Transitioning from equilibrium of glacier mass balance in the Upper Bhagirathi Valley, Central Himalaya

Abstract

This study analyses key glaciers in the Upper Bhagirathi Valley, Central Himalaya, India, between 1973 and 2024 based on multi-date digital elevation models (DEMs) to examine the shift from balanced to imbalanced glacier mass budget. From 1973 to 2000, ice loss was evident mainly at the terminus (-0.10 ± 0.04 m w.e. a⁻¹), with a data gap (∼16 km²) in the higher ablation and accumulation zone of Gangotri Glacier, where elevation changes through the main glacier flow path indicated a mass stability or a minor positive mass balance. A marginal increase in mass (+0.11 ± 0.04 m w.e. a⁻¹) was observed from 2000 to 2015, followed by substantial losses during 2015–2020 (-0.27 ± 0.09 m w.e. a⁻¹) and 2020–2024 (-0.62 ± 0.39 m w.e. a⁻¹). Notably, thinning and ice loss expanded from the low-altitude to high-altitude zones (∼>5200 m asl). Gangotri Glacier's mass loss intensified over time, increasing from −0.17 ± 0.01 m w.e. a⁻¹ (1973–2000) to −1.03 ± 0.58 m w.e. a⁻¹ (2020–2024). Similar trends were observed in former tributary glaciers like Raktavaran and Chaturangi. The contact zone between debris-mantled and bare ice exhibited the highest thinning rates, highlighting the influence of surface debris on melting rates. This study reveals surge activity for the first time in the Chaturangi Glacier's tributary (1973–2006), highlighting localized dynamics. The recent shift to an imbalanced state underscores climate change’s impact on ice melt, threatening water supply and ecosystems, and highlighting the need for regular monitoring and adaptive strategies in the warming Himalaya.