Editing IPCC:AR6/SROCC/Chapter-4 (section)

===== 4.2.3.3.1 Components of probabilistic global mean sea level projections =====

Thermal expansion: Global mean thermal expansion projections rely on coupled climate models projections (Kopp et al., 2014 <sup>[[#fn:r608|608]]</sup> ; Slangen et al., 2014a <sup>[[#fn:r609|609]]</sup> ; Jackson and Jevrejeva, 2016 <sup>[[#fn:r610|610]]</sup> ) or simple climate model projections (Perrette et al., 2013 <sup>[[#fn:r611|611]]</sup> ; Nauels et al., 2017b <sup>[[#fn:r612|612]]</sup> ; Wong et al., 2017 <sup>[[#fn:r613|613]]</sup> ) , and are substantively unchanged since AR5. For those studies relying on the CMIP5 GCM ensemble, interpretations of the model output differ mainly with regard to how the range is understood. For example, Kopp et al. (2014) <sup>[[#fn:r614|614]]</sup> , interprets the 5–95 percentile of CMIP5 values as a ''likely'' range of thermal expansion. The differences among the studies yield discrepancies smaller than 10 cm, e.g., Slangen et al. (2014a) <sup>[[#fn:r615|615]]</sup> use 20–36 cm in 2081–2100 with respect to 1986–2005, while (Kopp et al., 2014) project a ''likely'' range of 28–46 cm in 2081–2099 with respect to 1991–2009.

Glaciers: Projections of glacier mass change rely either on models of glacier SMB and geometry, forced by temperature and precipitation fields (Slangen and Van de Wal, 2011 <sup>[[#fn:r615|615]]</sup> ; Marzeion et al., 2012 <sup>[[#fn:r616|616]]</sup> ; Hirabayashi et al., 2013 <sup>[[#fn:r617|617]]</sup> ; Radić et al., 2014 <sup>[[#fn:r618|618]]</sup> ; Huss and Hock, 2015 <sup>[[#fn:r619|619]]</sup> ) , or simple scaling relationships with global mean temperature (Perrette et al., 2013 <sup>[[#fn:r620|620]]</sup> ; Bakker et al., 2017 <sup>[[#fn:r621|621]]</sup> ; Nauels et al., 2017a <sup>[[#fn:r622|622]]</sup> ) . Glacier mass change projections published since AR5, based on newly developed glacier models, confirm the overall assessment of AR5 (see also Section 4.2.3.2).

Land water storage: Projections of the GMSL rise contributions due to dam impoundment and groundwater withdrawal are generally either calibrated to hydrological models (e.g., Wada et al., 2012) or neglected. Recent coupled climate-hydrological modelling suggests that a significant minority of pumped groundwater remains on land, which may reduce total GMSL rise relative to studies assuming full drainage to the ocean (Wada et al., 2016 <sup>[[#fn:r623|623]]</sup> ) . Kopp et al. (2014) estimated land water storage based on population projections. However, there are no substantive updates to projections of the future land-water storage contribution to GMSL rise since AR5.

Ice sheets: GMSL projections in previous IPCC assessments were based on results from physical models of varying degree of complexity interpreted using expert judgment of the assessment authors (Meehl et al., 2007 <sup>[[#fn:r625|625]]</sup> ; Church et al., 2013 <sup>[[#fn:r626|626]]</sup> ) . AR5 (Church et al., 2013 <sup>[[#fn:r627|627]]</sup> ) used this approach and is partly based on the assessment of statistical-physical modelling of the Antarctic contribution (Little et al., 2013 <sup>[[#fn:r628|628]]</sup> ) . As an alternative to the model-based approach, several studies have applied structured expert elicitation to the GMSL contribution of ice sheets. This approach is based on a more formal expert elicitation protocol (Cooke, 1991 <sup>[[#fn:r629|629]]</sup> ; Bamber and Aspinall, 2013 <sup>[[#fn:r630|630]]</sup> ; Bamber et al., 2019 <sup>[[#fn:r631|631]]</sup> ) instead of physically based models. Combining the Antarctic contribution from the expert elicitation with the non-Antarctic components from AR5 as done for Table 4.4 leads to an estimated SLR of 0.95 m (median) for the high scenario and an upper ''likely'' range of 1.32 m (Figure 4.2), which is slightly higher than the process-based results. Results by Bamber and Aspinall (2013) <sup>[[#fn:r632|632]]</sup> were criticised because of their procedure for post-processing the expert data of individual ice sheets to a total sea level contribution from the ice sheets (de Vries and van de Wal, 2015; Bamber et al., 2016; de Vries and van de Wal, 2016) . Bamber et al. (2019) avoids this issue by eliciting expert judgments about ice sheet dependence. Alternatively, Horton et al. (2014) used a simpler elicitation protocol focusing on the total SLR rather than the ice sheet contribution alone. Finally, several probabilistic studies (e.g., Bakker et al., 2017; Kopp et al., 2017 <sup>[[#fn:r633|633]]</sup> ; Le Bars et al., 2017) used the results of a single ice sheet model study from DeConto and Pollard (2016) <sup>[[#fn:r634|634]]</sup> as the Antarctic contribution to GMSL.

Beside the total contribution of ice sheets several studies address the individual contribution of either Greenland or Antarctica (see Section 4.2.3.1.1 and 4.2.3.1.2) based on ice dynamical studies. Critical for GMSL projections is the low confidence in the dynamic contribution of the AIS beyond 2050 in previous assessments, as discussed in Section 4.2.3.1.2.

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