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

===== 3.2.1.1.2 Age and thickness =====

The proportion of Arctic sea ice at least 5 years old declined from 30% to 2% between 1979 and 2018; over the same period first-year sea ice proportionally increased from approximately 40% to 60–70% (Stroeve and Notz, 2018 <sup>[[#fn:r104|104]]</sup> ) ( ''very high confidence'' ) (Sections 3.2.1.1.3 and 3.2.1.1.4). Arctic sea ice has thinned through volume reductions in satellite altimeter retrievals (Laxon et al., 2013 <sup>[[#fn:r105|105]]</sup> ; Kwok, 2018 <sup>[[#fn:r106|106]]</sup> ), ocean–sea ice reanalyses (Chevallier et al., 2017 <sup>[[#fn:r107|107]]</sup> ) and ''in situ'' measurements (Renner et al., 2014 <sup>[[#fn:r108|108]]</sup> ; Haas et al., 2017 <sup>[[#fn:r109|109]]</sup> ) ( ''very high confidence'' ). Data from multiple satellite altimeter missions show declines in Arctic Basin ice thickness from 2000 to 2012 of –0.58 ± 0.07 m per decade (Lindsay and Schweiger, 2015 <sup>[[#fn:r110|110]]</sup> ). Integration of data from submarines, moorings, and earlier satellite radar altimeter missions shows ice thickness declined across the central Arctic by 65%, from 3.59 to 1.25 m between 1975 and 2012 (Lindsay and Schweiger, 2015 <sup>[[#fn:r111|111]]</sup> ). There is emerging evidence that this sea ice volume loss may be unprecedented over the past century (Schweiger et al., 2019 <sup>[[#fn:r112|112]]</sup> ). New estimates of ice thickness are available for the marginal seas (up to a maximum thickness of ~1 metre) from low-frequency satellite passive microwave measurements (Kaleschke et al., 2016 <sup>[[#fn:r113|113]]</sup> ; Ricker et al., 2017 <sup>[[#fn:r114|114]]</sup> ) but data are only available since 2010. The shift to thinner seasonal sea ice contributes to further ice extent reductions through enhanced summer season melt via increased energy absorption (Nicolaus et al., 2012 <sup>[[#fn:r115|115]]</sup> ), and it is vulnerable to fragmentation from the passage of intense Arctic cyclones in summer and increased ocean swell conditions (Zhang et al., 2013 <sup>[[#fn:r116|116]]</sup> ; Thomson and Rogers, 2014 <sup>[[#fn:r117|117]]</sup> ).

Surface observations of Antarctic sea ice thickness are extremely sparse (Worby et al., 2008 <sup>[[#fn:r118|118]]</sup> ). There are no consistent long-term observations from which trends in ice volume may be derived. Calibrated model simulations suggest that ice thickness trends closely follow those of ice concentration (Massonnet et al., 2013 <sup>[[#fn:r119|119]]</sup> ; Holland et al., 2014 <sup>[[#fn:r120|120]]</sup> ) ( ''medium confidence'' ). Satellite altimeter datasets of Antarctic sea ice thickness are emerging (Paul et al., 2018 <sup>[[#fn:r121|121]]</sup> ) but definitive trends are not yet available.

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