Projected Depletion of High Mountain Asia Glaciers by 2100: Linking Mass Loss to Asynchronous Peak Runoff

Glacierized-catchment runoff (GR) across High Mountain Asia (HMA) is approaching peak water, but the timing is not uniform. Glacier size, elevation, and regional climate push that threshold to different points in time, with direct implications for downstream water security. We used OGGM v1.6.1, forced by bias-corrected GSWP3-W5E5 historical data and an ensemble of 13 CMIP6 GCMs under four SSP scenarios, to simulate glacier mass balance, dynamics, and glacierized-catchment runoff across 17 major HMA basins from 1940 to 2100. By 2100, HMA-wide glacier mass declines by 70.7 ± 9.6%, relative to 2000 levels, and total GR falls by 10 ± 6.5% between the early (2001–2020) and late (2081–2100) century periods. Small, low-elevation glaciers have already passed peak runoff in many basins, some before 2020, and are losing volume rapidly. Large, high-elevation glaciers continue buffering downstream flows well into the late 21st century, with basin-wide peaks as late as 2058 in Tarim and 2064 in the Amu Darya, and individual large high-elevation glacier classes peaking as late as 2083 in Tarim under high-emission scenarios. We identify three distinct peak-GR regimes: early, transitional, and delayed, driven by the size and elevation composition of glacier classes within each basin, rather than average glacier behavior. Basin-averaged projections may obscure important class-specific runoff transitions, underscoring the need for class-resolved, basin-specific water resource planning across HMA