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

==== 9.4.2.3 Drivers of Future Antarctic Ice Sheet Change ====

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===== 9.4.2.3.1 Surface mass balance =====

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The AR5 projected a negative contribution from Antarctic surface mass balance (SMB) changes to sea level over the 21st century (i.e., mitigating sea level rise), due to increased snowfall associated with warmer air temperatures. Sensitivity of SMB to Antarctic surface air temperature change varied from 3.7 to 7% °C <sup>–1</sup> , and the sea level projections assumed a sensitivity of 5.1 ± 1.5% °C <sup>–1</sup> from CMIP3 era models ( [[#Gregory--2006|Gregory and Huybrechts, 2006]] ) to estimate SMB changes from Antarctic temperatures in the CMIP5 ensemble. Since the AR5, analyses of CMIP5 and CMIP6 models have found Antarctic temperature sensitivity for accumulation (precipitation minus sublimation) of 3.5 to 8.7% °C <sup>–1</sup> ( [[#Frieler--2015|Frieler et al., 2015]] ), for SMB of 6.0 to 9.9% °C <sup>–1</sup> ( [[#Previdi--2016|Previdi and Polvani, 2016]] ) and for precipitation of around 4 to 9% °C <sup>–1</sup> (±1 standard deviation ranges; [[#Bracegirdle--2020|Bracegirdle et al., 2020]] ). An accumulation sensitivity estimate derived from ice core data lies in the middle of the range, around 6% °C <sup>–1</sup> ( [[#Frieler--2015|Frieler et al., 2015]] ). These are consistent, within uncertainties, with each other and AR5, under the approximation that SMB is dominated by snowfall.

The AR5 found that the median and ''likely'' sea level contributions due to SMB from 1986–2005 to 2100 were –0.05 (–0.09 to –0.02) m under RCP8.5 and –0.02 (–0.05 to 0.00) m under RCP2.6. The SROCC did not present a separate SMB contribution, instead showing total Antarctic projections derived from ice-sheet models ( [[#9.4.2.5|Section 9.4.2.5]] ). Projections of the SMB contribution to sea level tend to be slightly more negative since AR5, due at least in part to the higher range in equilibrium climate sensitivity values in CMIP6 ( [[#Payne--2021|Payne et al., 2021]] ). Mean and ±1 standard deviation ranges for grounded Antarctic Ice Sheet SMB changes from 2000 to 2100 computed from CMIP5 models are –0.08 (–0.13 to –0.04) m sea level equivalent (SLE) for RCP8.5 and, similarly for CMIP6 models, are –0.07 (–0.11 to –0.03) m for SSP5-8.5 ( [[#Gorte--2020|Gorte et al., 2020]] ). The general circulation models (GCMs) used to drive ice-sheet models in ISMIP6 (Box 9.3) project mean grounded AIS SMB changes from 2005 to 2100 of –0.06 (range –0.08 to –0.03) m SLE under RCP8.5 for the six CMIP5 models ( [[#Seroussi--2020|Seroussi et al., 2020]] ) and –0.09 (range –0.10 to –0.07) m SLE under SSP5-8.5 for the four CMIP6 models, which have climate sensitivity values of 4.8°C –5.3°C ( [[#Payne--2021|Payne et al., 2021]] ). We apply the AR5 parametric AIS SMB model ( [[#9.6.3.2|Section 9.6.3.2]] ) to updated projections of global mean temperature from a two-layer energy budget emulator (Supplementary Material 7.SM.2), which gives a median –0.05 (5–95% range –0.07 to –0.02) m SLE for SSP5-8.5 ( [[#9.4.2.5|Section 9.4.2.5]] , Table 9.3), that is, similar to the AR5 assessment and slightly smaller than the CMIP6 estimate. This estimate is used to augment the LARMIP-2 dynamic projections (Box 9.3) in Sections 9.4.2.5 and 9.4.2.6. Overall, CMIP5 and CMIP6 GCM simulations of sea level fall by 2100 due to Antarctic SMB increases are around 2–4 cm greater than estimates derived with the statistical method used in AR5. Further details about projections of Antarctic temperature, precipitation and SMB are provided in Section Atlas.11.1.4, which assesses that, due to the challenges of model evaluation ( [[#9.4.2.2|Section 9.4.2.2]] ) and the possibility of increased meltwater runoff ( [[#Kittel--2021|Kittel et al., 2021]] ), there is only ''medium confidence'' that the future contribution of Antarctic SMB to sea level this century will be negative under all greenhouse gas emissions scenarios. Longer time scales are discussed in 9.4.2.6.

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===== 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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===== 9.4.2.3.3 Ice-shelf disintegration =====

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Antarctic ice shelves modulate grounded ice flow through buttressing, so their weakening or disintegration is crucial for the timing and magnitude of ice loss and onset of instabilities ( [[#9.4.2.4|Section 9.4.2.4]] ; Box 9.4). Projections of ice-shelf disintegration are uncertain in terms of atmospheric warming and the response of the shelf surface – that is, surface melting, and whether shelves then disintegrate due to hydrofracturing and flexing, or are resilient through refreezing or drainage ( [[#Bell--2018|Bell et al., 2018]] ). The SROCC stated it is not expected that widespread ice-shelf loss will occur before the end of the 21st century, but this was based on only one study, using a regional climate model forced by five GCMs ( [[#Trusel--2015|Trusel et al., 2015]] ), so there was ''low confidence'' in this assessment. The study of [[#DeConto--2016|DeConto and Pollard (2016)]] projected the appearance of extensive surface meltwater several decades earlier than [[#Trusel--2015|Trusel et al. (2015)]] and was therefore assessed to be too uncertain to include in SROCC projections of the AIS.

Since SROCC, further studies have highlighted the modelling uncertainties in this area. Coastal surface air temperature projections in CMIP6 models show large inter-model differences driven by sea ice retreat and exhibit more warming relative to global mean temperature under low emissions than high, due to delayed response of the Southern Ocean to stabilized emissions and stratospheric ozone recovery ( [[#Bracegirdle--2020|Bracegirdle et al., 2020]] ). The updated study of [[#DeConto--2021|DeConto et al. (2021)]] includes improvements to the climate simulations relative to those in [[#DeConto--2016|DeConto and Pollard (2016)]] , and the resulting surface meltwater projections are now consistent with [[#Trusel--2015|Trusel et al. (2015)]] . However, the net effect of meltwater feedbacks on ice shelves is uncertain. Ice discharge is expected to lead to surface ocean and atmosphere cooling: this increases ocean stratification and sub-shelf melting, but also reduces ice-shelf surface melting and delays hydrofracturing ( [[#Golledge--2019|Golledge et al., 2019]] ; [[#Sadai--2020|Sadai et al., 2020]] ; [[#DeConto--2021|DeConto et al., 2021]] ). The new studies are insufficient to change SROCC’s ''low confidence'' assessment on ice-shelf loss. The consequence of this uncertainty on projections is discussed in [[#9.4.2.5|Section 9.4.2.5]] and Box 9.4.

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