Health and sustainability of glaciers in High Mountain Asia

Glaciers and snow in High Mountain Asia (HMA) are vital water sources for approximately 250 million people. The region's vulnerability to climate change necessitates a comprehensive understanding of its ice and snow reservoirs. However, limited access and sparse field measurements have hindered accurate assessment of annual accumulation and ablation, especially at high altitudes. Existing models struggle with over-parameterization and the lack of spatially resolved data, failing to adequately capture complex processes like avalanching and debris cover influence. While recent remote sensing has improved regional-scale understanding, it cannot resolve spatial mass balance patterns across individual glaciers. This study aims to provide spatially distributed specific mass balance (SMB) data for HMA glaciers (2000-2016) to analyze glacier health, equilibrium line altitudes (ELAs), accumulation area ratios (AARs), and the sustainability of glacier ablation in major river basins, ultimately projecting future ice volume and discharge changes.

Previous research highlights the importance of HMA glaciers for water security and their vulnerability to climate change. Studies have shown challenges in accurately assessing glacier mass balance due to limited access and the complexities of high-altitude environments. Remote sensing studies have advanced our understanding, but high-precision elevation change measurements alone cannot resolve the spatial patterns of mass balance. Existing models often oversimplify mass balance, assuming linear altitudinal gradients and neglecting processes such as avalanching and debris cover, which are prevalent in HMA. The scarcity of in-situ measurements leads to significant uncertainty in projection of glacier volume change. This study builds upon previous work by utilizing a novel approach to account for the mass redistribution due to ice flow and better capture the complex dynamics of HMA glaciers.

This study employs a continuity equation approach to determine spatially distributed specific mass balance (SMB) for HMA glaciers from 2000-2016. The method combines estimates of ice thickness, ice surface motion observations, and a Monte Carlo-based depth-averaged correction factor to account for mass redistribution due to ice flow. Elevation change measurements are used in conjunction with the calculated ice flux divergence to derive altitudinal SMB. The method was first validated by comparison with available surface mass balance measurements and existing remote sensing-derived estimates. A total of 5527 glaciers were analyzed, representing 71% of the total volume of glaciers larger than 2 km². The density of snow, firn, and ice was carefully considered to account for surface density variations in accumulation and ablation zones. A Monte Carlo analysis was used to assess uncertainties in the input data and methods. The derived SMB data were then used to calculate glacier-specific ELAs and AARs, and to partition annual glacier ablation into “balance” and “imbalance” components. Finally, a glacier retreat and advance parameterization was used to estimate the changes in ice volume and ablation by 2100 implied by the current mass balance regimes. This involved iteratively updating ice thickness, glacier extent, SMB and elevation datasets with an annual timestep, ultimately projecting the volume change over 200 years.

The study revealed significant imbalances in the mass balance of HMA glaciers. Only 60% ± 10% of regional annual ablation was compensated by accumulation. 41% of glaciers accumulated mass over less than 20% of their area. The area-weighted mean ELA for the entire HMA region was 5283 m a.s.l., with considerable variations between glaciers. A substantial portion of glaciers (16%) showed no accumulation area, while 32% had very small accumulation areas. The AAR varied significantly across the region, reflecting the heterogeneous health of glaciers. The Karakoram Anomaly was apparent in some subregions exhibiting high AARs, but its influence was less clear in ELA distribution. Analysis of glacier ablation in major basins showed that 40% ± 11% was unsustainable. Basins fed by Karakoram Anomaly glaciers showed higher sustainability (over 50% balanced ablation), but even these basins face long-term reductions in ice mass. The Indus basin, a crucial water source, showed 65% ± 23% balanced ablation. Other basins, like the Ganges-Brahmaputra, showed mostly imbalanced ablation. Based on the current mass balance regimes, the study projects a -23% ± 1% change in glacier volume by 2100, with significant volume reduction in most subregions except the Kunlun Shan. Total annual ablation rates are projected to decrease by -28% ± 6%. The Karakoram Anomaly partly obscures mass losses, but without further climate warming, 35% of glaciers will lose at least half their volume by 2100.

The findings highlight the widespread imbalance in HMA glaciers, challenging the assumption of a uniform response to climate change. The substantial portion of unsustainable ablation underscores the vulnerability of water resources dependent on glacier meltwater. The contrast between basins influenced by the Karakoram Anomaly and others emphasizes the regional heterogeneity in glacier health. The study's projection of significant ice volume and ablation reductions, even without further warming, emphasizes the urgency of mitigation and adaptation efforts. The results have significant implications for water resource management, particularly in regions heavily reliant on glacier meltwater. The findings call for improved glacier modeling to better represent regional processes, enhancing prediction accuracy for future glacier changes.

This study provides a comprehensive assessment of HMA glacier health and sustainability using a novel spatially explicit approach. The findings reveal significant imbalances, unsustainable ablation in many basins, and substantial projected ice loss even without further warming. This emphasizes the need for improved glacier models capable of incorporating complex processes and for improved monitoring to reduce uncertainty. Future research should focus on improving model representation of debris cover, avalanches, and frontal ablation. Continued monitoring and refinement of input data are critical for accurate predictions of future glacier change.

The study's reliance on existing datasets (ice thickness, velocity, elevation change) introduces uncertainties, particularly in less well-sampled areas. The exclusion of glaciers smaller than 2 km² might underestimate the overall volume change. Assumptions made about basal conditions and ice rheology could affect SMB estimates. The projection of future volume change is based on a simplified model and may not capture all aspects of complex glacial dynamics. The analysis of the Karakoram Anomaly's influence is limited by the relatively small sample size of glaciers in that area.