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

==== 3.2.2.1 Sea Ice ====

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The multi-model ensemble of historical simulations from CMIP5 models identify declines in total Arctic sea ice extent and thickness (Sections 3.2.1.1.1; 3.2.1.1.2; Figure 3.3) which agree with observations (Massonnet et al., 2012 <sup>[[#fn:r372|372]]</sup> ; Stroeve et al., 2012a <sup>[[#fn:r373|373]]</sup> ; Stroeve et al., 2014a <sup>[[#fn:r374|374]]</sup> ; Stroeve and Notz, 2015 <sup>[[#fn:r375|375]]</sup> ). There is a range in the ability of individual models to simulate observed sea ice thickness spatial patterns and sea ice drift rates (Jahn et al., 2012 <sup>[[#fn:r376|376]]</sup> ; Stroeve et al., 2014a <sup>[[#fn:r377|377]]</sup> ; Tandon et al., 2018 <sup>[[#fn:r378|378]]</sup> ). Reductions in Arctic sea ice extent scale linearly with both global temperatures and cumulative CO 2 emissions in simulations and observations (Notz and Stroeve, 2016 <sup>[[#fn:r379|379]]</sup> ), although aerosols influenced historical sea ice trends (Gagné et al., 2017 <sup>[[#fn:r380|380]]</sup> ). The uncertainty in sea ice sensitivity (ice extent loss per unit of warming) is quite large (Niederdrenk and Notz, 2018 <sup>[[#fn:r381|381]]</sup> ) and the model sensitivity is too low in most CMIP5 models (Rosenblum and Eisenman, 2017 <sup>[[#fn:r382|382]]</sup> ). Emerging evidence suggests, however, that internal variability, including links between the Arctic and lower latitude, strongly influences the ability of models to simulate observed reductions in Arctic sea ice extent (Swart et al., 2015b <sup>[[#fn:r383|383]]</sup> ; Ding et al., 2018 <sup>[[#fn:r384|384]]</sup> ).

CMIP5 models project continued declines in Arctic sea ice through the end of the century (Figure 3.3) (Notz and Stroeve, 2016 <sup>[[#fn:r385|385]]</sup> ) ( ''high confidence'' ). There is a large spread in the timing of when the Arctic may become ice free in the summer, and for how long during the season (Massonnet et al., 2012 <sup>[[#fn:r386|386]]</sup> ; Stroeve et al., 2012a <sup>[[#fn:r387|387]]</sup> ; Overland and Wang, 2013 <sup>[[#fn:r388|388]]</sup> ) as a result of natural climate variability (Notz, 2015 <sup>[[#fn:r389|389]]</sup> ; Swart et al., 2015b <sup>[[#fn:r390|390]]</sup> ; Screen and Deser, 2019 <sup>[[#fn:r391|391]]</sup> ), scenario uncertainty (Stroeve et al., 2012a <sup>[[#fn:r392|392]]</sup> ; Liu et al., 2013 <sup>[[#fn:r393|393]]</sup> ), and model uncertainties related to sea ice dynamics (Rampal et al., 2011 <sup>[[#fn:r394|394]]</sup> ; Tandon et al., 2018 <sup>[[#fn:r395|395]]</sup> ) and thermodynamics (Massonnet et al., 2018 <sup>[[#fn:r396|396]]</sup> ). Internal climate variability results in an uncertainty of approximately 20 years in the timing of seasonally ice-free conditions (Notz, 2015 <sup>[[#fn:r397|397]]</sup> ; Jahn, 2018 <sup>[[#fn:r398|398]]</sup> ), but the clear link between summer sea ice extent and cumulative CO 2 emissions provides a basis for when consistent ice-free conditions may be expected ( ''high confidence'' ). For stabilised global warming of 1.5°C, sea ice in September is ''likely'' to be present at end of century with an approximately 1% chance of individual ice-free years (Notz and Stroeve, 2016 <sup>[[#fn:r399|399]]</sup> ; Sanderson et al., 2017 <sup>[[#fn:r400|400]]</sup> ; Jahn, 2018 <sup>[[#fn:r401|401]]</sup> ; Sigmond et al., 2018 <sup>[[#fn:r402|402]]</sup> ); after 10 years of stabilised warming at a 2°C increase, more frequent occurrence of an ice-free summer Arctic is expected (around 10-35%) (Mahlstein and Knutti, 2012 <sup>[[#fn:r403|403]]</sup> ; Jahn et al., 2016 <sup>[[#fn:r404|404]]</sup> ; Notz and Stroeve, 2016 <sup>[[#fn:r405|405]]</sup> ). Model simulations show that a temporary temperature overshoot of a warming target has no lasting impact on ice cover (Armour et al., 2011 <sup>[[#fn:r406|406]]</sup> ; Ridley et al., 2012 <sup>[[#fn:r407|407]]</sup> ; Li et al., 2013 <sup>[[#fn:r408|408]]</sup> ).

CMIP5 models show a wide range of mean states and trends in Antarctic sea ice (Turner et al., 2013 <sup>[[#fn:r409|409]]</sup> ; Shu et al., 2015 <sup>[[#fn:r410|410]]</sup> ). The ensemble mean across multiple models show a decrease in total Antarctic sea ice extent during the satellite era, in contrast to the lack of any observed trend (Figure 3.3; Section 3.2.1.1.1). Interannual sea ice variability in the models is larger than observations (Zunz et al., 2013 <sup>[[#fn:r411|411]]</sup> ), which may mask disparity between models and observations. Internal variability (Polvani and Smith, 2013 <sup>[[#fn:r412|412]]</sup> ; Zunz et al., 2013 <sup>[[#fn:r413|413]]</sup> ), and model sensitivity to warming (Rosenblum and Eisenman, 2017 <sup>[[#fn:r414|414]]</sup> ) are also important sources of uncertainty. During the historical period, regional trends of Antarctic sea ice are not captured by the models, particularly the decrease in the Bellingshausen Sea and the expansion in the Ross Sea (Hobbs et al., 2015 <sup>[[#fn:r415|415]]</sup> ). There is a very wide spread of model responses in the Weddell Sea (Hobbs et al., 2015 <sup>[[#fn:r416|416]]</sup> ; Ivanova et al., 2016 <sup>[[#fn:r417|417]]</sup> ), a region with complex ocean-sea ice interactions that many models do not replicate (de Lavergne et al., 2014).

There is ''low confidence'' in projections of Antarctic sea ice because there are multiple anthropogenic forcings (ozone and greenhouse gases) and complicated processes involving the ocean, atmosphere, and adjacent ice sheet (Section 3.2.1.1.). Model deficiencies are related to stratification (Sallée et al., 2013a <sup>[[#fn:r418|418]]</sup> ), freshening by ice shelf melt water (Bintanja et al., 2015 <sup>[[#fn:r419|419]]</sup> ), atmospheric processes including clouds (Schneider and Reusch, 2015 <sup>[[#fn:r420|420]]</sup> ; Hyder et al., 2018 <sup>[[#fn:r421|421]]</sup> ), and wind and ocean driven processes (Purich et al., 2016a <sup>[[#fn:r422|422]]</sup> ; Purich et al., 2016b <sup>[[#fn:r423|423]]</sup> ; Schroeter et al., 2017 <sup>[[#fn:r424|424]]</sup> ; Purich et al., 2018 <sup>[[#fn:r425|425]]</sup> ; Zhang et al., 2018a <sup>[[#fn:r426|426]]</sup> ). Uncertainty in sea ice projections reduces confidence in projections of Antarctic Ice Sheet surface mass balance because sea ice affects Antarctic temperature and precipitation trends (Bracegirdle et al., 2015 <sup>[[#fn:r427|427]]</sup> ), and impacts projected changes in the Southern Hemisphere westerly jet (Bracegirdle et al., 2018 <sup>[[#fn:r428|428]]</sup> ; England et al., 2018 <sup>[[#fn:r429|429]]</sup> ) with implications for the Southern Ocean overturning circulation (Cross-Chapter Box 7 in Chapter 3).

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