Thinning-induced glacier deceleration in the Zanskar Himalayas

Abstract. Trends in surface velocity provide insight into glacier response to climate change as well as local drivers of ice dynamics. The Zanskar Himalayas are heavily glacierised, and retreating glaciers pose a threat to local and regional water security. Satellite image correlation provides a tool for observing surface velocity over multiple glaciers in a remote and challenging area for field work, providing key observations for tracking changes in this important region. This study provides a comprehensive analysis of long-term (1992–2023) interannual glacier surface velocity and elevation change for 12 selected glaciers with varied morphological characteristics in the Zanskar Basin of the Ladakh Himalayas. We show a statistically significant, region-wide deceleration in glacier flow, with a mean rate of 2.43 m yr−1 per decade. This slowdown is accompanied by progressively increasing surface thinning, from ∼ 0.22 m yr−1 between 2000 and 2005 to ∼ 0.57 m yr−1 by 2015 and 2020, with the strongest thinning and velocity reductions occurring at lower elevations. The spatial and temporal agreement between thinning and velocity decline, together with the expected reduction in driving stress associated with thinning, suggests that ice mass loss is the dominant control on the observed deceleration. Glacier-specific characteristics such as geometry, topography, debris cover, and terminus type further influenced glacier response. For example, lake-terminating glaciers exhibited local increases in ice velocity near their termini. Overall, our results demonstrate a sustained multi-decadal slowdown in glacier flow that is closely linked to ice mass loss, providing new insight into the mechanisms governing glacier dynamics in the Zanskar Himalaya under ongoing climatic change.