Projected Glacier Mass Change in High Mountain Asia Through 2100 Using the Ice-sheet and Sea-level System Model

High Mountain Asia (HMA) glaciers are critical for downstream water resources and are an increasing contributor to global sea level rise. Yet, their 21st century evolution remains uncertain because of complex topography, heterogeneous climate forcing, and limited observational constraints. Ongoing glacier mass loss reflects a shift in their buffering capacity, with consequences for the timing and reliability of downstream meltwater supply, as well as for the stability of glacierized landscapes. Quantifying this glacier response requires physically based projections of glacier evolution that adequately capture ice flow and surface processes. Existing regional projections rely on simplified flow-line, shallow-ice flow models approximating ice-dynamic processes. In this study, we simulate the glacier mass change across HMA until the end of 2100 using the Ice-sheet and Sea-level System Model (ISSM). We use a MOno-Layer Higher-Order (MOLHO) ice flow approximation on a non-uniform triangular finite-element mesh at high spatial resolution (30–500 m), locally refined based on present-day observed ice velocities. Basal friction coefficients are inferred through inverse modeling by minimizing the misfit between observed and modeled surface velocities, with independent calibration performed for each HMA subregion using observations from 2022. We use a temperature index method for surface mass balance (SMB) that explicitly accounts for the spatial distribution of supraglacial debris cover. SMB is calibrated using geodetic estimates based on stereo-imagery for the period of 2000 to 2020. We project the glaciers evolution under SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5 climate scenarios, using bias corrected climate forcing from five CMIP6 global climate models referenced to ERA5-Land. Our results show that all regions experience mass loss by 2100, but hightlight pronounced spatial heterogeneity in glacier mass change across High Mountain Asia, with strongly varying magnitudes across subregions and climate scenarios. Under the low-emission scenario, projected mass loss remains between 11–40% compared to the glacier mass during 2000, whereas high-emission scenarios lead to substantial ice loss across most regions, ranging between 42–74% in regions such as Nyainqentanglha, Pamir, and eastern Hindu Kush. These results provide improved projections of HMA glacier change and offer valuable insights for assessing future water availability and supporting sustainable water-resource management in High Mountain Asia.