On the strongly imbalanced state of glaciers in the Sikkim, eastern Himalaya, India

This study evaluates multiple glacier parameters (length, area, debris cover, snowline altitude (SLA), glacial lakes, velocity, and surface elevation change) to comprehend the response of poorly understood glaciers of the Sikkim Himalaya to climate change. For the proposed task, 23 representative glaciers were selected from the region, and remotely acquired data from Landsat-TM/ETM/OLI (1991-2017), and Terra-ASTER (2007-2017) along with the SRTM DEMs were used for extraction of the various parameters. Results show that during 1991-2015 the studied glaciers have significantly retreated (17.78 ± 2.06 m a^-1), deglaciated (5.44 ± 0.87%), and experienced a considerable increase in SLA (~7 m a^-1) and debris cover (16.49 ± 2.96%). Glaciers slowed-down (by 24.90%) with sizable growth in number (23.81%) and area (48.78 ± 2.23%) of glacial lakes. They also exhibit a notable downwasting (-0.77 ± 0.08 m a^-1) during 2000-2007/17. The behavior of glaciers in the region is heterogeneous and found to be primarily determined by glacier size, debris cover and glacial lakes. Though a generalized mass loss is observed for both small- (<3 km²) and large-sized glaciers (>10 km²), they seem to adopt different mechanisms to cope with the ongoing climatic changes. While the first adjust mostly by retreat/deglaciation, the latter lose mass through downwasting. Comparing with other Himalayan regions, the magnitude of dimensional changes and debris growth are higher in the Sikkim. The SLA trends are comparable with the central and western Himalaya up to 2000, but a reverse trend is seen afterwards. Also, contrary to the western and central Himalaya, where glaciers are reported to have slowed-down in recent decade, the Sikkim glaciers have shown negligible deceleration after 2000. Climate analysis confirm almost double increase in summer temperature (24.47%) than winters (12.77%) during 1990-2016, which, given the 'summer-accumulation-type' nature of the Sikkim glaciers, seems to be the prime driver of the observed changes.