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

===== 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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