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

==== 9.6.2.1 Mid-Pliocene Warm Period ====

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During the mid-Pliocene Warm Period (MPWP), GMST was 2.5°C–4°C warmer than 1850–1900 ( ''medium confidence'' ) and GMSL was between 5 and 25 m higher than today ( ''medium confidence'' ) (Table 9.6 and [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.3|Section 2.3.3.3]] ). The AR5 ( [[#Masson-Delmotte--2013|Masson-Delmotte et al., 2013]] ) concluded that ice-sheet models consistently produce near-complete deglaciation of the Greenland and West Antarctic ice sheets, and multi-meter loss of the East Antarctic Ice Sheet (EAIS) in response to MPWP climate conditions. Studies since AR5 have yielded a consistent but broader range, due in part to larger ensembles exploring more parameters ( [[#DeConto--2016|DeConto and Pollard, 2016]] ; [[#Yan--2016|Yan et al., 2016]] ; [[#DeConto--2021|DeConto et al., 2021]] ). Partly on the basis of these studies, SROCC proposed a ‘plausible’ upper bound on GMSL of 25 m ( ''low confidence'' ) with evidence suggesting an Antarctic contribution of anywhere between 5.4 and 17.8 m.

The MPWP climate had substantial polar amplification, up to 8°C above pre-industrial levels in Arctic Russia ( [[IPCC:Wg1:Chapter:Chapter-7#7.4.4.1|Section 7.4.4.1]] ; [[#Fischer--2018|Fischer et al., 2018]] ). Ice-sheet model simulations indicate that Northern Hemisphere glaciation was limited to high-elevation regions in eastern and southern Greenland ( ''medium confidence'' ) (Figure 9.17; [[#De%20Schepper--2014|De Schepper et al., 2014]] ; [[#Yan--2014|Yan et al., 2014]] ; [[#Koenig--2015|Koenig et al., 2015]] ; [[#Dowsett--2016|Dowsett et al., 2016]] ; [[#Berends--2019|Berends et al., 2019]] ) with Northern Hemisphere glaciation only becoming more widespread from the (cooler) late Pliocene ( [[#Bachem--2017|Bachem et al., 2017]] ; [[#Blake-Mizen--2019|Blake-Mizen et al., 2019]] ; [[#Knutz--2019|Knutz et al., 2019]] ; [[#Sánchez-Montes--2020|Sánchez-Montes et al., 2020]] ). Southern Hemisphere glaciation was characterized by an Antarctic Ice Sheet (AIS) reduced in volume from the present ( ''medium confidence'' ) (Figure 9.18; [[#Dowsett--2016|Dowsett et al., 2016]] ; [[#Berends--2019|Berends et al., 2019]] ; [[#Grant--2019|Grant et al., 2019]] ; [[#Miller--2020|Miller et al., 2020]] ) with mountain ice fields in the Andes of South America ( [[#De%20Schepper--2014|De Schepper et al., 2014]] ). Ice-sheet models are inconsistent in the magnitude of the sea level contribution from Antarctica ( [[#DeConto--2016|DeConto and Pollard, 2016]] ; [[#Yan--2016|Yan et al., 2016]] ; [[#Golledge--2017b|Golledge et al., 2017b]] ; [[#Berends--2019|Berends et al., 2019]] ; [[#DeConto--2021|DeConto et al., 2021]] ) but near-field sedimentological reconstructions support precessionally modulated and eccentricity-paced multi-metre sea level contributions from the Wilkes Subglacial Basin over 3–5 kyr ( [[#Patterson--2014|Patterson et al., 2014]] ; [[#Bertram--2018|Bertram et al., 2018]] ). Insummary, under a past warming level of around 2.5°C–4°C, ice sheets in both hemispheres were reduced in extent compared to present ( ''high confidence'' ). Proxy-based evidence ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.3|Section 2.3.3.3]] ) combined with numerical modelling indicates that, on millennial time scales, the GMSL contribution arising from ice sheets was &gt;5 m ( ''high confidence'' ) or &gt;10 m ( ''medium confidence'' ) (Figures 9.17 and 9.18; [[#Moucha--2017|Moucha and Ruetenik, 2017]] ; [[#Berends--2019|Berends et al., 2019]] ; [[#Dumitru--2019|Dumitru et al., 2019]] ).

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