possible significance of the increase in mean Antarctic sea ice area over the shorter period 1979 to 2014 (Figure 2.20; [[#Simmonds--2015|Simmonds, 2015]] ; [[#Comiso--2017b|Comiso et al., 2017b]] ) is unclear. This is because of observational uncertainty (see [[#9.3.1.1|Section 9.3.1.1]] ), large year-to-year fluctuations in all months (Figure 9.15), and limited understanding of the processes and reliability of year-to-year correlation of Antarctic sea ice area ( [[#Yuan--2017|Yuan et al., 2017]] ).

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[[File:acf3192464d6175b170565c4ccacb36f IPCC_AR6_WGI_Figure_9_15.png]]

'''Figure''' '''9.15 |''' '''Antarctic sea ice historical records and Coupled Model Intercomparison Project Phase 6 (CMIP6) projections.''' '''(Left)''' Absolute anomaly of observed monthly mean Antarctic sea ice area during the period 1979–2019 relative to the average monthly mean Antarctic sea ice area during the period 1979–2008. '''(Right)''' Sea ice coverage in the Antarctic as given by the average of the three most widely used satellite-based estimates for September and February, which usually are the months of maximum and minimum sea ice coverage, respectively. First column: Mean sea ice coverage during the decade 1979–1988. Second column: Mean sea ice coverage during the decade 2010–2019. Third column: Absolute change in sea ice concentration between these two decades, with grid lines indicating non-significant differences. Fourth column: Number of available CMIP6 models that simulate a mean sea ice concentration above 15% for the decade 2045–2054. The average observational record of sea ice area is derived from the UHH sea ice area product ( [[#Doerr--2021|Doerr et al., 2021]] ), based on the average sea ice concentration of OSISAF/CCI (OSI-450 for 1979–2015, OSI-430b for 2016–2019) ( [[#Lavergne--2019|Lavergne et al., 2019]] ), NASA Team (version 1, 1979–2019) ( [[#Cavalieri--1996|Cavalieri et al., 1996]] ) and Bootstrap (version 3, 1979–2019) ( [[#Comiso--2017|Comiso, 2017]] ) that is also used for the figure panels showing observed sea ice concentration. Further details on data sources and processing are available in the chapter data table (Table 9.SM.9).

As assessed by SROCC, the evolution of mean Antarctic sea ice area is the result of opposing regional trends ( ''high confidence'' ), with slightly decreasing sea ice cover during the period 1979 to 2019 in the Amundsen and Bellingshausen Seas, particularly during summer, and slightly increasing sea ice cover in the eastern parts of the Weddell and Ross Seas (Figure 9.15). With the exception of the Ross Sea, these trends are not significant, considering the large variability of the time series ( [[#Yuan--2017|Yuan et al., 2017]] ).

The SROCC assessed that the regional trends are closely related to meridional wind trends ( ''high confidence'' ). This is the case as the regional trends in the maximum northward extent of the ice cover (Figure 9.15) are determined by the balance between the northward advection of the ice that is formed in polynyas near the continental margin, and the lateral and subsurface melting through oceanic heat fluxes. The advection of the sea ice is strongly correlated with winds and cyclones ( [[#Schemm--2018|Schemm, 2018]] ; [[#Vichi--2019|Vichi et al., 2019]] ; [[#Alberello--2020|Alberello et al., 2020]] ). Accordingly, the increasing sea ice area in the Ross Sea can be linked to a strengthening of the Amundsen Sea low (e.g., [[#Holland--2017b|Holland et al., 2017b]] , 2018), while other regional sea ice trends in the austral autumn can be linked to changes in westerly winds, cyclone activity and the Southern Annular Mode (SAM) in summer and spring ( [[#Doddridge--2017|Doddridge and Marshall, 2017]] ; [[#Holland--2017a|Holland et al., 2017a]] ; [[#Schemm--2018|Schemm, 2018]] ). In addition to the wind-driven changes, increased near-surface ocean stratification ( [[#9.2.1.3|Section 9.2.1.3]] ) has contributed to the observed increase in sea ice coverage (e.g., [[#Purich--2018|Purich et al., 2018]] ; L. [[#Zhang--2019|]] [[#Zhang--2019|]] [[#Zhang--2019|Zhang et al., 2019]] ) as it tends to cool the surface ocean (Sections 9.2.1.1 and 9.2.3.2). The changes in stratification result partly from surface freshening ( [[#De%20Lavergne--2014|De Lavergne et al., 2014]] ), associated with increased northward sea ice advection ( [[#Haumann--2020|Haumann et al., 2020]] ) and/or melting of the Antarctic ice sheet ( ''medium confidence'' ) (e.g., [[#Haumann--2020|Haumann et al., 2020]] ; [[#Jeong--2020|Jeong et al., 2020]] ; [[#Mackie--2020|Mackie et al., 2020]] ), and amplified by local ice–ocean feedbacks ( [[#Goosse--2014|Goosse and Zunz, 2014]] ; [[#Lecomte--2017|Lecomte et al., 2017]] ; [[#Goosse--2018|Goosse et al., 2018]] ). In the Amundsen Sea, strong ice shelf melting can cause local sea ice melt next to the ice shelf front by entraining warm circumpolar deep water to the ice shelf cavity and surface ocean ( ''medium confidence'' ) (Sections 9.2.3.2 and 9.4.2.2; [[#Jourdain--2017|Jourdain et al., 2017]] ; [[#Merino--2018|Merino et al., 2018]] ). It has also been suggested that the observed regional increase in sea ice coverage since 1979 results from a long-term Southern Ocean surface cooling trend (e.g., [[#Kusahara--2019|Kusahara et al., 2019]] ; [[#Jeong--2020|Jeong et al., 2020]] ) but the importance of this mechanism for the observed sea ice evolution is unclear owing to intricate feedbacks between sea ice change and surface cooling ( [[#Haumann--2020|Haumann et al., 2020]] ). The importance of changing wave activity ( [[#9.6.4.2|Section 9.6.4.2]] ; [[#Kohout--2014|Kohout et al., 2014]] ; [[#Bennetts--2017|Bennetts et al., 2017]] ; [[#Roach--2018b|Roach et al., 2018b]] ) on sea ice is unclear due to limited process understanding. In summary, there is ''high confidence'' that regional Antarctic trends are primarily caused by changes in sea ice drift and decay, with ''medium confidence'' in a dominating role of changing wind pattern. The precise relative contribution of individual drivers remains uncertain because of limited observations, disagreement between models, unresolved processes, and temporal and spatial remote linkages caused by sea ice drift ( [[#9.2.3.2|Section 9.2.3.2]] ; [[#Pope--2017|Pope et al., 2017]] ).

Recent research has confirmed SROCC assessment of atmospheric and oceanic drivers of the sea ice decline from 2014 to 2017, which can be linked to changes in both subsurface ocean heat flux ( [[#Meehl--2019|Meehl et al., 2019]] ; [[#Purich--2019|Purich and England, 2019]] ) and atmospheric circulation, with the latter partly related to teleconnections with the tropics ( [[#Meehl--2019|Meehl et al., 2019]] ; [[#Purich--2019|Purich and England, 2019]] ; G. [[#Wang--2019|]] [[#Wang--2019|]] [[#Wang--2019|Wang et al., 2019]] ). In the Weddell Sea, these changes caused in 2017 the re-emergence of the largest polynya over the Maud Rise since the 1970s ( [[#9.2.3.2|Section 9.2.3.2]] ; [[#Campbell--2019|Campbell et al., 2019]] ; [[#Jena--2019|Jena et al., 2019]] ; [[#Turner--2020|Turner et al., 2020]] ).

The AR5 ( [[#Collins--2013|Collins et al., 2013]] ) and SROCC found ''low confidence'' in future projections of Antarctic sea ice. This includes the projected mitigation of the sea ice loss by stratospheric ozone recovery ( [[#Smith--2012|Smith et al., 2012]] ) and by an increased freshwater input from melting of the Antarctic Ice Sheet ( [[#Bronselaer--2018|Bronselaer et al., 2018]] ). Compared to the interannual variability during the satellite record from 1979 onwards, models simulate too much variability in both CMIP5 ( [[#Zunz--2013|Zunz et al., 2013]] ) and CMIP6 ( [[#Roach--2020|Roach et al., 2020]] ). The seasonal cycle in sea ice coverage is misrepresented in most CMIP5 (e.g., [[#Holmes--2019|Holmes et al., 2019]] ) and CMIP6 models ( [[#Roach--2020|Roach et al., 2020]] ), but the multi-model mean seasonal cycle in CMIP5 and CMIP6 agrees well with observations ( [[#Shu--2015|Shu et al., 2015]] ; [[#Roach--2020|Roach et al., 2020]] ). Most CMIP5 models do not realistically simulate the evolution of Antarctic sea ice volume ( [[#Shu--2015|Shu et al., 2015]] ) and consistently overestimate the amount of low concentration sea ice, and underestimate the amount of high concentration sea ice ( [[#Roach--2018a|Roach et al., 2018a]] ). In contrast, CMIP6 models simulate a more realistic distribution of regional sea ice coverage ( [[#Roach--2020|Roach et al., 2020]] ). Most CMIP5 models poorly represent Antarctic sea ice drift (e.g., [[#Schroeter--2018|Schroeter et al., 2018]] ; [[#Holmes--2019|Holmes et al., 2019]] ), affecting simulated historical trends, with models that simulate a strong sea ice motion showing more variability in sea ice coverage than models with weaker sea ice motion ( [[#Schroeter--2018|Schroeter et al., 2018]] ). Owing to ''limited agreement'' between model simulations and observations, limited reliable observations on a process level, and a lack of process understanding of the substantial spread in CMIP5 and CMIP6 model simulations, there remains ''low confidence'' in existing future projections of Antarctic sea ice decrease and lack of decrease.

The discrepancy between the modelled and observed evolution of Antarctic sea ice has been related by SROCC to deficiencies in modelled stratification, freshening by ice-shelf meltwater, clouds, and other wind- and ocean-driven processes. Recent studies highlight the possible mis-representation of freshwater fluxes from ice shelves ( [[#Jeong--2020|Jeong et al., 2020]] ), and the possible effect of the low resolution of most models ( [[#Sidorenko--2019|Sidorenko et al., 2019]] ), even though lower-resolution models are, in principle, capable of a realistic simulation of the seasonal sea ice budgets in the Southern Ocean ( [[#Holmes--2019|Holmes et al., 2019]] ). The relative importance of these possible reasons for the models’ shortcomings remains unclear (see [[IPCC:Wg1:Chapter:Chapter-3#3.4.1.2|Section 3.4.1.2]] for details).

The analysis and understanding of the long-term evolution of the Antarctic sea ice cover is hindered by the scarcity of observational records before the satellite period, and the scarcity of paleorecords (see [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.1.2|Section 2.3.2.1.2]] for further details). Such long records are particularly relevant given that the Southern Ocean response to external forcing takes longer than the length of the available direct observational record ( [[#Goosse--2001|Goosse and Renssen, 2001]] ; [[#Armour--2016|Armour et al., 2016]] ). There is only ''limited evidence'' for large-scale decadal fluctuations in sea ice coverage caused by large-scale temperature and wind forcing. Sparse direct pre-satellite observations suggest a decrease in sea ice coverage from the 1950s to the 1970s ( [[#Fan--2014|Fan et al., 2014]] ). Paleo-proxy data indicate that, on multi-decadal to multi-centennial time scales, sea ice coverage of the Southern Ocean follows large-scale temperature trends (e.g., [[#Crosta--2018|Crosta et al., 2018]] ; [[#Chadwick--2020|Chadwick et al., 2020]] ; [[#Lamping--2020|Lamping et al., 2020]] ), for example linked to fluctuations in the El Niño–Southern Oscillation and Southern Annular Mode ( [[#Crosta--2021|Crosta et al., 2021]] ), and that during the Last Glacial Maximum, Antarctic sea ice extended to about the polar front latitude in most regions during winter, whereas the extent during summer is less well understood (e.g., [[#Benz--2016|Benz et al., 2016]] ; [[#Xiao--2016|Xiao et al., 2016]] ; [[#Nair--2019|Nair et al., 2019]] ).

Regionally, proxy data from ice cores consistently indicate that the increase of sea ice area in the Ross Sea and the decrease of sea ice area in the Bellingshausen Sea are part of longer centennial trends and exceed internal variability on multi-decadal time scales ( ''medium confidence'' ) (e.g., [[#Thomas--2019|Thomas et al., 2019]] ; [[#Tesi--2020|Tesi et al., 2020]] ). These centennial trends are consistent with simulations from CMIP5 models ( [[#Hobbs--2016b|Hobbs et al., 2016b]] ; J.M. [[#Jones--2016|]] [[#Jones--2016|Jones et al., 2016]] ; [[#Kimura--2017|Kimura et al., 2017]] ).

There is ''low confidence'' in the attribution of the observed changes in Antarctic sea ice area ( [[IPCC:Wg1:Chapter:Chapter-3#3.4.1.2|Section 3.4.1.2]] ). Based on the available evidence, the lack of a negative trend of Antarctic sea ice area, despite substantial global warming in recent decades, has been attributed to internal variability in analyses of the observational record ( [[#Meier--2013|Meier et al., 2013]] ; [[#Gallaher--2014|Gallaher et al., 2014]] ; [[#Gagné--2015b|Gagné et al., 2015b]] ), reconstructions from early observations ( [[#Fan--2014|Fan et al., 2014]] ; [[#Edinburgh--2016|Edinburgh and Day, 2016]] ) and proxy data ( [[#Hobbs--2016b|Hobbs et al., 2016b]] ) in model simulations ( [[#Turner--2013|Turner et al., 2013]] ; [[#Zunz--2013|Zunz et al., 2013]] ; L. [[#Zhang--2019|]] [[#Zhang--2019|]] [[#Zhang--2019|Zhang et al., 2019]] ). Nonetheless, without accurate simulations of observed changes, the possible contribution of anthropogenic forcing to the regional changes in sea ice area remains unclear ( [[#Hosking--2013|Hosking et al., 2013]] ; [[#Turner--2013|Turner et al., 2013]] ; [[#Haumann--2014|Haumann et al., 2014]] ; L. [[#Zhang--2019|]] [[#Zhang--2019|]] [[#Zhang--2019|Zhang et al., 2019]] ).

The attribution of the observed trends in atmospheric and oceanic forcing is also uncertain because of limited observational records and discrepancies between modelled and observed evolution of the sea ice cover. More specifically, there is contrasting evidence for a direct role of stratospheric ozone depletion on the observed changes in atmospheric circulation ( [[#Haumann--2014|Haumann et al., 2014]] ; [[#England--2016|England et al., 2016]] ; [[#Landrum--2017|Landrum et al., 2017]] ). In contrast, there is ''high confidence'' that multi-decadal variations in the tropical Pacific and in the Atlantic affect the Amundsen Sea low ( [[#Li--2014|Li et al., 2014]] ; [[#Kwok--2016|Kwok et al., 2016]] ; [[#Meehl--2016|Meehl et al., 2016]] ; [[#Purich--2016|Purich et al., 2016]] ; [[#Simpkins--2016|Simpkins et al., 2016]] ), while other modes of climate variability (Annex IV) affect, for example, Southern Ocean cyclone activity ( [[#Simpkins--2012|Simpkins et al., 2012]] ; [[#Cerrone--2017|Cerrone et al., 2017]] ; [[#Schemm--2018|Schemm, 2018]] ) ''.''

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