Energy and mass balance sensitivities of glaciers to climate across High Mountain Asia

Meltwater from High Mountain Asia (HMA) provides water resources to large downstream populations of central, southern and eastern Asia. Recent decades have seen critical changes in the health of the high mountain cryosphere of the region, which poses risks to those dependent upon its water supply. Glaciers of HMA have been observed to change mass at varying rates regionally, likely caused by a combination of heterogeneous climate change and region-specific glacier sensitivities. Regional studies on glacier sensitivities have been limited by computational power to rely on degree day models with higher dependence on calibration and equifinality issues. The same computational limits and data availability have limited energy balance studies to short periods, localised domains or coarsely resolved glacier regions. Explicit representation of glacier specific hypsometry and surface characteristics allows the inclusion of non-linear sensitivity of mass balance with elevation, as well as the inclusion of effects of debris cover. Here we provide a new regional picture of HMA glacier sensitivity to climate using a physically-based high resolution energy and mass balance model (Tethys & Chloris).

We run the model for 50 glaciers in each glacier subregion of HMA, chosen to be representative in terms of glacier area, median elevation, debris area and mean debris thickness. Each glacier is modelled using a clustering of points to discretise its surface, where the number of points is optimised depending on glacier size. We first build a baseline of glacier response to present climate (2000-2010) by forcing the model with bias-corrected hourly ERA5-Land data. For the bias correction, a precipitation factor and temperature bias is optimised for each glacier so that model results match against remotely sensed observations of glacier mass balance, snow line altitude and surface albedo.

To determine glacier sensitivities and their drivers, we perturb temperature and precipitation annually to quantify the mass balance changes at the glacier and regional scale. Further experiments perturbing seasonal climatic signals reveal the relevance and relative impacts of temperature and precipitation changes for regions previously identified as experiencing anomalous mass balance behaviour, such as the Pamir-Karakoram. We analyse the contribution of the main energy fluxes and melt components to the glacier-wide mass balance and quantify the relative influence of precipitation solid fraction and sublimation under these different climate perturbation scenarios. We study the sensitivity of the accumulation area ratio and the glacier equilibrium line altitude as indicators of the regional patterns in glacier health, its year-to-year variability and susceptibility to abrupt shifts under climatic extremes. With this, we identify and explain which regions may be most susceptible to future climate change signals, with an unprecedented attribution to the underlying glacier characteristics and changes in physical processes.