Editing IPCC:AR6/WGI/Chapter-9 (section)

==== 9.6.2.6 Holocene ====

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Around half (50–60 m) of the GMSL rise since the LGM occurred during the early Holocene at a sustained rate of about 15 m kyr <sup>–1</sup> from around 11.4–8.2 ka ( [[#Lambeck--2014|Lambeck et al., 2014]] ), possibly punctuated by abrupt meltwater pulses ( [[#Smith--2011|Smith et al., 2011]] ; [[#Carlson--2012|Carlson and Clark, 2012]] ; [[#Törnqvist--2012|Törnqvist and Hijma, 2012]] ; [[#Harrison--2019|Harrison et al., 2019]] ). An abrupt (about 1.1 m) sea level rise around 8.2 ka was associated with drainage of the pro-glacial Agassiz and Ojibway lakes, attributed to accelerated melt from collapsing Laurentide Ice Sheet ice saddles ( [[#Matero--2017|Matero et al., 2017]] ). The Laurentide Ice Sheet provided the greatest contribution (27 m) to early Holocene GMSL ( [[#Peltier--2015|Peltier et al., 2015]] ; [[#Roy--2017|Roy and Peltier, 2017]] ), the Scandinavian Ice Sheet contributed about 2 m from the beginning of the Holocene until its demise by around 10.5 ka, ( [[#Cuzzone--2016|Cuzzone et al., 2016]] ), while the Barents Sea Ice Sheet contributed a small but unknown amount ( [[#Patton--2015|Patton et al., 2015]] , 2017; [[#Auriac--2016|Auriac et al., 2016]] ). The Greenland Ice Sheet contributed about 4 m, consistent with ice thinning rates inferred from the Camp Century ice core ( [[#Lecavalier--2017|Lecavalier et al., 2017]] ; [[#McFarlin--2018|McFarlin et al., 2018]] ). Recent estimates of Antarctic contributions during the early Holocene vary considerably from about 1.2 m to 8.5 m ( [[#Whitehouse--2012|Whitehouse et al., 2012]] ; [[#Ivins--2013|Ivins et al., 2013]] ; [[#Argus--2014|Argus et al., 2014]] ; [[#Briggs--2014|Briggs et al., 2014]] ; [[#Golledge--2014|Golledge et al., 2014]] ; [[#Pollard--2016|Pollard et al., 2016]] ; [[#Roy--2017|Roy and Peltier, 2017]] ; [[#Albrecht--2020|Albrecht et al., 2020]] ). In summary, the early Holocene was characterized by steadily rising GMSL as global ice sheets continued to retreat from their LGM extents. This steady rise was punctuated by abrupt pulses during episodes of rapid meltwater discharge.

In the middle Holocene, GMST peaked at 0.2°C–1.0°C higher than 1850–1900 temperature between 7 and 6 ka ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.1.2|Section 2.3.1.1.2]] ). GMSL rise slowed coincidently with final melting of the Laurentide ice sheet by 6.7 ± 0.4 ka ( [[#Ullman--2016|Ullman et al., 2016]] ), after which only Greenland and Antarctic ice sheets could have contributed significantly. At 6 ka, GMSL was –3.5 to +0.5 m ( ''medium confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.3|Section 2.3.3.3]] ). Simulations of the Holocene Thermal Maximum give a Greenland Ice Sheet broadly consistent with geological reconstructions so, despite uncertainties regarding the timing of minimum ice-sheet volume and extent, there is ''medium confidence'' that minima were reached at different times in different areas during the period 8–3 ka BP ( [[#Larsen--2015|Larsen et al., 2015]] ; [[#Young--2015|Young and Briner, 2015]] ; [[#Briner--2016|Briner et al., 2016]] ). Geochronological and numerical modelling studies indicate that it is ''likely'' ( ''medium confidence'' ) that the period of smaller-than-present ice extent in all sectors of Greenland persisted for at least 2000 to 3000 years ( [[#Larsen--2015|Larsen et al., 2015]] ; [[#Young--2015|Young and Briner, 2015]] ; [[#Briner--2016|Briner et al., 2016]] ; [[#Nielsen--2018|Nielsen et al., 2018]] ). Based on ice-sheet modelling and carbon-14 ( <sup>14</sup> C) dating ( [[#Kingslake--2018|Kingslake et al., 2018]] ) suggested that West Antarctic grounding lines retreated prior to around 10 ka BP, followed by a readvance. Other studies from the same region conclude that retreat was fastest from 9–8 ka BP ( [[#Spector--2017|Spector et al., 2017]] ), or from 7.5–4.8 ka BP ( [[#Venturelli--2020|Venturelli et al., 2020]] ). Marine geological evidence indicates open marine conditions east of Ross Island by 8.6 ± 0.2 ka BP ( [[#McKay--2016|McKay et al., 2016]] ). In the western Weddell Sea, [[#Johnson--2019|Johnson et al. (2019)]] reported rapid glacier thinning from 7.5–6 ka BP. [[#Hein--2016|Hein et al. (2016)]] concluded that the fastest thinning further south took place from 6.5–3.5 ka BP, potentially contributing 1.4–2 m to GMSL. Geophysical data indicate stabilization or readvance in this area around 6 ± 2 ka BP ( [[#Wearing--2019|Wearing and Kingslake, 2019]] ). In coastal Dronning Maud Land (East Antarctica) rapid thinning occurred 9–5 ka BP ( [[#Kawamata--2020|Kawamata et al., 2020]] ), whereas glaciers in the Northern Antarctic Peninsula receded during the period 11–8 ka BP and readvanced to their maximal extents by 7–4 ka BP ( [[#Kaplan--2020|Kaplan et al., 2020]] ). In summary, higher-than-pre-industrial GMST during the mid-Holocene coincided with recession of the Greenland Ice Sheet to a smaller-than-present extent ( ''high confidence'' ). Multiple lines of evidence give ''high confidence'' that thinning or retreat in parts of Antarctica during the Holocene took place at different times in different places. However, limited data means there is only ''low confidence'' in whether or not the ice sheet as a whole was smaller than present during the mid-Holocene.

In summary, both proxies and model simulations indicate that GMSL changes during the early to mid-Holocene were the result of episodic pulses, due to drainage of meltwater lakes, superimposed on a trend of steady rise due to continued ice-sheet retreat ( ''high confidence'' ).

The combination of tide gauge observations and geological reconstructions indicates that a sustained increase of GMSL began between 1820–1860 and led to a 20th-century GMSL rise that was ''very likely'' ( ''high confidence'' ) faster than in any preceding century in the last 3000 years ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.3|Section 2.3.3.3]] ). At a regional level, tide gauge and geological data from the North Atlantic and Australasia show inflections in RSL trends between 1895–1935, with an increase of 0.8 to 2.5 mm yr <sup>–1</sup> across the inflection ( [[#Gehrels--2013|Gehrels and Woodworth, 2013]] ). A statistical meta-analysis of globally distributed geological and tide gauge data ( [[#Kopp--2016|Kopp et al., 2016]] ) found that, in all 20 examined regions with geological records stretching back at least 2000 years, the rate of RSL rise in the 20th century was greater than the local average over 0–1700 CE. In four of the 20 regions, all in the North Atlantic (Connecticut, New Jersey, North Carolina, and Iceland), the 19th century rate was also greater than the 0–1700 CE average (90% confidence interval). In summary, rates of RSL rise exceeding the pre-industrial background rate of rise are apparent in parts of the North Atlantic in the 19th century ( ''medium confidence'' ), and in most of the world in the 20th century ( ''high confidence'' ).

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