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

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

<div id="h4-24-siblings" class="h4-siblings"></div>

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.

<div id="2.3.2.2" class="h3-container"></div>

<span id="terrestrial-snow-cover"></span>