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

===== 9.6.3.2.9 Glacial isostatic adjustment and other drivers of vertical land motion =====

<div id="h4-15-siblings" class="h4-siblings"></div>

Glacial Isostatic Adjustment (GIA) leads to vertical land motion (VLM; see Box 9.1) and changes in sea surface height, both of which contribute to RSL change. GIA uncertainty is caused by uncertainty in the rheological structure of the solid Earth, which drives the longer-term viscous Earth deformation, as well as uncertainty in the modelled global ice history (e.g., [[#Whitehouse--2018|Whitehouse, 2018]] ). In AR5 and SROCC, GIA contributions to RSL change were calculated using a sea level equation solver with an ice-sheet history taken as the mean of the ICE5G ( [[#Peltier--2015|Peltier et al., 2015]] ) and ANU ( [[#Lambeck--2014|Lambeck et al., 2014]] ) ice-sheet models. Since AR5, new global models are emerging that more rigorously treat ice and Earth structure uncertainty ( [[#Caron--2018|Caron et al., 2018]] ). However, there is also a growing recognition that lateral variations in Earth structure limit the utility of global models that treat the solid Earth as though it were laterally uniform ( [[#Love--2016|Love et al., 2016]] ; [[#Huang--2019|Huang et al., 2019]] ; T. [[#Li--2020|]] [[#Li--2020|]] [[#Li--2020|Li et al., 2020]] ).

As noted by SROCC, VLM from sources other than GIA – including tectonics and mantle dynamic topography, volcanism, compaction, and anthropogenic subsidence – can be locally important, producing VLM rates comparable to or greater than rates of GMSL change. Complete global projections of these processes are not available because of the small spatial scales, the sensitivity of subsidence to local human activities, and the stochasticity of tectonics ( [[#Wöppelmann--2016|Wöppelmann and Marcos, 2016]] ; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ). Therefore, integrated RSL projections to date have either included only the component of VLM associated with GIA (as in AR5 and SROCC), or used a constant long-term background rate of change (including both GIA and other long-term drivers of VLM) estimated from historical tide gauge trends (e.g., [[#Kopp--2014|Kopp et al., 2014]] ). The updated projections use the second approach and extrapolate the field of long-term background rates of RSL change, including long-term VLM derived from tide gauges, to global coverage using a spatio-temporal statistical approach (Supplementary Material 9.SM.4.6; [[#Kopp--2014|Kopp et al., 2014]] ). The combined GIA and long-term VLM is assumed to be scenario independent and constant over the projected period. In areas where rapid subsidence occurs in a cluster of tide gauges (e.g., the western Gulf of Mexico), the associated rates are interpolated between the tide gauges. In areas where the available tide gauges exhibit large, tectonically driven VLM that changes considerably in rate over short distances (e.g., Alaska and the Bering Strait), a sizable uncertainty propagates into the RSL projections (Figure 9.26). Rates of RSL rise are likely to be underestimated due to subsidence in shallow strata that are not recorded by tide gauges ( [[#Keogh--2019|Keogh and Törnqvist, 2019]] ) and in some locations may therefore be minimum values, especially if anomalously high subsidence rates associated with fluid extraction are also considered (e.g., [[#Minderhoud--2017|Minderhoud et al., 2017]] ). Therefore, depending on location, there is ''low'' to ''medium confidence'' in the GIA and VLM projections employed in this Report. In many regions, higher-fidelity projections would require more detailed regional analysis.

<div id="9.6.3.3" class="h3-container"></div>

<span id="sea-level-projections-to-2150-based-on-shared-socio-economic-pathway-scenarios"></span>