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

==== 9.5.1.2 Model Evaluation ====

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Since AR5, glacier mass projections have been coordinatedby the Glacier Model Intercomparison Project (GlacierMIP; [[#Hock--2019a|Hock et al., 2019a]] ; [[#Marzeion--2020|Marzeion et al., 2020]] ). The SROCC ( [[#Hock--2019b|Hock et al., 2019b]] ) relied on six global-scale glacier models based on previously published glacier model projections ( [[#Hock--2019a|Hock et al., 2019a]] ). It found with ''high confidence'' that glaciers will lose substantial mass by the end of the century, but assigned ''medium confidence'' to the magnitude and timing of the projected glacier mass loss, because of the simplicity of the models, the limited observations in some regions to calibrate them, and the diverging initial glacier volumes.

Since SROCC, [[#Marzeion--2020|Marzeion et al. (2020)]] projected 21st century global-scale glacier mass changes based on seven global-scale and four regional-scale glacier models (Annex II). All models used the same initial and boundary conditions, forming a more coherent ensemble of projections compared to SROCC. Nevertheless, challenges remain because of scarcity of glacier thickness, surface mass balance (SMB) and frontal ablation data for model calibration, but also due to uncertainties in glacier outlines, surface elevations and ice velocities. The global SMB models are of varying complexity, including mass balance sensitivity approaches (van de Wal and Wild, 2001), temperature-index methods ( [[#Anderson--2012|Anderson and Mackintosh, 2012]] ; [[#Marzeion--2012|Marzeion et al., 2012]] ; [[#Radić--2014|Radić et al., 2014]] ; [[#Huss--2015|Huss and Hock, 2015]] ; [[#Kraaijenbrink--2017|Kraaijenbrink et al., 2017]] ; [[#Maussion--2019|Maussion et al., 2019]] ; [[#Zekollari--2019|Zekollari et al., 2019]] ; [[#Rounce--2020|Rounce et al., 2020]] ) and simplified energy balance calculations ( [[#Sakai--2017|Sakai and Fujita, 2017]] ; [[#Shannon--2019|Shannon et al., 2019]] ). Compared to simpler, empirical parametrizations, full energy-balance models are not necessarily the most appropriate choice for simulating future glacier response to climate change, even at the local scale ( [[#Réveillet--2017|Réveillet et al., 2017]] , 2018), because of parameter and forcing uncertainties. All models account for glacier retreat and advance, but only two models ( [[#Anderson--2012|Anderson and Mackintosh, 2012]] ; [[#Huss--2015|Huss and Hock, 2015]] ) include frontal ablation.

Secondary processes such as debris-cover thickening (e.g., [[#Herreid--2020|Herreid and Pellicciotti, 2020]] ), albedo changes due to light-absorbing particles (e.g., [[#Magalhães--2019|Magalhães et al., 2019]] ; [[#Williamson--2019|Williamson et al., 2019]] ), trends of refreezing and water storage in firn (e.g., [[#Ochwat--2021|Ochwat et al., 2021]] ), dynamic instabilities such as surges (e.g., [[#Thøgersen--2019|Thøgersen et al., 2019]] ) or glacier collapse (e.g., [[#Kääb--2018|Kääb et al., 2018]] ), are not represented in global glacier models, resulting in both underestimated and overestimated sensitivity to warming that is currently not possible to quantify. Furthermore, challenges for future projections are caused by the low-resolution and high-spatial variability at sub-grid scale of the precipitation amount provided by general circulation models (GCMs), which requires downscaling to the spatial scale of a glacier ( [[#Maussion--2019|Maussion et al., 2019]] ; [[#Zekollari--2019|Zekollari et al., 2019]] ; [[#Marzeion--2020|Marzeion et al., 2020]] ). In summary, in agreement with SROCC, progress in global scale glacier modelling efforts allows ''medium confidence'' in the capability of current-generation glacier models to simulate the magnitude and timing of glacier mass changes as a response to climatic forcing.

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