Increased up-glacier thinning in four major glaciers of High Mountain Asia revealed by geodetic mass balance estimates

Given its profound implications for future water security, the response of High Mountain Asia (HMA) glaciers to climate change remains a topic of critical concern. While recent studies have primarily focused on small glaciers (with glacierised areas of less than 1 to ~20 km²) to understand the impact of climate change on HMA water resources, these studies assume that smaller glaciers are regionally representative. However, it has been shown that smaller glaciers respond differently due to their smaller accumulation area particularly during warm years and lesser hydrological contribution. On the other hand, large glaciers in a catchment often serve as major contributors to runoff, thus influencing long-term water availability. Hence, larger glaciers may offer a more representative understanding of regional changes and are essential for future water security.

In this study, we calculate the geodetic mass balance of four very large glaciers —Fedchenko (with a total area of 664 km²), Baltoro (809 km²), Bara Shigri (112 km²), and Gangotri (122 km²)— covering the period from 2009 to 2022. The analysis is conducted over two different time intervals, utilizing digital elevation models generated from ASTER stereo imagery. We examined distinct glacier surfaces — debris-covered, clean-ice, and accumulation areas — to discern mass loss patterns with elevations. Bias-corrected in-situ meteorological data along with surface ice velocity data (from ITS_LIVE and Sentinel-1) were used to elucidate recent mass balance patterns.

Results reveal an amplified mass loss rate during the recent period of ~2015-2022 compared to the preceding period of ~2009-2015. Bara Shigri is an exception, experiencing a slight reduction in mass loss during ~2015-2022. The increased mass loss is driven by rising local summer temperatures and declining winter precipitation. Thinning was prominent at higher elevations (> 5000 m a.s.l.) across all four glaciers, with its intensity increasing over time. This indicates warming in the accumulation areas of these glaciers. Furthermore, upper glacier areas near the equilibrium line altitude exhibited stable ice velocities in certain glaciers, and the lower ablation zones experienced gradual slowdown, likely due to persistent mass loss.

These findings highlight the propagation of up-glacier thinning in large HMA glaciers, indicating mass loss across higher elevations and underscoring the vulnerability of local river systems and water resources.