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

==== 9.3.1.2 Arctic Sea Ice Volume and Thickness ====

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The SROCC assessed with ''very high confidence'' that Arctic sea ice has become thinner over the satellite period from 1979 onwards, and this assessment is confirmed for the updated time series ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.1.1|Section 2.3.2.1.1]] ). Sea ice area has also decreased substantially over this period ( [[#9.3.1.1|Section 9.3.1.1]] ), leading to the assessment that Arctic sea ice volume has also decreased with ''very high confidence'' over the satellite period since 1979. There is, however, only ''low confidence'' in quantitative estimates of the sea ice volume loss over this period because of a lack of reliable, long-term, pan-Arctic observations and substantial spread in available reanalyses ( [[#Chevallier--2017|Chevallier et al., 2017]] ). Current best estimates from reanalyses suggest a reduction of September Arctic sea ice volume of 55 to 65% over the period 1979–2010, and of about 72% over the period 1979–2016, with the latter deemed a conservative estimate ( [[#Schweiger--2019|Schweiger et al., 2019]] ).

For the more recent past, ice thickness can be directly estimated from satellite retrievals of sea ice freeboard ( [[#Kwok--2015|Kwok and Cunningham, 2015]] ; [[#Kwok--2018|Kwok, 2018]] ). Based on these retrievals, there is ''medium confidence'' that Arctic sea ice volume has decreased since 2003. There is ''low confidence'' in the amount of decrease over this period and over the CryoSat-2 period from 2011 onwards, primarily because of snow-induced uncertainties in the retrieval algorithms, the shortness of the record, and the small identified trend (e.g., [[#Bunzel--2018|Bunzel et al., 2018]] ; [[#Petty--2018|Petty et al., 2018]] , 2020).

Observations of regional changes in sea ice thickness vary in quality. Analysis of submarine data in the central Arctic Ocean suggests that its sea ice has thinned by about 75 cm compared to the mid-1970s ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.1.1|Section 2.3.2.1.1]] ). For smaller regions, data are too sparse to allow for quantitative estimates of long-term trends ( [[#King--2017|King et al., 2017]] ; [[#Rösel--2018|Rösel et al., 2018]] ), but a clear thinning signal over 10 to 20 years has been found for sea ice in the Fram Strait ( [[#Spreen--2020|Spreen et al., 2020]] ), north of Canada ( [[#Haas--2017|Haas et al., 2017]] ) and for landfast ice in the Kongsfjorden/Svalbard Arctic border ( [[#Pavlova--2019|Pavlova et al., 2019]] ). The CMIP5 models and reanalyses fail to capture the observed distribution ( [[#Stroeve--2014|Stroeve et al., 2014]] ; [[#Shu--2015|Shu et al., 2015]] ) and evolution ( [[#Chevallier--2017|Chevallier et al., 2017]] ) of Arctic sea ice thickness. Most CMIP6 models do not capture the observed spatial distribution of sea ice thickness realistically ( [[#Wei--2020|Wei et al., 2020]] ). This leads to ''low confidence'' in estimates of thickness from reanalyses and from CMIP5 and CMIP6 models and in their projections of sea ice volume.

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