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

=== 3.2.1 Observed Changes in Sea Ice and Ocean ===

<div id="section-3-2-1-1-sea-ice"></div>

<span id="sea-ice"></span>
==== 3.2.1.1 Sea Ice ====

<div id="section-3-2-1-1-sea-ice-block-1"></div>

Sea ice reflects a high proportion of incoming solar radiation back to space, provides thermal insulation between the ocean and atmosphere, influences thermohaline circulation, and provides habitat for ice-associated species. Sea ice characteristics differ between the Arctic and Antarctic. Expansion of winter sea ice in the Arctic is limited by land, and ice circulates within the central Arctic basin, some of which survives the summer melt season to form multi-year ice. Arctic sea ice variability and impacts on communities includes indigenous knowledge and local knowledge from across the circumpolar Arctic (Cross-Chapter Box 3 in Chapter 1). The Antarctic continent is surrounded by sea ice which interacts with adjacent ice shelves; winter season expansion is limited by the influence of the Antarctic Circumpolar Current (ACC).

<div id="section-3-2-1-1-sea-ice-block-2"></div>

<span id="extent-and-concentration"></span>
===== 3.2.1.1.1 Extent and concentration =====

The pan-Arctic loss of sea ice cover is a prominent indicator of climate change. Sea ice extent (the total area of the Arctic with at least 15% sea ice concentration) has declined since 1979 in each month of the year ( ''very high confidence'' ) (Barber et al., 2017 <sup>[[#fn:r41|41]]</sup> ; Comiso et al., 2017b <sup>[[#fn:r42|42]]</sup> ; Stroeve and Notz, 2018 <sup>[[#fn:r43|43]]</sup> ) (Figure 3.3) ''.'' Changes are largest in summer and smallest in winter, with the strongest trends in September (1979–2018; summer month with the lowest sea ice cover) of –83,000 km <sup>2</sup> yr <sup>–1</sup> (–12.8% per decade ± 2.3% relative to 1981–2010 mean), and –41,000 km <sup>2</sup> yr <sup>–1</sup> (–2.7% per decade ± 0.5% relative to 1981–2010 mean) for March (1979–2019; winter month with the greatest sea ice cover) (Onarheim et al., 2018 <sup>[[#fn:r44|44]]</sup> ). Regionally, summer ice loss is dominated by reductions in the East Siberian Sea (explains 22% of the September trend), and large declines in the Beaufort, Chukchi, Laptev and Kara seas (Onarheim et al., 2018 <sup>[[#fn:r45|45]]</sup> ). Winter ice loss is dominated by reductions within the Barents Sea, responsible for 27% of the pan-Arctic March sea ice trends (Onarheim and Årthun, 2017 <sup>[[#fn:r46|46]]</sup> ). Summer Arctic sea ice loss since 1979 is unprecedented in 150 years based on historical reconstructions (Walsh et al., 2017 <sup>[[#fn:r47|47]]</sup> ) and more than 1000 years based on palaeoclimate evidence (Polyak et al., 2010 <sup>[[#fn:r48|48]]</sup> ; Kinnard et al., 2011 <sup>[[#fn:r49|49]]</sup> ; Halfar et al., 2013 <sup>[[#fn:r50|50]]</sup> ) ( ''medium confidence'' ).

<span id="figure-3.3"></span>
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'''Figure 3.3'''

<span id="maps-of-linear-trends-in-oc-per-decade-of-arctic-a-c-and-antarctic-e-g-sea-surface-temperature-sst-for-19822017-in-march-a-e-and-september-c-g.-b-d-f-h-same-as-a-c-e-g-but-for-the-linear-trends-of-sea-ice-concentration-in-per-decade.-stippled-regions"></span>
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'''Maps of linear trends (in oC per decade) of Arctic (a, c) and Antarctic (e, g) sea surface temperature (SST) for 1982−2017 in March (a, e) and September (c, g). (b, d, f, h) same as (a, c, e, g), but for the linear trends of sea ice concentration (in % per decade). Stippled regions […]'''
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[[File:142cb65f02b265ad385c70283141521f IPCC-SROCC-CH_3_3.jpg]]

Maps of linear trends (in oC per decade) of Arctic (a, c) and Antarctic (e, g) sea surface temperature (SST) for 1982−2017 in March (a, e) and September (c, g). (b, d, f, h) same as (a, c, e, g), but for the linear trends of sea ice concentration (in % per decade). Stippled regions indicate the trends that are statistically insignificant. Dashed circles indicate the Arctic/Antarctic Circle. Beneath each map of linear trend shows the time series of SST (area-averaged north of 40oN/south of 40oS) or sea ice extent in the northern/southern hemisphere. Black, green, blue, orange, and red curves indicate observations, Coupled Model Intercomparison Project Phase 5 (CMIP5) historical simulation, Representative Concentration Pathway (RCP)2.6, RCP4.5, and RCP8.5 projections respectively; shading indicates ± standard deviation of multi-models. SST trend was calculated from Hadley Centre Sea Ice and Sea Surface Temperature data set (Version 1, HadISST1; Rayner, 2003). Sea ice concentration trend was calculated from the NOAA/NSIDC Climate Data Record of Passive Microwave Sea Ice Concentration, Version 3 (https://nsidc.org/data/g02202). The time series of observed SST are averages of HadISST1 and NOAA Optimum Interpolation SST dataset (version 2; Reynolds et al., 2002). The time series of observed sea ice extent are the averages of HadISST, the NOAA/NSIDC Climate Data Record of Passive Microwave Sea Ice Concentration, and the Global sea ice concentration reprocessing dataset from EUMETSAT (http://osisaf.met.no/p/ice/ice_conc_reprocessed.html).

Approximately half of the observed Arctic summer sea ice loss is driven by increased concentrations of atmospheric greenhouse gases, with the remainder attributed to internal climate variability (Kay et al., 2011 <sup>[[#fn:r51|51]]</sup> ; Notz and Marotzke, 2012 <sup>[[#fn:r52|52]]</sup> ) ( ''medium confidence'' ). The sea ice albedo feedback (increased air temperature reduces sea ice cover, allowing more energy to be absorbed at the surface, fostering more melt) is a key driver of sea ice loss (Perovich and Polashenski, 2012 <sup>[[#fn:r53|53]]</sup> ; Stroeve et al., 2012b; Serreze et al., 2016 <sup>[[#fn:r54|54]]</sup> ) and is exacerbated by the transition from perennial to seasonal sea ice (Haine and Martin, 2017 <sup>[[#fn:r55|55]]</sup> ; see Section 3.2.1.1.2). Other drivers include increased warm, moist air intrusions into the Arctic during both winter (Box 3.1) and spring (Boisvert et al., 2016 <sup>[[#fn:r56|56]]</sup> ; Cullather et al., 2016 <sup>[[#fn:r57|57]]</sup> ; Kapsch et al., 2016 <sup>[[#fn:r58|58]]</sup> ; Mortin et al., 2016 <sup>[[#fn:r59|59]]</sup> ; Graham et al., 2017 <sup>[[#fn:r60|60]]</sup> ; Hegyi and Taylor, 2018 <sup>[[#fn:r61|61]]</sup> ), radiative feedbacks associated with cloudiness and humidity (Kapsch et al., 2013 <sup>[[#fn:r62|62]]</sup> ; Pithan and Mauritsen, 2014 <sup>[[#fn:r63|63]]</sup> ; Hegyi and Deng, 2016 <sup>[[#fn:r64|64]]</sup> ; Morrison et al., 2018 <sup>[[#fn:r65|65]]</sup> ), and increased exchanges of sensible and latent heat flux from the ocean to the atmosphere (Serreze et al., 2012 <sup>[[#fn:r66|66]]</sup> ; Taylor et al., 2018 <sup>[[#fn:r67|67]]</sup> ). A lack of complete process understanding limits a more definitive differentiation between anthropogenic versus internal drivers of summer Arctic sea ice loss (Serreze et al., 2016 <sup>[[#fn:r68|68]]</sup> ; Ding et al., 2017 <sup>[[#fn:r69|69]]</sup> ; Meehl et al., 2018 <sup>[[#fn:r70|70]]</sup> ). The unabated reduction in Arctic summer sea ice since AR5 means contributions to additional global radiative forcing (Flanner et al., 2011 <sup>[[#fn:r71|71]]</sup> ) have continued, with estimates of up to an additional 6.4 ± 0.9 W/m <sup>2</sup> of solar energy input to the Arctic Ocean region since 1979 (Pistone et al., 2014 <sup>[[#fn:r72|72]]</sup> ).

Although Arctic ice freeze-up is occurring later (Section 3.2.1.1.3), rapid thermodynamic ice growth occurs over thin ice areas after air temperatures drop below freezing in autumn. Later freeze-up also delays snowfall accumulation on sea ice, leading to a thinner and less insulating snowpack (Section 3.2.1.1.6) (Sturm and Massom, 2016 <sup>[[#fn:r73|73]]</sup> ). These two negative feedbacks help to mitigate sudden and irreversible loss of Arctic sea ice (Armour et al., 2011 <sup>[[#fn:r74|74]]</sup> ).

Total Antarctic sea ice cover exhibits no significant trend over the period of satellite observations (Figure 3.3; 1979–2018) ( ''high confidence'' ) (Ludescher et al., 2018 <sup>[[#fn:r75|75]]</sup> ). A significant positive trend in mean annual ice cover between 1979 and 2015 (Comiso et al., 2017a <sup>[[#fn:r76|76]]</sup> ) has not persisted, due to three consecutive years of below average ice cover (2016–2018) driven by atmospheric and oceanic forcing (Turner et al., 2017b <sup>[[#fn:r77|77]]</sup> ; Kusahara et al., 2018 <sup>[[#fn:r78|78]]</sup> ; Meehl et al., 2019 <sup>[[#fn:r79|79]]</sup> ; Wang et al., 2019 <sup>[[#fn:r80|80]]</sup> ). The overall Antarctic sea ice extent trend is composed of near-compensating regional changes, with rapid ice loss in the Amundsen and Bellingshausen seas counteracted by rapid ice gain in the Weddell and Ross seas (Holland, 2014 <sup>[[#fn:r81|81]]</sup> ) (Figure 3.3). These regional trends are strongly seasonal in character (Holland, 2014 <sup>[[#fn:r82|82]]</sup> ); only the western Ross Sea has a trend that is statistically significant in all seasons, relative to the variance during the period of satellite observations.

Multiple factors contribute to the regionally variable nature of Antarctic sea ice extent trends (Matear et al., 2015 <sup>[[#fn:r83|83]]</sup> ; Hobbs et al., 2016b <sup>[[#fn:r84|84]]</sup> ). Sea ice trends are closely related to meridional wind trends ( ''high confidence'' ) (Holland and Kwok, 2012 <sup>[[#fn:r85|85]]</sup> ; Haumann et al., 2014 <sup>[[#fn:r86|86]]</sup> ): poleward wind trends in the Bellingshausen Sea push sea ice closer to the coast (Holland and Kwok, 2012 <sup>[[#fn:r87|87]]</sup> ) and advect warm air to the sea ice zone (Kusahara et al., 2017 <sup>[[#fn:r88|88]]</sup> ), and the reverse is true over much of the Ross Sea. These meridional wind trends are linked to Pacific variability (Coggins and McDonald, 2015 <sup>[[#fn:r89|89]]</sup> ; Meehl et al., 2016 <sup>[[#fn:r90|90]]</sup> ; Purich et al., 2016b <sup>[[#fn:r91|91]]</sup> ). Ozone depletion may also affect meridional winds (Fogt and Zbacnik, 2014 <sup>[[#fn:r92|92]]</sup> ; England et al., 2016 <sup>[[#fn:r93|93]]</sup> ), but there is ''low confidence'' that this explains observed sea ice trends (Landrum et al., 2017 <sup>[[#fn:r94|94]]</sup> ).

Coupled climate models indicate that anthropogenic warming at the surface is delayed by the Southern Ocean circulation, which transports heat downwards into the deep ocean (Armour et al., 2016 <sup>[[#fn:r95|95]]</sup> ). This overturning circulation (Cross-Chapter Box 7 in Chapter 3), along with differing cloud and lapse rate feedbacks (Goosse et al., 2018 <sup>[[#fn:r96|96]]</sup> ), may explain the weak response of Antarctic sea ice cover to increased atmospheric greenhouse gas concentrations compared to the Arctic ( ''medium confidence'' ). Because Antarctic sea ice extent has remained below climatological values since 2016, there is still potential for longer-term changes to emerge in the Antarctic (Meehl et al., 2019 <sup>[[#fn:r97|97]]</sup> ), similar to the Arctic.

Historical surface observations (Murphy et al., 2014 <sup>[[#fn:r98|98]]</sup> ), reconstructions (Abram et al., 2013b <sup>[[#fn:r99|99]]</sup> ), ship records (de la Mare, 2009; Edinburgh and Day, 2016 <sup>[[#fn:r100|100]]</sup> ), early satellite images (Gallaher et al., 2014 <sup>[[#fn:r101|101]]</sup> ), and model simulations (Gagné et al., 2015 <sup>[[#fn:r102|102]]</sup> ) indicate a decrease in overall Antarctic sea ice cover since the early 1960s which is too modest to be separated from natural variability (Hobbs et al., 2016a <sup>[[#fn:r103|103]]</sup> ) ( ''high confidence'' ).

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<div id="section-3-2-1-1-sea-ice-block-3"></div>

<span id="errata-figure-3.3"></span>
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'''Errata Figure 3.3'''

<span id="maps-of-linear-trends-in-oc-per-decade-of-arctic-a-c-and-antarctic-e-g-sea-surface-temperature-sst-for-19822017-in-march-a-e-and-september-c-g.-b-d-f-h-same-as-a-c-e-g-but-for-the-linear-trends-of-sea-ice-concentration-in-per-decade.-stippled-regions-1"></span>
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'''Maps of linear trends (in oC per decade) of Arctic (a, c) and Antarctic (e, g) sea surface temperature (SST) for 1982−2017 in March (a, e) and September (c, g). (b, d, f, h) same as (a, c, e, g), but for the linear trends of sea ice concentration (in % per decade). Stippled regions […]'''
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[[File:1eb311662e7f1e4f811ad2cb8b9fc956 Figure_Chapter_3_3_errata-3000x2397.jpg]]

Maps of linear trends (in oC per decade) of Arctic (a, c) and Antarctic (e, g) sea surface temperature (SST) for 1982−2017 in March (a, e) and September (c, g). (b, d, f, h) same as (a, c, e, g), but for the linear trends of sea ice concentration (in % per decade). Stippled regions indicate the trends that are statistically insignificant. Dashed circles indicate the Arctic/Antarctic Circle. Beneath each map of linear trend shows the time series of SST (area-averaged north of 40oN/south of 40oS) or sea ice extent in the northern/southern hemisphere. Black, green, blue, orange, and red curves indicate observations, Coupled Model Intercomparison Project Phase 5 (CMIP5) historical simulation, Representative Concentration Pathway (RCP)2.6, RCP4.5, and RCP8.5 projections respectively; shading indicates ± standard deviation of multi-models. SST trend was calculated from Hadley Centre Sea Ice and Sea Surface Temperature data set (Version 1, HadISST1; Rayner, 2003). Sea ice concentration trend was calculated from the NOAA/NSIDC Climate Data Record of Passive Microwave Sea Ice Concentration, Version 3 (https://nsidc.org/data/g02202). The time series of observed SST are averages of HadISST1 and NOAA Optimum Interpolation SST dataset (version 2; Reynolds et al., 2002). The time series of observed sea ice extent are the averages of HadISST, the NOAA/NSIDC Climate Data Record of Passive Microwave Sea Ice Concentration, and the Global sea ice concentration reprocessing dataset from EUMETSAT (http://osisaf.met.no/p/ice/ice_conc_reprocessed.html).

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<div id="section-3-2-1-1-sea-ice-block-4"></div>

<span id="age-and-thickness"></span>
===== 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.

<div id="section-3-2-1-1-sea-ice-block-5"></div>

<span id="seasonality"></span>
===== 3.2.1.1.3 Seasonality =====

There is ''high confidence'' that the Arctic sea ice melt season has extended by 3 days per decade since 1979 due earlier melt onset, and 7 days per decade due to later freeze-up (Stroeve and Notz, 2018 <sup>[[#fn:r122|122]]</sup> ). This longer melt season is consistent with the observed loss of sea ice extent and thickness (Sections 3.2.1.1.1; 3.2.1.1.2). While the melt onset trends are smaller, they play a large role in the earlier development of open water (Stroeve et al., 2012b <sup>[[#fn:r123|123]]</sup> ; Serreze et al., 2016 <sup>[[#fn:r124|124]]</sup> ) and melt pond development (Perovich and Polashenski, 2012 <sup>[[#fn:r125|125]]</sup> ) which enhance the sea ice albedo feedback (Stroeve et al., 2014b <sup>[[#fn:r126|126]]</sup> ; Liu et al., 2015a <sup>[[#fn:r127|127]]</sup> ). Observed reductions in the duration of seasonal sea ice cover are reflected in community-based observations of decreased length of time in which activities can safely take place on sea ice (Laidler et al., 2010 <sup>[[#fn:r128|128]]</sup> ; Eisner et al., 2013 <sup>[[#fn:r129|129]]</sup> ; Fall et al., 2013 <sup>[[#fn:r130|130]]</sup> ; Ignatowski and Rosales, 2013 <sup>[[#fn:r131|131]]</sup> ).

Changes in the duration of Antarctic sea ice cover over 1979–2011 largely followed the spatial pattern of sea ice extent trends with reduced ice cover duration in the Amundsen/Bellingshausen Sea region in summer and autumn owing to earlier retreat and later advance, and increases in the Ross Sea due to later ice retreat and earlier advance (Stammerjohn et al., 2012 <sup>[[#fn:r132|132]]</sup> ).

<div id="section-3-2-1-1-sea-ice-block-6"></div>

<span id="motion"></span>
===== 3.2.1.1.4 Motion =====

Winds associated with the climatological Arctic sea level pressure pattern drive the Beaufort Gyre (Dewey et al., 2018 <sup>[[#fn:r133|133]]</sup> ; Meneghello et al., 2018 <sup>[[#fn:r134|134]]</sup> ) and the Transpolar Drift Stream (Vihma et al., 2012 <sup>[[#fn:r135|135]]</sup> ), which retains sea ice within the central Arctic Basin, and exports sea ice out of the Fram Strait, respectively. There is ''high confidence'' that sea ice drift speeds have increased since 1979, both within the Arctic Basin and through Fram Strait (Rampal et al., 2009 <sup>[[#fn:r136|136]]</sup> ; Krumpen et al., 2019 <sup>[[#fn:r137|137]]</sup> ), attributed to thinner ice (Spreen et al., 2011 <sup>[[#fn:r138|138]]</sup> ) and changes in wind forcing (Olason and Notz, 2014 <sup>[[#fn:r139|139]]</sup> ). Fram Strait sea ice area export estimates range between 600,000 to 1 million km 2 of ice annually, which represents approximately 10% of the ice within the Arctic Basin ( ''medium confidence'' ) (Kwok et al., 2013 <sup>[[#fn:r140|140]]</sup> ; Krumpen et al., 2016 <sup>[[#fn:r141|141]]</sup> ; Smedsrud et al., 2017 <sup>[[#fn:r142|142]]</sup> ; Zamani et al., 2019 <sup>[[#fn:r143|143]]</sup> ). Sea ice volume flux estimates through Fram Strait are now available from satellite altimeter datasets (Ricker et al., 2018 <sup>[[#fn:r144|144]]</sup> ), but they cover too short a time period for robust trend analysis. Observations of extreme Arctic sea ice deformation is attributed to the combination of decreased ice thickness and increased ice motion (Itkin et al., 2017 <sup>[[#fn:r145|145]]</sup> ).

Satellite estimates of sea ice drift velocity show significant trends in Antarctic ice drift (Holland and Kwok, 2012 <sup>[[#fn:r146|146]]</sup> ). Increased northward drift in the Ross Sea and decreased northward drift in the Bellingshausen and Weddell seas agree with the respective ice extent gains and losses in these regions, but there is only ''medium confidence'' in these trends due to a small number of ice drift data products derived from temporally inconsistent satellite records (Haumann et al., 2016 <sup>[[#fn:r147|147]]</sup> ).

<div id="section-3-2-1-1-sea-ice-block-7"></div>

<span id="landfast-ice"></span>
===== 3.2.1.1.5 Landfast ice =====

Immobile sea ice anchored to land or ice shelves is referred to as ‘landfast’. The few long term surface (auger hole) records of Arctic landfast sea ice thickness all exhibit thinning trends in springtime maximum sea ice thickness since the mid-1960s ( ''high confidence'' ): declines of 11 cm per decade in the Barents Sea (Gerland et al., 2008 <sup>[[#fn:r148|148]]</sup> ), 3.3 cm per decade along the Siberian Coast (Polyakov et al., 2010 <sup>[[#fn:r149|149]]</sup> ), and 3.5 cm per decade in the Canadian Arctic Archipelago (Howell et al., 2016 <sup>[[#fn:r150|150]]</sup> ). Over a shorter 1976–2007 period, winter season landfast sea ice extent from measurements across the Arctic significantly decreased at a rate of 7% per decade, with the largest decreases in the regions of Svalbard (24% per decade) and the northern coast of the Canadian Arctic Archipelago (20% per decade) (Yu et al., 2013 <sup>[[#fn:r151|151]]</sup> ). Svalbard and the Chukchi Sea regions are experiencing the largest declines in landfast sea ice duration (~1 week per decade) since the 1970s (Yu et al., 2013 <sup>[[#fn:r152|152]]</sup> ; Mahoney et al., 2014 <sup>[[#fn:r153|153]]</sup> ). While most Arctic landfast sea ice melts completely each summer, perennial landfast ice (also termed an ‘ice-plug’) occurs in Nansen Sound and the Sverdrup Channel in the Canadian Arctic Archipelago. These ice-plugs were in place continuously from the start of observations in the early 1960s, until they disappeared during the anomalously warm summer of 1998, and they have rarely re-formed since 2005 (Pope et al., 2017 <sup>[[#fn:r154|154]]</sup> ). The loss of this perennial sea ice is associated with reduced landfast ice duration in the northern Canadian Arctic Archipelago (Galley et al., 2012 <sup>[[#fn:r155|155]]</sup> ; Yu et al., 2013 <sup>[[#fn:r156|156]]</sup> ) and increased inflow of multi-year ice from the Arctic Ocean into the northern Canadian Arctic Archipelago (Howell et al., 2013 <sup>[[#fn:r157|157]]</sup> ).

Arctic landfast ice is important to northern residents as a platform for travel, hunting, and access to offshore regions (Sections 3.4.3.3, 3.5.2.2). Reports of thinning, less stable, and less predictable landfast ice have been documented by residents of coastal communities in Alaska (Eisner et al., 2013 <sup>[[#fn:r158|158]]</sup> ; Fall et al., 2013 <sup>[[#fn:r159|159]]</sup> ; Huntington et al., 2017 <sup>[[#fn:r160|160]]</sup> ), the Canadian Arctic (Laidler et al., 2010 <sup>[[#fn:r161|161]]</sup> ), and Chukotka (Inuit Circumpolar Council, 2014). The impact of changing prevailing wind forcing on local ice conditions has been specifically noted (Rosales and Chapman, 2015 <sup>[[#fn:r163|163]]</sup> ) including impacts on the landfast ice edge and polynyas (Box 3.3) (Gearheard et al., 2013 <sup>[[#fn:r164|164]]</sup> ). Long-term records of Antarctic landfast ice are limited in space and time (Stammerjohn and Maksym, 2016 <sup>[[#fn:r165|165]]</sup> ), with a high degree of regional variability in trends (Fraser et al., 2011 <sup>[[#fn:r166|166]]</sup> ) ( ''low confidence'' ).

<div id="section-3-2-1-1-sea-ice-block-8"></div>

<span id="snow-on-ice"></span>
===== 3.2.1.1.6 Snow on ice =====

Snow accumulation on sea ice inhibits sea ice melt through a high albedo, but the insulating properties limit sea ice growth (Sturm and Massom, 2016 <sup>[[#fn:r167|167]]</sup> ) and inhibits photosynthetic light (important for in- and under-ice biota) from reaching the bottom of the ice (Mundy et al., 2007 <sup>[[#fn:r168|168]]</sup> ). If snow on first-year ice is sufficiently thick, it can depress the ice below the sea level surface, which forms snow-ice due to surface flooding. This process is widespread in the Antarctic (Maksym and Markus, 2008 <sup>[[#fn:r169|169]]</sup> ) and the Atlantic Sector of the Arctic (Merkouriadi et al., 2017 <sup>[[#fn:r170|170]]</sup> ), and may become more common across the Arctic (with implications for sea ice ecosystems) as the ice regime shifts to thinner seasonal ice (Olsen et al., 2017 <sup>[[#fn:r171|171]]</sup> ; Granskog et al., 2018 <sup>[[#fn:r172|172]]</sup> ) ( ''medium confidence'' ).

Despite the importance of snow on sea ice (Webster et al., 2018 <sup>[[#fn:r173|173]]</sup> ), surface or satellite derived observations of snowfall over sea ice, and snow depth on sea ice are lacking (Webster et al., 2014 <sup>[[#fn:r174|174]]</sup> ). The primary source of snow depth on Arctic sea ice are based on observations collected decades ago (Warren et al., 1999 <sup>[[#fn:r175|175]]</sup> ) the utility of which are impacted by the rapid loss of multi-year ice across the central Arctic (Stroeve and Notz, 2018 <sup>[[#fn:r176|176]]</sup> ), and large interannual variability in snow depth on sea ice (Webster et al., 2014 <sup>[[#fn:r177|177]]</sup> ). Airborne radar retrievals of snow depth on sea ice provide more recent estimates, but spatial and temporal sampling is highly discontinuous (Kurtz and Farrell, 2011 <sup>[[#fn:r178|178]]</sup> ). Multi-source time series provide evidence of declining snow depth on Arctic sea ice (Webster et al., 2014 <sup>[[#fn:r179|179]]</sup> ) consistent with estimates of higher fractions of liquid precipitation since 2000 (Boisvert et al., 2018 <sup>[[#fn:r180|180]]</sup> ) but there is ''low confidence'' because surface measurements for validation are extremely limited and suggest a high degree of regional variability (Haas et al., 2017 <sup>[[#fn:r181|181]]</sup> ; Rösel et al., 2018 <sup>[[#fn:r182|182]]</sup> ).

Although there are regional estimates of snow depth on Antarctic sea ice from satellite (Kern and Ozsoy-Çiçek, 2016 <sup>[[#fn:r183|183]]</sup> ), airborne remote sensing (Kwok and Maksym, 2014 <sup>[[#fn:r184|184]]</sup> ), field measurements (Massom et al., 2001 <sup>[[#fn:r185|185]]</sup> ) and ship-based observations (Worby et al., 2008 <sup>[[#fn:r186|186]]</sup> ), data are not sufficient in time nor space to assess changes in snow accumulation on Antarctic sea ice.

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