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

==== 3.4.1.2 Antarctic Sea Ice ====

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AR5 concluded that ‘there is ''low confidence'' in the attribution of the observed increase in Antarctic SIE since 1979’ ( [[#Bindoff--2013|Bindoff et al., 2013]] ) due to the limited understanding of the external forcing contribution as well as the role of internal variability. Based on a difference between the first and last decades, Antarctic sea ice cover exhibited a small increase in summer and winter over the 1979–2017 period ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.1.2|Section 2.3.2.1.2]] , and Figures 3.20 and 3.21). However, these changes are not statistically significant and starting in late 2016, anomalously low sea ice area has been observed ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.1.2|Section 2.3.2.1.2]] ). The mean hemispheric sea ice changes result from much larger, but partially compensating, regional changes with increases in the western Ross Sea and Weddell Sea and declines in the Bellingshausen and Amundsen Seas ( [[#Hobbs--2016|Hobbs et al., 2016]] ). Observed regional trends have been particularly large in austral autumn (see [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.1.2|Section 2.3.2.1.2]] , and also Section 9.3.2.1 for more details of regional changes and related physical processes). Starting in austral spring of 2016, the ice extent decreased strongly ( [[#Turner--2017|Turner et al., 2017]] ) and has since remained anomalously low (Figure 3.21 and Figure 2.20). This decrease has been associated with anomalous atmospheric conditions associated with teleconnections from warming in the eastern Indian Ocean and a negative Southern Annular Mode ( [[#Chenoli--2017|Chenoli et al., 2017]] ; [[#Stuecker--2017|Stuecker et al., 2017]] ; [[#Schlosser--2018|Schlosser et al., 2018]] ; [[#Meehl--2019|Meehl et al., 2019]] ; [[#Purich--2019|Purich and England, 2019]] ; G. [[#Wang--2019|]] [[#Wang--2019|Wang et al., 2019]] ). A decadal-scale warming of the near-surface ocean that resulted from strengthened westerlies may also have contributed to and helped to sustain the sea ice loss ( [[#Meehl--2019|Meehl et al., 2019]] ). Before satellites and on even longer time scales, very limited observational data and proxy coverage leads to ''low confidence'' in all aspects of Antarctic sea ice (Sections 2.3.2.1.2 and 9.3.2.1).

CMIP5 climate models generally simulate Antarctic sea ice loss over the satellite era since 1979 ( [[#Mahlstein--2013|Mahlstein et al., 2013]] ; [[#Turner--2013|Turner et al., 2013]] ) in contrast to the observed change, and CMIP6 models also simulate Antarctic ice loss ( [[#Roach--2020|Roach et al., 2020]] ; Figure 3.20 and 3.21). A number of studies have suggested that this discrepancy may be in part due to the role of internal variability in the observed change ( [[#Mahlstein--2013|Mahlstein et al., 2013]] ; [[#Polvani--2013|Polvani and Smith, 2013]] ; [[#Zunz--2013|Zunz et al., 2013]] ; [[#Meehl--2016c|Meehl et al., 2016c]] ; [[#Turner--2016|Turner et al., 2016]] ), including teleconnections associated with tropical Pacific variability ( [[#Meehl--2016c|Meehl et al., 2016c]] ) and changing surface conditions resulting from multi-decadal ocean circulation variations ( [[#Singh--2019|Singh et al., 2019]] ). However, when the spatial pattern is considered, trends in the summer and autumn (from 1979–2005) appear outside the range of internal variability ( [[#Hobbs--2015|Hobbs et al., 2015]] ). This suggests that the models may exhibit an unrealistic simulation of the Antarctic sea ice forced response or the internal variability of the system. Discrepancies among the models in simulated sea ice variability ( [[#Zunz--2013|Zunz et al., 2013]] ), the sea ice climatological state ( [[#Roach--2018|Roach et al., 2018]] ), upper ocean temperature trends ( [[#Schneider--2018|Schneider and Deser, 2018]] ), Southern Hemisphere westerly wind trends ( [[#Purich--2016|Purich et al., 2016]] ), or the sea ice response to Southern Annular Mode variations ( [[#Ferreira--2014|Ferreira et al., 2014]] ; [[#Holland--2017|Holland et al., 2017]] ; [[#Kostov--2017|Kostov et al., 2017]] ; [[#Landrum--2017|Landrum et al., 2017]] ) may all play some role in explaining these differences with the observed trends. Increased fresh water fluxes caused by mass loss of the Antarctic Ice Sheet (either by melting at the front of ice shelves or via iceberg calving) have been suggested as a possible mechanism driving the multi-decadal Antarctic sea ice expansion ( [[#Bintanja--2015|Bintanja et al., 2015]] ; [[#Pauling--2016|Pauling et al., 2016]] ) but there is a lack of consensus on this mechanism’s impacts ( [[#Pauling--2017|Pauling et al., 2017]] ). A recent study based on a decadal prediction system suggests that initializing the state of the Antarctic Bottom Water cell allows the system to reproduce the observed Antarctic sea ice increase ( [[#Zhang--2017|Zhang et al., 2017]] ), consistent with the suggestion that multi-decadal variability associated with variations in deep convection has contributed to the observed increase in Antarctic sea ice since 1979 ( [[#Latif--2013|Latif et al., 2013]] ; [[#Zhang--2017|Zhang et al., 2017]] ; L. [[#Zhang--2019|]] [[#Zhang--2019|Zhang et al., 2019]] ) (see also Section 9.3.2.1).

There have been several studies that aimed to identify causes of the observed Antarctic SIE changes. [[#Gagné--2015|Gagné et al. (2015)]] assessed the consistency of observed and simulated changes in Antarctic SIE for an extended period using recovered satellite-based estimates, and found that the observed trends since the mid-1960s are not inconsistent with model simulated trends. Studies based on the satellite period also indicate that the observed trends are largely within the range of simulated internal variability ( [[#Hobbs--2016|Hobbs et al., 2016]] ). A few distinct factors that led to the weak signal-to-noise ratio in Antarctic SIE trends have been further identified, which include large multi-decadal variability ( [[#Monselesan--2015|Monselesan et al., 2015]] ), the short observational record (e.g., [[#Abram--2013|Abram et al., 2013]] ), and the limited model performance at representing the complex Antarctic climate system as discussed above ( [[#Bintanja--2013|Bintanja et al., 2013]] ; [[#Uotila--2014|Uotila et al., 2014]] ). The short period of comprehensive satellite observations, beginning in 1979, makes it challenging to set the observed increase between 1979 and 2015, or the subsequent decrease, in a long-term context, and to assess whether the difference in trend between observations and models, which mostly simulate long-term decreases, is systematic or a rare expression of internal variability on decadal to multi-decadal time scales.

In conclusion, the observed small increase in Antarctic sea ice extent during the satellite era is not generally captured by global climate models, and there is ''low confidence'' in attributing the causes of the change.

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