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

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 accumulation 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. • This study examines the shift from balanced to imbalanced glacier mass balance in the Upper Bhagirathi Basin, Garhwal Himalaya (1973–2024) using multi-temporal DEMs and the geodetic method, highlighting key findings for the first time. • It provides the region's longest record of glacier health (1973-2024) and highlights the accelerating impact of climate warming on glacier dynamics. • Reveal a significant shift in glacier mass balance trends, with an initial period (1973-2000) of stability (-0.10 ± 0.04 m w.e. a⁻¹) followed by increasing mass loss (-0.62 ± 0.16 m w.e. a⁻¹) particularly in recent decades (2015-2024). • Notably, thinning and mass loss expanded from the terminus to the upper part of the ablation and high-altitude accumulation zones in recent decades (2015-2024). • This study provides the first account of surge activity identified in a Chaturangi Glacier tributary, highlighting localized glacier dynamics.