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

==== 2.3.2.4 Ice-sheet Mass and Extent ====

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During glacial periods, ice sheets were more extensive and the state of knowledge on their paleo-reconstruction can be found in recent publications (e.g., [[#Stokes--2015|Stokes et al., 2015]] ; [[#Batchelor--2019|Batchelor et al., 2019]] ). This section focuses only on the large-scale aspects of those ice sheets, Greenland and Antarctic, that still exist today.

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===== 2.3.2.4.1 Greenland Ice Sheet (GrIS) =====

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The AR5 concluded the volume of the Greenland Ice Sheet (GrIS) was reduced compared to present during periods of the past few million years that were globally warmer than present ( ''high confidence'' ). It reported that the GrIS had lost ice during the prior two decades ( ''very high confidence'' ), that the ice loss had occurred in several sectors, and that high rates of mass loss had both expanded to higher elevations ( ''high confidence)'' and ''very likely'' accelerated since 1992. The SROCC concluded that it was ''extremely lik'' e ''ly'' that ice loss increased through the early 21st century. The SROCC also found that summer melting rate had increased since the 1990s to a rate unprecedented over the last 350 years ''(very high confidence'' ), being two to five times greater than the pre-industrial rates ( ''medium confidence'' ).

Details of the history of the GrIS fluctuations during warm interglacials continue to be elucidated. Oscillations over the past 7.5 Myr, including the Pliocene and through the glacial – interglacial cycles of the Pleistocene are not well-constrained, but most studies indicate that Greenland was at least partially glaciated over this time with extended periods when it was predominantly deglaciated ( [[#Bierman--2016|Bierman et al., 2016]] ; [[#Schaefer--2016|Schaefer et al., 2016]] ). Geological evidence and modelling studies suggest periods of glacial intensification during the Pliocene at 4.9 Myr, 4.0 Myr, 3.6 Myr and 3.3 Myr ( [[#De%20Schepper--2014|De Schepper et al., 2014]] ; [[#Bierman--2016|Bierman et al., 2016]] ; [[#Bachem--2017|Bachem et al., 2017]] ). Retreat of the GrIS occurred during the MPWP and GrIS extent was reduced compared to today with some studies suggesting that the ice sheet was limited to the highest elevations ( [[#De%20Schepper--2014|De Schepper et al., 2014]] ; [[#Koenig--2015|Koenig et al., 2015]] ; [[#Haywood--2016|Haywood et al., 2016]] ; [[#Blake-Mizen--2019|Blake-Mizen et al., 2019]] ). There is apparent glacial intensification following the MPWP, 2.75–2.72 Myr ( [[#Nielsen--2013|Nielsen and Kuijpers, 2013]] ; [[#De%20Schepper--2014|De Schepper et al., 2014]] ; [[#Blake-Mizen--2019|Blake-Mizen et al., 2019]] ; [[#Knutz--2019|Knutz et al., 2019]] ). Several studies agree that during the LIG the total GrIS extent was ''likely'' less than present day (Section 9.4.1, Figure 9.17) with the total mass loss ranging from 0.3–6.2 m sea level equivalent (SLE), although timing and magnitude of this mass loss are not well constrained ( [[#Helsen--2013|Helsen et al., 2013]] ; [[#Stone--2013|Stone et al., 2013]] ; [[#Vasskog--2015|Vasskog et al., 2015]] ; [[#Goelzer--2016|Goelzer et al., 2016]] ; [[#Sinclair--2016|Sinclair et al., 2016]] ; [[#Yau--2016|Yau et al., 2016]] ; [[#Clark--2020|Clark et al., 2020]] ). During the LGM, the GrIS reached a peak ice volume greater than present (2–5 m SLE), as revealed by the limited number of available geological records ( [[#Simpson--2009|Simpson et al., 2009]] ; [[#Lecavalier--2014|Lecavalier et al., 2014]] ; [[#Batchelor--2019|Batchelor et al., 2019]] ).

Recent studies of marine and lake sediments, glacier ice, and geomorphic features show that the GrIS retreated rapidly during the early Holocene but halted periodically, with a complex ice-margin chronology ( [[#Carlson--2014|Carlson et al., 2014]] ; [[#Larsen--2014|Larsen et al., 2014]] , 2015; [[#Young--2015|Young and Briner, 2015]] ; [[#Briner--2016|Briner et al., 2016]] ; [[#Young--2020|Young et al., 2020]] ). It is probable that its total volume during 8–3 ka was smaller than today ( [[#Larsen--2015|Larsen et al., 2015]] ; [[#Young--2015|Young and Briner, 2015]] ; [[#Briner--2016|Briner et al., 2016]] ), but uncertainties exist regarding precisely when the minimum MH extent and volume was reached, due to uncertainties in reconstructions. The GrIS then re-advanced reaching its maximum extent in most places during 1450–1850 CE, although the timing and extent of this maximum differed by sector ( [[#Larsen--2015|Larsen et al., 2015]] ; [[#Briner--2016|Briner et al., 2016]] ).

Greenland-wide estimates of mass change based on direct observations were limited prior to 1992 at the time of AR5 ( [[#Kjeldsen--2015|Kjeldsen et al., 2015]] ). Combined records based on airborne observations, model-based estimates and geodetic approaches indicate an average mass loss of 75 ± 29.4 Gt yr <sup>–1</sup> for 1900–1983 ( [[#Kjeldsen--2015|Kjeldsen et al., 2015]] ). Integration of proxies and modelling indicates that the last time the rate of mass loss of the GrIS was plausibly similar to 20th century rates was during the early Holocene ( [[#Buizert--2018|Buizert et al., 2018]] ; [[#Briner--2020|Briner et al., 2020]] ).

Since AR5, a combination of remote sensing, in situ observations and modelling has provided new insights regarding surface processes and their contribution to recent GrIS mass changes ( [[#AMAP--2017|AMAP, 2017]] ; [[#van%20den%20Broeke--2017|van den Broeke et al., 2017]] ; [[#Bamber--2018|Bamber et al., 2018]] ; [[#Mouginot--2019|Mouginot et al., 2019]] ; [[#IMBIE%20Consortium--2020|IMBIE Consortium, 2020]] ; [[#Khan--2020|Khan et al., 2020]] ). Estimates of total ice loss during the post-1850 period ( [[#Kjeldsen--2015|Kjeldsen et al., 2015]] ) and recent observations show that the rate of loss has increased since the beginning of the 21st century ( [[#IMBIE%20Consortium--2020|IMBIE Consortium, 2020]] ; [[#Sasgen--2020|Sasgen et al., 2020]] ; [[#Velicogna--2020|Velicogna et al., 2020]] ) (Section 9.4.1.1 and Figures 2.24 and 9.17).

The GrIS lost 4890 [4140 to 5640] Gt (SLE 13.5 [11.4 to 15.6] mm) of ice between 1992 and 2020 (Section 9.4.1 and Figure 2.24; [[#IMBIE%20Consortium--2020|IMBIE Consortium, 2020]] ). The ice sheet was close to mass balance in the 1990s, but increases in mass loss have occurred since ( [[#Bamber--2018|Bamber et al., 2018]] ; [[#WCRP%20Global%20Sea%20Level%20Budget%20Group--2018|WCRP Global Sea Level Budget Group, 2018]] ; [[#Mouginot--2019|Mouginot et al., 2019]] ; [[#IMBIE%20Consortium--2020|IMBIE Consortium, 2020]] ). The rate of ice-sheet (including peripheral glaciers) mass loss rose from 120 [70 to 170] Gt yr <sup>–1</sup> (SLE 0.33 [0.18 to 0.47] mm yr <sup>–1</sup> ) in 1901–1990 to 330 [290 to 370] Gt yr <sup>–1</sup> (SLE 0.91 [0.79 to 1.02] mm yr <sup>–1</sup> ) for 2006–2018 (Section 9.4.1, Table 9.5).

In summary, the GrIS was smaller than present during the MPWP ( ''medium confidence'' ), LIG ( ''high confidence'' ) and the MH ( ''high confidence'' ). GrIS mass loss began following a peak volume attained during the 1450–1850 period and the rate of loss has increased substantially since the turn of the 21st century ( ''high confidence'' ).

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===== 2.3.2.4.2 Antarctic Ice Sheet (AIS) =====

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The AR5 reported that there was ''high confidence'' that the Antarctic Ice Sheet (AIS) was losing mass. The average ice mass loss from Antarctica was 97 [58 to 135] Gt yr <sup>–1</sup> (GMSL equivalent of 0.27 [0.16 to 0.37] mm yr <sup>–1</sup> ) over 1993–2010, and 147 [74 to 221] Gt yr <sup>–1</sup> (0.41 [0.20 to 0.61] mm yr <sup>–1</sup> ) over 2005–2010. These assessments included the Antarctic peripheral glaciers. The AR5 reported with ''high confidence'' that the volume of the West Antarctic Ice Sheet (WAIS) was reduced during warm periods of the past few million years. The SROCC concluded that over 2006–2015, the AIS lost mass at an average rate of 155 ± 19 Gt yr <sup>–1</sup> ( ''very high confidence'' ). The SROCC also stated that it is ''virtually certain'' that the Antarctic Peninsula and WAIS combined have cumulatively lost mass since widespread measurements began in 1992, and that the rate of loss has increased since around 2006.

Process understanding and, to some extent, paleoclimate records show that changes in parts of the AIS can occur over multi-century time scales (&lt;2kyr; Sections 9.4.2.3 and 9.6.2; e.g., [[#Dowdeswell--2020|Dowdeswell et al., 2020]] ). Based on physical understanding, paleo evidence and numerical simulations, it is ''very likely'' that the AIS has been smaller than today during at least some past warm climates (such as MCO and LIG), in particular the WAIS (Figure 9.18; [[#Golledge--2014|Golledge et al., 2014]] ; [[#de%20Boer--2015|de Boer et al., 2015]] ; [[#DeConto--2016|DeConto and Pollard, 2016]] ; [[#Levy--2016|Levy et al., 2016]] ). Results from sediment studies suggest a smaller AIS during the MPWP compared with current levels, with main differences in the WAIS (Section 9.6.2; SROCC, [[#IPCC--2019|IPCC, 2019]] ; [[#Bertram--2018|Bertram et al., 2018]] ; [[#Shakun--2018|Shakun et al., 2018]] ). Marine sediments indicate that during the Pleistocene repeated ungrounding and loss of large marine-based parts of the AIS occurred during interglacial periods, with at least seven transitions between floating and grounded ice in the Ross Sea during the last 780 kyr ( [[#McKay--2012|McKay et al., 2012]] ) and at least three reductions in ice volume in the Wilkes Basin during the last 500 kyr ( [[#Wilson--2018|Wilson et al., 2018]] ). Proxies, modelling and process understanding ( [[#Rohling--2019|Rohling et al., 2019]] ; [[#Clark--2020|Clark et al., 2020]] ) indicate that the AIS was smaller during the LIG than present.

Geological evidence has been used to reconstruct Holocene glacial fluctuations of the ice sheet margin and lowerings of its surface, which occurred at different times in different places, as recently reviewed by [[#Noble--2020|Noble et al. (2020)]] . In West Antarctica, marine sediments below the ice sheet ( [[#Kingslake--2018|Kingslake et al., 2018]] ) corroborate a previous glacial isostatic adjustment modelling study ( [[#Bradley--2015|Bradley et al., 2015]] ), which suggests that ice had retreated behind the present grounding line prior to about 10 ka, and then readvanced. Geophysical imaging indicates a readvance in this area around 6 ± 2 ka ( [[#Wearing--2019|Wearing and Kingslake, 2019]] ). Other studies from the region conclude that ice-sheet retreat and thinning was fastest from 9 to 8 ka ( [[#Johnson--2014|Johnson et al., 2014]] ; [[#McKay--2016|McKay et al., 2016]] ; [[#Spector--2017|Spector et al., 2017]] ), or millennia later, during the MH ( [[#Hein--2016|Hein et al., 2016]] ; [[#Johnson--2019|Johnson et al., 2019]] ), with indications of a subsequent readvance ( [[#Venturelli--2020|Venturelli et al., 2020]] ). In East Antarctica, rapid ice-sheet thinning occurred between around 9 and 5 ka ( [[#Jones--2015|Jones et al., 2015]] ), consistent with previous work indicating that the ice sheet in many regions was at or close to its current position by 5 ka ( [[#Bentley--2014|Bentley et al., 2014]] ). Overall, during the MH, the AIS was retreating, but remained more extensive than present, while some parts of the ice sheet might have been smaller than now ( ''low confidence'' ).

Improved estimates of surface mass balance (SMB) in Antarctica from 67 ice core records do not show any substantial changes in accumulation rates over most of Antarctica since 1200 CE ( [[#Frezzotti--2013|Frezzotti et al., 2013]] ). The SMB growth rate in Antarctica is estimated to be 7.0 ± 0.1 Gt per decade between 1800 and 2010 and 14.0 ± 1.8 Gt per decade since 1900 ( [[#Thomas--2017|Thomas et al., 2017]] ). For the period 1979–2000, an insignificant Antarctic-wide negative SMB trend has been estimated ( [[#Medley--2019|Medley and Thomas, 2019]] ). The Antarctic Ice Sheet lost 2670 [1800 to 3540] Gt (SLE 7.4 [5.0 to 9.8] mm) of ice between 1992 and 2020. The rate of ice-sheet (including peripheral glaciers) mass loss rose from 0 [–36 to +40] Gt yr <sup>–1</sup> (SLE 0.0 [–0.10 to 0.11] mm yr <sup>–1</sup> ) in 1901–1990 to 192 [145 to 239] Gt yr <sup>–1</sup> (SLE 0.54 [0.47 to 0.61] mm yr <sup>–1</sup> ) for 2006–2018 (Section 9.4.2, Figure 2.24, and Table 9.5). Within quantified uncertainties, this estimate agrees with other recent estimates ( [[#Rignot--2019|Rignot et al., 2019]] ; [[#Smith--2020|]] [[#Smith--2020|B. Smith et al., 2020]] ; [[#Velicogna--2020|Velicogna et al., 2020]] ). There is therefore ''very high confidence'' that the AIS has been losing mass over 1992–2020 (Section 9.4.2.1 and Figure 2.24). Major contributions to recent AIS changes arise from West Antarctica and Wilkes Land in East Antarctica ( [[#Rignot--2019|Rignot et al., 2019]] ). For the East Antarctic most studies suggest that the mass balance is not significantly different from zero ( [[#Bamber--2018|Bamber et al., 2018]] ; [[#IMBIE%20Consortium--2018|IMBIE Consortium, 2018]] ; [[#Mohajerani--2018|Mohajerani et al., 2018]] ; [[#Rignot--2019|Rignot et al., 2019]] ).

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[[File:8246f75a6d42767749dc6960bd8237e0 IPCC_AR6_WGI_Figure_2_24.png]]

'''Figure 2.''' '''24 |''' '''Cumulative Antarctic Ice Sheet (AIS) and Greenland Ice Sheet (GrIS) mass changes.''' Values shown are in gigatons and come from satellite-based measurements ( [[#IMBIE%20Consortium--2018|IMBIE Consortium, 2018]] , 2020) for the period 1992–2020. The estimated uncertainties, ''very likely'' range, for the respective cumulative changes are shaded. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

In summary, the AIS has lost mass between 1992 and 2020 ( ''very high confidence'' ), and there is ''medium confidence'' that this mass loss has increased. During the MPWP and LIG, the ice sheet was smaller than present ( ''medium confidence'' ). There is ''low confidence'' as to whether the total mass of the ice sheet was larger or smaller around 6 ka compared to now.

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