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

==== 9.3.2.2 Antarctic Sea Ice Thickness ====

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The SROCC assessed that observations are too sparse to reliably estimate long-term trends in Antarctic sea ice thickness. This remains true, and only qualitative statements on prevailing thicknesses are possible. Data from ICESat-1 laser altimetry ( [[#Kurtz--2012|Kurtz and Markus, 2012]] ), from Operation IceBridge ( [[#Kwok--2018|Kwok and Kacimi, 2018]] ), and long-term shipboard observations collected in the Antarctic Sea Ice Processes and Climate (ASPeCt) dataset ( [[#Worby--2008|Worby et al., 2008]] ) suggest that sea ice thicker than 1 m prevails in regions of multi-year ice along the eastern coast of the Antarctic Peninsula in the Weddell Sea, in the high-latitude embayment of the Weddell Sea, and along the coast of the Amundsen Sea, with remaining regions dominated by thinner first-year sea ice ( ''high confidence'' ). Regional patterns in ice thickness are affected by areas of high snow deposition and resulting snow-ice formation ( [[#Massom--2001|Massom et al., 2001]] ; [[#Maksym--2008|Maksym and Markus, 2008]] ), and deformation, ridging, and rafting that regionally cause formation of very thick sea ice ( [[#Massom--2006|Massom et al., 2006]] ; G. [[#Williams--2015|]] [[#Williams--2015|Williams et al., 2015]] ). In addition, near ice shelves a sub-ice platelet layer from supercooled water can significantly increase sea ice thickness ( [[#Hoppmann--2020|Hoppmann et al., 2020]] ; [[#Haas--2021|Haas et al., 2021]] ). Regarding snow thickness, observations are too sparse in space and time to reliably estimate changes across Southern Ocean sea ice ( [[#Webster--2018|Webster et al., 2018]] ).

There is ''low confidence'' in the long-term trend of Antarctic sea ice thickness. Both ASPeCt and ICESat-1 measurements are biased low in regions with thick ice ( [[#Kern--2015|Kern and Spreen, 2015]] ), compared to results from reanalyses ( [[#Massonnet--2013|Massonnet et al., 2013]] ; [[#Haumann--2016|Haumann et al., 2016]] ) and observations with autonomous vehicles under sea ice (G. [[#Williams--2015|]] [[#Williams--2015|Williams et al., 2015]] ). Estimates of sea ice thickness from CryoSat-2 do not substantially reduce uncertainty, primarily because of the unknown snow thickness and radar scattering above the snow–ice interface ( [[#Bunzel--2018|Bunzel et al., 2018]] ; [[#Kwok--2018|Kwok and Kacimi, 2018]] ; [[#Kacimi--2020|Kacimi and Kwok, 2020]] ). Isolated in situ time series show no clear long-term trend in landfast ice thickness in the Weddell Sea ( [[#Arndt--2020|Arndt et al., 2020]] ). Reanalyses suggest overall increasing sea ice thickness and volume between 1980 and 2010 ( [[#Holland--2014|Holland et al., 2014]] ; [[#Zhang--2014|Zhang, 2014]] ; [[#Massonnet--2015|Massonnet et al., 2015]] ), while CMIP5 ( [[#Shu--2015|Shu et al., 2015]] ; [[#Schroeter--2018|Schroeter et al., 2018]] ) and CMIP6 models simulate a decrease in Antarctic sea ice volume over the historical period. Because of this discrepancy, and the unclear reliability of the reanalyses ( [[#Uotila--2019|Uotila et al., 2019]] ), there is ''low confidence'' in CMIP5 and CMIP6 simulated future Antarctic sea ice thickness.

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