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

===== 9.4.2.3.2 Sub-shelf melting =====

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The SROCC highlighted that an important ongoing deficiency in projections of Antarctic sub-shelf melting is the lack of ice–ocean coupling in most continental-scale studies. Increased basal melting is mainly caused by warmer CDW ( [[#9.2.2.3|Section 9.2.2.3]] ) on the continental shelves, and warming surface waters intruding under ice shelves ( [[#Naughten--2018|Naughten et al., 2018]] ). Predicting whether or not open ocean water masses will freely penetrate ice shelf cavities, or will be partially blocked by ocean density gradients, is complex ( [[#Wåhlin--2020|Wåhlin et al., 2020]] ); while melting related to CDW inflow is currently dominant in the Amundsen Sea Embayment, melt in other embayments is limited by deep inflows of high-salinity shelf water or seasonally warmed shallow incursions of Antarctic Surface Water ( [[#Stewart--2019|Stewart et al., 2019]] ; [[#Adusumilli--2020|Adusumilli et al., 2020]] ). There is little consensus regarding future change in CDW ( [[#9.2.2.3|Section 9.2.2.3]] ), and more generally ''low confidence'' in future change in the temperature of Antarctic ice-shelf cavities ( [[#9.2.3.2|Section 9.2.3.2]] ).

The response of sub-shelf melting to ocean warming is also poorly constrained. A key unknown is whether, and when, cold ice-shelf cavities might become more similar to the Amundsen Sea Embayment, not only in ocean temperature but also ice–ocean heat exchange, which depends on the cavity geometry and ocean circulation ( [[#Little--2009|Little et al., 2009]] ). Only two ocean models with ice-shelf cavities have been used to make sub-shelf basal melting projections for Special Report on Emissions Scenarios and Representative Concentration Pathway (RCP) scenarios ( [[#Hellmer--2012|Hellmer et al., 2012]] ; [[#Timmermann--2013|Timmermann and Hellmer, 2013]] ; [[#Timmermann--2017|Timmermann and Goeller, 2017]] ; [[#Naughten--2018|Naughten et al., 2018]] ). The FESOM simulation, forced by a CMIP5 multi-model mean under RCP8.5, projects a 90% increase in melting (Figure 9.19), although this could be overestimated due to an underestimation of present-day melt rates ( [[#9.4.2.2|Section 9.4.2.2]] ; [[#Naughten--2018|Naughten et al., 2018]] ). The temperature–melt relationship was parameterized by ISMIP6 in terms of heat exchange velocity in m a <sup>–1</sup> , and by LARMIP-2 as basal melt sensitivity in m a <sup>–1</sup> °C <sup>–1</sup> (Box 9.3; [[#Jourdain--2020|Jourdain et al., 2020]] ; [[#Levermann--2020|Levermann et al., 2020]] ; [[#Reese--2020|Reese et al., 2020]] ), and both vary widely around the continent, depending on cavity type. Median values of ISMIP6 heat exchange velocity vary by a factor of 5–10 when calibrating to either mean Antarctic or high Pine Island Glacier observed melt rates ( [[#9.4.2.2|Section 9.4.2.2]] ; Box 9.3; [[#Jourdain--2020|Jourdain et al., 2020]] ). Basal melt sensitivities near the grounding line estimated by [[#Reese--2020|Reese et al. (2020)]] with a box model of ocean overturning range from 3.9 m a <sup>–1</sup> °C <sup>–1</sup> for the Weddell Sea to 10.5 m a <sup>–1</sup> °C <sup>–1</sup> for the Amundsen Sea region, with a continental mean of 5.3 m a <sup>–1</sup> °C <sup>–1</sup> . Similarly high Amundsen Sea sensitivities are estimated in coupled ice–ocean simulations of Thwaites Glacier (mean 9.4 m a <sup>–1</sup> °C <sup>–1</sup> ; range 6–16 m a <sup>–1</sup> °C <sup>–1</sup> ) ( [[#Seroussi--2017|Seroussi et al., 2017]] ). These large variations lead to large differences in basal melt rates and projected sea level contributions when applied to the whole ice sheet in ISMIP6 and LARMIP-2 (Box 9.3). Projections of melt rates from the two ISMIP6 calibrations are higher than those from FESOM, driven by a CMIP5 multi-model mean (Figure 9.19; [[#Jourdain--2020|Jourdain et al., 2020]] ). The ISMIP6 ensemble mostly uses the mean Antarctic calibration, but includes some simulations with the Pine Island Glacier calibration, and the ISMIP6 emulator samples more of these higher values; LARMIP-2 uses basal melt sensitivities (7–16 m a <sup>–1</sup> °C <sup>–1</sup> ) consistent with estimates for the Amundsen Sea Embayment. Due to the limited availability of cavity-resolving ocean models, and the wide regional variation in estimates of basal melt sensitivity to ocean temperature, there is only ''low confidence'' in projected future sub-ice-shelf melt rates. The impact of this uncertainty on AIS model projections to 2100 is discussed in [[#9.4.2.5|Section 9.4.2.5]] .

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