Editing IPCC:AR6/WGI/Chapter-2 (section)

==== 2.3.2.1 Sea Ice Coverage and Thickness ====

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===== 2.3.2.1.1 Arctic sea ice =====

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The AR5 reported that the annual mean Arctic sea-ice extent (SIE) ''very likely'' decreased by 3.5–4.1% per decade between 1979 and 2012 with the summer sea-ice minimum (perennial sea ice) ''very likely'' decreasing by 9.4–13.6% per decade. This was confirmed by SROCC reporting the strongest reductions in September (12.8 ± 2.3% per decade; 1979–2018) and stating that these changes were ''likely'' unprecedented in at least 1 kyr ( ''medium confidence'' ). The spatial extent had decreased in all seasons, with the largest decrease for September ( ''high confidence'' ). The AR5 reported also that the average winter sea ice thickness within the Arctic Basin had ''likely'' decreased by between 1.3 m and 2.3 m from 1980 to 2008 ( ''high confidence'' ), consistent with the decline in multi-year and perennial ice extent. The SROCC stated further that it was ''virtually certain'' that Arctic sea ice had thinned, concurrent with a shift to younger ice. Lower sea ice volume in 2010–2012 compared to 2003–2008 was documented in AR5 ( ''medium confidence'' ). There was ''high confidence'' that, where the sea ice thickness had decreased, the sea-ice drift speed had increased.

Proxy records are used in combination with modelling to assess Arctic paleo sea ice conditions to the extent possible. For the Pliocene, ''limited'' proxy ''evidence'' of a reduced sea ice cover compared to ‘modern’ winter conditions ( [[#Knies--2014|Knies et al., 2014]] ; [[#Clotten--2018|Clotten et al., 2018]] ) and model simulations of a largely ice-free Arctic Ocean during summer ( [[#Howell--2016|Howell et al., 2016]] ; [[#Feng--2019|Feng et al., 2019]] ; F. [[#Li--2020|]] [[#Li--2020|Li et al., 2020]] ) imply ''medium confidence'' that the Arctic Ocean was seasonally ice covered. Over the LIG, sparse proxy reconstructions ( [[#Stein--2017|Stein et al., 2017]] ; [[#Kremer--2018|Kremer et al., 2018]] ) and proxy evidence from marine sediments ( [[#Kageyama--2021b|Kageyama et al., 2021b]] ) provide ''medium confidence'' of perennial sea ice cover.

Over the past 13 kyr proxy records suggest extensive sea-ice coverage during the Younger Dryas (at the end of the LDT), followed by a decrease in sea ice coverage during the Early Holocene, and increasing sea-ice coverage from the MH to the mid-15th century ( [[#De%20Vernal--2013|De Vernal et al., 2013]] ; [[#Belt--2015|Belt et al., 2015]] ; [[#Cabedo-Sanz--2016|Cabedo-Sanz et al., 2016]] ; [[#Armand--2017|Armand et al., 2017]] ; [[#Belt--2018|Belt, 2018]] ). There is ''limited evidence'' that the Canadian Arctic had less multiyear sea ice during the Early Holocene than today ( [[#Spolaor--2016|Spolaor et al., 2016]] ). For more regional details on paleo arctic sea ice see Section 9.3.1.1.

Pan-Arctic SIE conditions (annual means and late summer) during the last decade were unprecedented since at least 1850 (Figure 2.20a; [[#Walsh--2017|Walsh et al., 2017]] , 2019; [[#Brennan--2020|Brennan et al., 2020]] ), while, as reported in SROCC, there remains ''medium confidence'' that the September (late summer) Arctic sea ice loss during the last decade was unprecedented during the past 1 kyr. Sea-ice charts since 1850 ( [[#Walsh--2017|Walsh et al., 2017]] , 2019) suggest that there was no significant trend before the 1990s, but the uncertainty of these estimates is large and could mask a trend, a possibility illustrated by [[#Brennan--2020|Brennan et al. (2020)]] , who found a loss of Arctic sea ice between 1910 and 1940 in an estimate based on a data assimilation approach.

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[[File:4236ee7cfe6bd3c131388169f24b7c4e IPCC_AR6_WGI_Figure_2_20.png]]

'''Figure 2.2''' '''0 |''' '''Changes in Arctic and Antarctic sea ice area. (a)''' Three time series of Arctic sea-ice area (SIA) for March and September from 1979 to 2020 (passive microwave satellite era). In addition, the range of SIA from 1850–1978 is indicated by the vertical bar to the left. '''(b)''' Three time series of Antarctic sea ice area for September and February (1979–2020). In both (a) and (b), decadal means for the three series for the first and most recent decades of observations are shown by horizontal lines in grey (1979–1988) and black (2010–2019). SIA values have been calculated from sea ice concentration fields. Available data for 2020 (OSISAF) is shown in both (a) and (b). Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

There has been a continuing decline in SIE and Arctic sea ice area (SIA) in recent years (Figure 2.20a). To reduce grid-geometry associated biases and uncertainties ( [[#Notz--2014|Notz, 2014]] ; [[#Ivanova--2016|Ivanova et al., 2016]] ; [[#Meier--2019|Meier and Stewart, 2019]] ) SIA is used in addition to, or instead of SIE herein (see also section 9.3.1). A record-low Arctic SIA since the start of the satellite era (1979) occurred in September 2012 (Figure 2.20a). Decadal SIA means based on the average of three different satellite products decreased from 6.23 to 3.76 million km <sup>2</sup> for September and 14.52 to 13.42 million km <sup>2</sup> for March SIA (Figure 2.20a). Initial SIA data for 2020 (OSISAF) are within the range of these recent decadal means or slightly below (Figure 2.20a). SIA has declined since 1979 across the seasonal cycle (Figure 9.13). Most of this decline in SIA has occurred after 2000, and is superimposed by substantial interannual variability. The sharp decline in Arctic summer SIA coincides with earlier surface melt onset ( [[#Mortin--2016|Mortin et al., 2016]] ; [[#Bliss--2017|Bliss et al., 2017]] ), later freeze-up, and thus a longer ice retreat and open water period ( [[#Stammerjohn--2012|Stammerjohn et al., 2012]] ; [[#Parkinson--2014|Parkinson, 2014]] ; [[#Peng--2018|Peng et al., 2018]] ).

Over the past two decades, first-year sea ice has become more dominant and the oldest multiyear ice (older than 4 years) which in March 1985 made up 33% of the Arctic sea-ice cover, has nearly disappeared, making up 1.2% in March 2019 ( [[#Perovich--2020|Perovich et al., 2020]] ). The loss of older ice is indicative of a thinning overall of ice cover ( [[#Tschudi--2016|Tschudi et al., 2016]] ), but also the remaining older ice has become thinner (E. [[#Hansen--2013|]] [[#Hansen--2013|Hansen et al., 2013]] ). Since in situ ice thickness measurements are sparse, information about ice thickness is mainly based on airborne and satellite surveys. Records from a combination of different platforms show for the central and western Arctic Ocean (Arctic Ocean north of Canada and Alaska) negative trends since the mid-1970s ( [[#Lindsay--2015|Lindsay and Schweiger, 2015]] ; [[#Kwok--2018|Kwok, 2018]] ), with a particularly rapid decline during the 2000s, which coincided with a large loss of multiyear sea ice. Direct observations from 2004 and 2017 indicate a decrease of modal ice thickness in the Arctic Ocean north of Greenland by 0.75 m, but with little thinning between 2014 and 2017 ( [[#Haas--2017|Haas et al., 2017]] ). This agrees with data based on satellite altimetry and airborne observations, showing no discernible thickness trend since 2010 ( [[#Kwok--2015|Kwok and Cunningham, 2015]] ; [[#Kwok--2018|Kwok, 2018]] ; [[#Kwok--2018|Kwok and Kacimi, 2018]] ; see Figure 2.21). However, sea-ice thickness derived from airborne and spaceborne data is still subject to uncertainties imposed by snow loading. For radar altimeters, insufficient penetration of radar signal into the snowpack results in overestimation of ice thickness (e.g., [[#Ricker--2015|Ricker et al., 2015]] ; [[#King--2018|King et al., 2018]] ; [[#Nandan--2020|Nandan et al., 2020]] ). Negative trends in ice thickness since the 1990s are also reported from the Fram Strait in the Greenland Sea, and north of Svalbard (E. [[#Hansen--2013|]] [[#Hansen--2013|Hansen et al., 2013]] ; [[#Renner--2014|Renner et al., 2014]] ; [[#King--2018|King et al., 2018]] ; [[#Rösel--2018|Rösel et al., 2018]] ; [[#Spreen--2020|Spreen et al., 2020]] ). Thickness data collected in the Fram Strait originate from ice exported from the interior of the Arctic Basin and are representative of a larger geographical area upstream in the transpolar drift. A reduction of survival rates of sea ice exported from the Siberian shelves by 15% per decade has interrupted the transpolar drift and affected the long-range transport of sea ice ( [[#Krumpen--2019|Krumpen et al., 2019]] ). The thinner and on average younger ice has less resistance to dynamic forcing, resulting in a more dynamic ice cover ( [[#Hakkinen--2008|Hakkinen et al., 2008]] ; [[#Spreen--2011|Spreen et al., 2011]] ; [[#Vihma--2012|Vihma et al., 2012]] ; [[#Kwok--2013|Kwok et al., 2013]] ).

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[[File:3d0cd64b5ae223c19653df37531731aa IPCC_AR6_WGI_Figure_2_21.png]]

'''Figure''' '''2.21 |''' '''Arctic sea ice thickness changes (means) for autumn (red/dotted red) and winter (blue/dotted blue).''' Shadings (blue and red) show 1 standard error (S.E.) ranges from the regression analysis of submarine ice thickness and expected uncertainties in satellite ice thickness estimates. Data release area of submarine data ice thickness data is shown in inset. Satellite ice thickness estimates are for the Arctic south of 88°N. Thickness estimates from more localized airborne/ground electromagnetic surveys near the North Pole (diamonds) and from Operation IceBridge (circles) are shown within the context of the larger scale changes in the submarine and satellite records. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

The SROCC noted the lack of continuous records of snow on sea ice. Nevertheless in recent decades, more snow on sea ice has been observed in the Atlantic sector in the Arctic than in the western Arctic Ocean ( [[#Webster--2018|Webster et al., 2018]] ). Previously, [[#Warren--1999|Warren et al. (1999)]] showed that over 1954–1991 there were weak trends towards declining snow depth on sea ice in the Pacific sector. Recent observations indicate a substantial thinning of the spring snowpack in the western Arctic ( [[#Cavalieri--2012|Cavalieri et al., 2012]] ; [[#Brucker--2013|Brucker and Markus, 2013]] ; [[#Kurtz--2013|Kurtz et al., 2013]] ; [[#Laxon--2013|Laxon et al., 2013]] ; [[#Webster--2018|Webster et al., 2018]] ). In contrast, thick snow over Arctic sea ice in the Atlantic sector north of Svalbard (snow thickness around 0.4 m or more) has been observed in the 1970s and since the 1990s ( [[#Rösel--2018|Rösel et al., 2018]] ), but data are too sparse to detect trends.

In summary, over 1979–2019 Arctic SIA has decreased for all months, with the strongest decrease in summer ( ''very high confidence'' ). Decadal means for SIA decreased from the first to the last decade in that period from 6.23 to 3.76 million km <sup>2</sup> for September, and from 14.52 to 13.42 million km <sup>2</sup> for March. Arctic sea ice has become younger, thinner and faster moving ( ''very high confidence'' ). Snow thickness on sea ice has decreased in the western Arctic Ocean ( ''medium confidence'' ). Since the Younger Dryas at the end of the LDT, proxy indicators show that Arctic sea ice has fluctuated on multiple time scales with a decrease in sea ice coverage during the Early Holocene and an increase from the MH to the mid-15th century. Current pan-Arctic sea ice coverage levels (annual mean and late summer) are unprecedentedly low since 1850 ( ''high confidence),'' and with ''medium confidence'' for late summer for at least the past 1 kyr.

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===== 2.3.2.1.2 Antarctic sea ice =====

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The AR5 reported a small but significant increase in the total annual mean Antarctic SIE that was ''very likely'' in the range of 1.2–1.8% per decade between 1979 and 2012 (0.13–0.20 million km <sup>2</sup> per decade) ( ''very high confidence'' ), while SROCC reported that total Antarctic sea ice coverage exhibited no significant trend over the period of satellite observations (1979–2018) ( ''high confidence'' ). The SROCC noted that a significant positive trend in mean annual sea ice cover between 1979 and 2015 had not persisted, due to three consecutive years of below-average sea ice cover (2016–2018). The SROCC stated also that historical Antarctic sea ice data from different sources indicated a decrease in overall Antarctic sea ice cover since the early 1960s, but was too small to be separated from natural variability ( ''high confidence'' ).

There is only ''limited evidence'' from predominantly regional paleo proxies for the evolution of Southern Ocean sea ice before the instrumental record and estimates are not available for all proxy target periods (Section 9.3.2). Proxies from marine sediments for intervals preceding and following the MPWP indicate open water conditions with less sea ice than modern conditions ( [[#Taylor-Silva--2018|Taylor-Silva and Riesselman, 2018]] ; [[#Ishino--2020|Ishino and Suto, 2020]] ). During the LGM, proxies indicate that austral winter sea ice coverage reached the polar ocean front (e.g., [[#Nair--2019|Nair et al., 2019]] ). More recently, sea ice coverage appears to have fluctuated substantially throughout the Holocene (e.g., for the western Amundsen Sea, [[#Lamping--2020|Lamping et al., 2020]] ). At the beginning of the CE, regional summer sea ice coverage in the north-western Ross Sea was lower than today ( [[#Tesi--2020|Tesi et al., 2020]] ). [[#Crosta--2021|Crosta et al. (2021)]] suggest, based on different proxies, four different phases with 7–10 months periods of sea ice occurrence per year in the Antarctic region off Adelie Land during the CE, where each phase was several hundred years long.

More recent sea ice reconstructions are based on diverse sources including whaling records ( [[#de%20La%20Mare--1997|de La Mare, 1997]] , 2009; [[#Cotté--2007|Cotté and Guinet, 2007]] ), old ship logbooks ( [[#Ackley--2003|Ackley et al., 2003]] ; [[#Edinburgh--2016|Edinburgh and Day, 2016]] ), and ice core records ( [[#Curran--2003|Curran et al., 2003]] ; [[#Abram--2010|Abram et al., 2010]] ; [[#Sinclair--2014|Sinclair et al., 2014]] ), amongst other methods (e.g., [[#Murphy--2014|Murphy et al., 2014]] ). These reconstructions, in combination with recent satellite-based observations indicate: (i) a decrease in summer SIE across all Antarctic sectors since the early- to mid-20th century; (ii) a decrease in winter SIE in the East Antarctic and Amundsen-Bellingshausen Seas sectors starting in the 1960s; and (iii) small fluctuations in winter SIE in the Weddell Sea over the 20th century ( [[#Hobbs--2016a|Hobbs et al., 2016a]] , b). There are also ice-core indications that the pronounced Ross Sea increase dates back to the mid-1960s ( [[#Sinclair--2014|Sinclair et al., 2014]] ; [[#Thomas--2016|Thomas and Abram, 2016]] ). While there is reasonable broad-scale concurrence across these estimates, the uncertainties are large, there is considerable interannual variability, and reconstructions require further validation ( [[#Hobbs--2016a|Hobbs et al., 2016a]] , b). New reconstructions ( [[#Thomas--2019|Thomas et al., 2019]] ) from Antarctic land ice cores show that SIE in the Ross Sea had increased between 1900 and 1990, while the Bellingshausen Sea had experienced a decline in SIE; this dipole pattern is consistent with satellite-based observations from 1979 to 2019 ( [[#Parkinson--2019|Parkinson, 2019]] ), but the recent rate of change then has been larger. Records of Antarctic SIE for the late 19th and early 20th centuries ( [[#Edinburgh--2016|Edinburgh and Day, 2016]] ), show SIE comparable with the satellite era, although with marked spatial heterogeneity (e.g., [[#Thomas--2019|Thomas et al., 2019]] ).

Early Nimbus satellite visible and infrared imagery from the 1960s ( [[#Meier--2013|Meier et al., 2013]] ; [[#Gallaher--2014|Gallaher et al., 2014]] ) indicate higher overall SIE compared to 1979–2013 ( [[#Hobbs--2016a|Hobbs et al., 2016a]] , b), but with large uncertainties and poorly quantified biases ( [[#NA%20SEM--2017|NA SEM, 2017]] ). The continuous satellite passive-microwave record shows that there was a modest increase in overall Antarctic SIA of 2.5% ± 0.2% per decade (1 standard error over 1979–2015; [[#Comiso--2017|Comiso et al., 2017]] ). For overall ice coverage and for this period, positive long-term trends were most pronounced during austral autumn advance ( [[#Maksym--2019|Maksym, 2019]] ), being moderate in summer and winter, and lowest in spring ( [[#Holland--2014|Holland, 2014]] ; [[#Turner--2015|Turner et al., 2015]] ; [[#Hobbs--2016a|Hobbs et al., 2016a]] , b; [[#Comiso--2017|Comiso et al., 2017]] ). Since 2014, overall Antarctic SIE (and SIA) has exhibited major fluctuations from record-high to record-low satellite era extents ( [[#Massonnet--2015|Massonnet et al., 2015]] ; [[#Reid--2015|Reid and Massom, 2015]] ; [[#Reid--2015|Reid et al., 2015]] ; [[#Comiso--2017|Comiso et al., 2017]] ; [[#Parkinson--2019|Parkinson, 2019]] ). After setting record-high extents each September from 2012 through 2014, Antarctic SIE (and SIA) dipped rapidly in mid-2016 and remained predominantly below average through 2019 ( [[#Reid--2020|Reid et al., 2020]] ). For the most recent decade of observations (2010–2019), the decadal means of three SIA products (Figure 2.20b) were 2.17 million km <sup>2</sup> for February and 15.75 million km <sup>2</sup> for September, respectively. The corresponding levels for the means for the first decade of recordings (1979–1988) were 2.04 million km <sup>2</sup> for February and 15.39 million km <sup>2</sup> for September indicating little overall change. Initial SIA data for 2020 (OSISAF) show SIA for September above, and for February slightly below the recent decadal means (Figure 2.20b). The 2020 September level (OSISAF) remains below the levels observed over 2012–2014.

In summary, Antarctic sea ice has experienced both increases and decreases in SIA over 1979–2019, and substantively lower levels since 2016, with only minor differences between decadal means of SIA for the first (for February 2.04 million km <sup>2</sup> , for September 15.39 million km <sup>2</sup> ) and last decades (for February 2.17 million km <sup>2</sup> , for September 15.75 million km <sup>2</sup> ) of satellite observations ( ''high confidence'' ). There remains ''low confidence'' in all aspects of Antarctic sea ice prior to the satellite era owing to a paucity of records that are highly regional in nature and often seemingly contradictory.

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