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

==== 9.6.2.3 Last Interglacial ====

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The AR5 found that the Last Interglacial (LIG) GMSL was &gt;5 m ( ''very high confidence'' ) but &lt;10 m ( ''high confidence'' ). Their best estimate of 6 m was based on two studies ( [[#Kopp--2009|Kopp et al., 2009]] ; [[#Dutton--2012|Dutton and Lambeck, 2012]] ). The SROCC concluded that, during the LIG, Greenland’s contribution to the GMSL highstand (the highest sea levels during the LIG) of 6–9 m increased gradually, whereas the Antarctic contribution occurred early, from about 129 ka. Due to widely varying reconstructions from model studies (Greenland) and the paucity of direct evidence of ice-sheet change (Antarctic), the magnitude of sea level contributions from both ice sheets was assigned ''low confidence.''

Since AR5, information has improved about the LIG, when GMST was about 0.5°C–1.5°C above 1850–1900 ( ''medium confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.1|Section 2.3.1.1]] ). The LIG had higher summer insolation than present and polar amplified sea surface and surface air temperatures that reached &gt;1°C–4°C and &gt;3°C–11 °C in the Arctic respectively ( [[#Landais--2016|Landais et al., 2016]] ; [[#Capron--2017|Capron et al., 2017]] ; [[#Fischer--2018|Fischer et al., 2018]] ). Mean annual and maximum summer ocean temperatures peaked early (129–125 ka) in the interglacial period, reaching 1.1 ± 0.3 °C above the modern global mean ( [[#Shackleton--2020|Shackleton et al., 2020]] ) with summer anomalies of 2.5°C–3.5 °C in the Southern Ocean ( [[#Bianchi--2002|Bianchi and Gersonde, 2002]] ) and spatially variable timing ( [[#Chadwick--2020|Chadwick et al., 2020]] ). It is ''virtually certain'' that GMSL was higher than today, ''likely'' by 5–10 m ( ''medium confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.3|Section 2.3.3.3]] ). Global mean thermal expansion peaked at about 0.9 ± 0.3 m early in the LIG (about 129 ka), declining to modern levels by about 127 ka ( [[#Shackleton--2020|Shackleton et al., 2020]] ). With no more than 0.3 ± 0.1 m of GMSL rise from glaciers ( [[#9.5.1|Section 9.5.1]] ), at most 1.0 ± 0.3 m of the GMSL rise originated from sources other than the polar ice sheets.

Recent LIG ice-sheet simulations agree that peak loss from the Greenland Ice Sheet occurred late (125–120 ka; [[#Goelzer--2016|Goelzer et al., 2016]] ; [[#Tabone--2018|Tabone et al., 2018]] ; [[#Plach--2019|Plach et al., 2019]] ) when Northern Hemisphere insolation was greater than at present ( ''medium confidence'' ) ( [[#Capron--2017|Capron et al., 2017]] ), consistent with inferences from marine sediment records ( [[#Hatfield--2016|Hatfield et al., 2016]] ; [[#Irvalı--2020|Irvalı et al., 2020]] ) and far-field GMSL indicators ( [[#Rohling--2019|Rohling et al., 2019]] ). Best estimates of the GMSL contribution from Greenland (Figure 9.17) differ between models: ≤1 m ( [[#Albrecht--2020|Albrecht et al., 2020]] ; [[#Clark--2020|Clark et al., 2020]] ), 1–2 m ( [[#Calov--2015|Calov et al., 2015]] ; [[#Goelzer--2016|Goelzer et al., 2016]] ; [[#Bradley--2018|Bradley et al., 2018]] ), up to 3 m ( [[#Tabone--2018|Tabone et al., 2018]] ; [[#Plach--2019|Plach et al., 2019]] ), and &gt;5 m ( [[#Yau--2016|Yau et al., 2016]] ). There is ''high confidence'' that the response time of the Greenland Ice Sheet to LIG warming was multi-millennial, and ''high confidence'' that it contributed to LIG GMSL change, but ''low agreement'' in the contribution magnitude.

Far-field GMSL records suggest that the AIS contributed to LIG sea level from 129.5–125 ka (Figure 9.18) but direct evidence is sparse. Thinning of part of the WAIS is interpreted from a 130–80 ka hiatus in the Patriot Hills horizontal ice core record ( [[#Turney--2020|Turney et al., 2020]] ). Marine sediment records suggest a dynamic response of the Wilkes Subglacial Basin (WSB) of the EAIS during this period, indicating a response time scale of 1000–2500 yr ( [[#Wilson--2018|Wilson et al., 2018]] ), consistent with modelling studies ( [[#Mengel--2014|Mengel and Levermann, 2014]] ; [[#Golledge--2017b|Golledge et al., 2017b]] ; [[#Sutter--2020|Sutter et al., 2020]] ). Isotopic changes in the Talos Dome ice core are inconsistent with local surface lowering, limiting retreat to 0.4–0.8 m SLE from this sector ( [[#Sutter--2020|Sutter et al., 2020]] ). Ice-sheet models forced with unmodified atmosphere–ocean models ( [[#Goelzer--2016|Goelzer et al., 2016]] ; [[#Clark--2020|Clark et al., 2020]] ) simulate 3–4.4 m SLE mass loss, primarily from the WAIS, with no retreat in WSB (e.g., Figure 9.18). Models forced with proxy-based or ad hoc LIG ocean temperature anomalies ( [[#DeConto--2016|DeConto and Pollard, 2016]] ; [[#Sutter--2016|Sutter et al., 2016]] ) indicate collapse of West Antarctica under 2°C–3°C ocean forcing yielding 3–7.5 m sea level contribution, but modest or no retreat in the WSB. Based on ''limited evidence'' and ''limited agreement'' between models, there is ''low confidence'' in both the magnitude and timing of LIG mass loss from the AIS.

In summary, paleo-environmental and modelling studies indicate that, under past warming of the level achieved during the LIG (ca. 0.5°C–1.5°C), it is ''likely'' that both the Greenland and Antarctic ice sheets responded dynamically over multiple millennia ( ''high confidence'' ).

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