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

==== Atlas.11.1.2 Assessment and Synthesis of Observations, Trends and Attribution ====

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Figure Atlas.30 (Antarctic map inset) shows near-surface air temperature trends for 1957–2016 and 1979–2016 at the stations where observations are available for at least 50 years and the detected trends have statistical significance of at least 90% according to the most recent (after SROCC) studies ( [[#Jones--2019|Jones et al., 2019]] ; [[#Turner--2020|Turner et al., 2020]] ). It is ''very likely'' that the western and northern AP has been warming significantly since the 1950s (0.49°C ± 0.28°C per decade during 1957–2016 and 0.46°C ± 0.15°C during 1951–2018 at Faraday-Vernadsky station; 0.29°C ± 0.16°C per decade during 1957–2016 at Esperanza station), with no significant trends reported in the eastern AP during the same period ( [[#Gonzalez--2018|Gonzalez and Fortuny, 2018]] ; [[#Jones--2019|Jones et al., 2019]] ; [[#Turner--2020|Turner et al., 2020]] ). Short-term cooling trends, strongest during austral summer, have been reported at AP stations during 1999–2016, but the absence of warming and cooling at some stations during 1999–2016 is consistent with natural variability, and there is no evidence of a shift in the overall warming trend observed since the 1950s ( [[#Turner--2016|Turner et al., 2016]] , 2020; [[#Gonzalez--2018|Gonzalez and Fortuny, 2018]] ; [[#Jones--2019|Jones et al., 2019]] ; [[#Bozkurt--2020|Bozkurt et al., 2020]] ).

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[[File:a9ef0d8eab44f07a928142d2fca84de7 IPCC_AR6_WGI_Atlas_Figure_30.png]]

'''Figure Atlas.30''' '''|''' '''(Upper panels) Time series of annual surface mass balance (SMB) rates (in Gt a''' –1 ''') for the Greenland Ice Sheet and its regions (shown in the inset map) for the periods 1972–2018 ( [[#Mouginot--2019|Mouginot et al., 2019]] ) and 1980–2012 ( [[#Fettweis--2020|Fettweis et al., 2020]] ) using 13 different models.''' '''(Lower panels)''' Time series of annual SMB rates (in Gt a <sup>–1</sup> ) for the grounded Antarctic Ice Sheet (excluding ice shelves) and its regions (shown in the inset map) for the periods 1979–2019 ( [[#Rignot--2019|Rignot et al., 2019]] ) and 1980–2016 ( [[#Mottram--2021|Mottram et al., 2021]] ) using five Polar-CORDEX regional climate models. The Antarctic inset map also shows the location of the stations discussed in [[#Atlas.11.1.2|Atlas.11.1.2]] where observations are available for at least 50 years. Colours indicate near-surface air temperature trends for 1957–2016 (circles) and 1979–2016 (diamonds) statistically significant at 90% (Jones et al. 2019; Turner et al. 2020). Stations with an asterisk (*) are where significance estimates disagree between the two publications. Further details on data sources and processing are available in the chapter data table (Table Atlas.SM.15).

Significant warming at the Byrd station (0.29°C ± 0.19°C per decade during 1957–2016) confirms and extends earlier trend estimates (0.42°C ± 0.24°C per decade during 1958–2010) and is representative of the entire WAN warming (0.22°C ± 0.12°C per decade from 1958 to 2012 averaged over WAN excluding AP, ''medium confidence'' due to lack of observations) ( [[#Bromwich--2013|Bromwich et al., 2013]] , 2014; [[#Jones--2019|Jones et al., 2019]] ). WAN and AP show statistically significant warming in the HadCRUTv5 observational dataset (Figure 2.11b). There is ''high confidence'' in the long-term warming trend at the AP and WAN, and also at the century scale based on reconstructions ( [[#Zagorodnov--2012|Zagorodnov et al., 2012]] ; [[#Stenni--2017|Stenni et al., 2017]] ; [[#Lyu--2020|Lyu et al., 2020]] ), confirming the trends estimated by earlier studies assessed in the SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ). The century-scale warming trend in the AP is ''very likely'' an emerging signal compared to natural variability, while the WAN warming trend falls in the high end of century-scale trends over the last 2000 years ( ''medium confidence'' ) ( [[#Stenni--2017|Stenni et al., 2017]] ).

In EAN, during 1957–2016, three stations showed significant warming (Scott 0.22°C ± 0.15°C, Novolazarevskaya 0.13°C ± 0.09°C, and Vostok 0.15°C ± 0.13°C per decade), while other stations with long-term observations indicated no statistically significant trends (Figure Atlas.3 0). During 1979–2016, three coastal stations showed cooling, while at the South Pole a warming trend was detected, increasing to 0.61°C ± 0.34°C per decade during 1989–2018 (Figure Atlas.3 0; [[#Jones--2019|Jones et al., 2019]] ; [[#Clem--2020|Clem et al., 2020]] ; [[#Turner--2020|Turner et al., 2020]] ). The century-scale warming in Queen Maud Land coast based on ice-core reconstructions is within the range of centennial internal variability ( [[#Stenni--2017|Stenni et al., 2017]] ).

While a trend towards a positive phase of the SAM since the 1970s ''likely'' explains a significant part of the warming at the northern AP, it had a cooling effect on continental WAN and EAN (particularly strong in DJF; Table Atlas.1). Warming in western AP and over WAN during 1957–2016 (Figure Atlas.3 0) and through to 2020 (Figure 2.11) is ''likely'' due to significant contribution of other factors, such as tropical Pacific forcing through PDV, ENSO, Amundsen Sea Low position/strength and also anthropogenic climate change ( [[#Jones--2019|Jones et al., 2019]] ; [[#Scott--2019|Scott et al., 2019]] ; [[#Wille--2019|Wille et al., 2019]] ; [[#Donat-Magnin--2020|Donat-Magnin et al., 2020]] ; [[#Turner--2020|Turner et al., 2020]] ). Since SROCC, new studies confirmed the influence of foehn wind and cloud radiative forcing on Larsen C surface melt ( [[#Elvidge--2020|Elvidge et al., 2020]] ; [[#Gilbert--2020|Gilbert et al., 2020]] ; [[#Turton--2020|Turton et al., 2020]] ). In WAN, summer surface-melt occurrence over ice shelves may have increased since the late 2000s ( [[#Scott--2019|Scott et al., 2019]] ) ''.'' It is ''likely'' that increased meltwater ponding and resulting hydrofracturing have been important mechanisms of the rapid disintegration of the Larsen B ice shelf ( [[#Banwell--2013|Banwell et al., 2013]] ; [[#MacAyeal--2013|MacAyeal and Sergienko, 2013]] ; [[#Robel--2019|Robel and Banwell, 2019]] ). Ice-shelf disintegration and relevant processes are discussed in Sections 9.4.2.1 and 9.4.2.3.

Direct observations of snowfall in Antarctica using traditional gauges are highly uncertain and records from precipitation radars ( [[#Gorodetskaya--2015|Gorodetskaya et al., 2015]] ; [[#Grazioli--2017|Grazioli et al., 2017]] ; [[#Scarchilli--2020|Scarchilli et al., 2020]] ) are not long enough to assess trends. Estimates of precipitation and SMB are largely model-based due to the paucity of in situ observations in Antarctica ( [[#Lenaerts--2019|Lenaerts et al., 2019]] ; [[#Hanna--2020|Hanna et al., 2020]] ). Antarctic SMB is dominated by precipitation and removal by sublimation with very small amounts of melt mostly important only on the ice shelves. Climate models and satellite records (IMBIE team et al., 2018; [[#Rignot--2019|Rignot et al., 2019]] ; [[#Mottram--2021|Mottram et al., 2021]] ) suggest that strong interannual variability of Antarctic-wide SMB over the satellite period currently masks any existing trend (Figure Atlas.3 0) in spite of a possible ozone depletion-related precipitation increase over the 1991–2005 period ( [[#Lenaerts--2018|Lenaerts et al., 2018]] ). No significant Antarctic-wide SMB trend is inferred since 1979 (IMBIE team et al., 2018; [[#Medley--2019|Medley and Thomas, 2019]] ). While ice-core reconstructions show a significant increase in the western AP SMB since the 1950s ( [[#Thomas--2017|Thomas et al., 2017]] ; [[#Medley--2019|Medley and Thomas, 2019]] ; [[#Wang--2019|Wang et al., 2019]] ), this trend is not reproduced by regional climate models or the reanalyses used to drive them (Figure Atlas.3 0; [[#van%20Wessem--2016|van Wessem et al., 2016]] ; [[#Wang--2019|Wang et al., 2019]] ).

According to the ice-core reconstructions, SMB over WAN (including AP) has ''likely'' increased during the 20th century with trends of 5.4 ± 2.9 Gt yr <sup>–1</sup> per decade (1900–2010; [[#Wang--2019|Wang et al., 2019]] ) mitigating global mean sea level rise by, respectively, 0.28 ± 0.17 mm per decade (WAN excluding AP, during 1901–2000) and 0.62 ± 0.17 mm per decade (AP, during 1979–2000; [[#Medley--2019|Medley and Thomas, 2019]] ). Significant spatial heterogeneity in SMB trends has been observed over AP and WAN:

* Western AP has ''likely'' experienced a significant increase in SMB beginning around 1930 and accelerating during 1970–2010, which is outside of the natural variability range of the past 300 years ( [[#Thomas--2017|Thomas et al., 2017]] ; [[#Medley--2019|Medley and Thomas, 2019]] ; [[#Wang--2019|Wang et al., 2019]] );
* eastern AP has no significant SMB trends during the same period ( ''low confidence'' , observations limited to one ice core and large interannual variability) ( [[#Thomas--2017|Thomas et al., 2017]] ; [[#Engel--2018|Engel et al., 2018]] );
* overall WAN SMB (excluding AP) was stable during 1980–2009 but exhibited high regional variability ( [[#Medley--2013|Medley et al., 2013]] ): significant increases (5–15 mm per decade during 1957–2000) to the east of the West Antarctic Ice Sheet divide and a significant decrease (–1 to –5 mm per decade during 1901–1956, and –5 to –15 mm per decade during 1957–2000) to the west ( [[#Medley--2019|Medley and Thomas, 2019]] ; [[#Wang--2019|Wang et al., 2019]] ).

The SMB of EAN increased during the 20th century which mitigated global mean sea level rise by 0.77 ± 0.40 mm per decade during 1901–2000 ( ''medium confidence'' ) ( [[#Medley--2019|Medley and Thomas, 2019]] ). EAN SMB has been increasing at a much lower rate since 1979 as shown by observations, while regional climate models show strong interannual variability masking any trend ( ''low confidence'' due to limited observations) (Figure Atlas.3 0; [[#Medley--2019|Medley and Thomas, 2019]] ; [[#Rignot--2019|Rignot et al., 2019]] ). EAN SMB changes during the 20th century and recent decades showed large spatial heterogeneity:

* With significant increases ''likely'' in Queen Maud Land (QML): 5.2 ± 3.7% per decade during 1920–2011 measured in ice cores near the Kohnen station ( [[#Medley--2018|Medley et al., 2018]] ), an increase on the plateau ( [[#Altnau--2015|Altnau et al., 2015]] ), and stable conditions during 1993–2010 along the annual stake line from Syowa (coast) to Dome F (plateau) (Y. [[#Wang--2015|]] [[#Wang--2015|Wang et al., 2015]] ); increases during 1911–2010 ( [[#Thomas--2017|Thomas et al., 2017]] ) with anomalously high SMB observed in 2009 and 2011 ( [[#Boening--2012|Boening et al., 2012]] ; [[#Lenaerts--2013|Lenaerts et al., 2013]] ; [[#Gorodetskaya--2014|Gorodetskaya et al., 2014]] );
* increases in Wilkes Land and Queen Mary Land during 1957–2000 ( ''low confidence'' due to limited observations and strong spatial variability) ( [[#Thomas--2017|Thomas et al., 2017]] ; [[#Medley--2019|Medley and Thomas, 2019]] );
* a ''likely'' stable SMB in the interior of the east Antarctic plateau during the 1901–2000 period and the last decades ( [[#Thomas--2017|Thomas et al., 2017]] ; [[#Medley--2019|Medley and Thomas, 2019]] );
* stable in Adelie Land (annual stake line during 1971–2008) ( ''low confidence'' due to ''limited evidence'' ) ( [[#Agosta--2012|Agosta et al., 2012]] ).

Regional trends during recent 50 year (1961–2010) and 100 year (1911–2010) periods are within the centennial variability of the past 1000 years, except for coastal QML (unusual 100-year increase in accumulation) and for coastal Victoria Land (unusual 100-year decrease in accumulation) ( [[#Thomas--2017|Thomas et al., 2017]] ). Nevertheless, the current EAN SMB is not unusual compared to the past 800 years ( [[#Frezzotti--2013|Frezzotti et al., 2013]] ).

The geographic pattern of accumulation changes since the 1950s bears a strong imprint of a trend towards a more positive phase of the SAM (e.g., [[#Medley--2019|Medley and Thomas, 2019]] ), which could be linked to ozone depletion ( [[#Lenaerts--2018|Lenaerts et al., 2018]] ) or large-scale atmospheric warming ( [[#Frieler--2015|Frieler et al., 2015]] ; [[#Medley--2019|Medley and Thomas, 2019]] ). More evidence has emerged showing the importance of the Pacific–South American pattern, ENSO and Pacific Ocean convection, and large-scale blocking causing warm-air intrusions and both extreme precipitation and melt events, responsible for large interannual SMB variability ( ''high confidence'' ) ( [[#Gorodetskaya--2014|Gorodetskaya et al., 2014]] ; [[#Bodart--2019|Bodart and Bingham, 2019]] ; [[#Scott--2019|Scott et al., 2019]] ; [[#Turner--2019|Turner et al., 2019]] ; [[#Wille--2019|Wille et al., 2019]] ; [[#Adusumilli--2021|Adusumilli et al., 2021]] ). This strengthens evidence for an important connection between Antarctic climate and tropical sea surface temperature stated by SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ). [[IPCC:Wg1:Chapter:Chapter-3#3.4.3|Section 3.4.3]] and SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ) provide a discussion of attribution of Antarctic ice-sheet changes.

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