Tracing glacier changes since the 1960s on the south slope of Mt. Everest (central Southern Himalaya) using optical satellite imagery

This contribution examines glacier changes on the south side of Mt. Everest from 1962 to 2011 considering five intermediate periods using optical satellite imagery. The investigated glaciers cover ∼ 400 km2 and present among the largest debris coverage (32%) and the highest elevations (5720 m) of the world. We found an overall surface area loss of 13.0 ± 3.1% (median 0.42 ± 0.06 % a^-1), an upward shift of 182 ± 22 m (3.7 ± 0.5 m a^-1) in snow-line altitude (SLA), a terminus retreat of 403 ± 9 m (median 6.1 ± 0.2 m a^-1), and an increase of 17.6 ± 3.1% (median 0.20 ± 0.06% a^-1) in debris coverage between 1962 and 2011. The recession process of glaciers has been relentlessly continuous over the past 50 years. Moreover, we observed that (i) glaciers that have increased the debris coverage have experienced a reduced termini retreat (r = 0.87, p < 0.001). Furthermore, more negative mass balances (i.e., upward shift of SLA) induce increases of debris coverage (r = 0.79, p < 0.001); (ii) since early 1990s, we observed a slight but statistically insignificant acceleration of the surface area loss (0.35 ± 0.13% a^-1 in 1962-1992 vs 0.43 ± 0.25% a^-1 in 1992-2011), but an significant upward shift of SLA which increased almost three times (2.2 ± 0.8 m a^-1 in 1962-1992 vs 6.1 ± 1.4 m a^-1 in 1992-2011). However, the accelerated shrinkage in recent decades (both in terms of surface area loss and SLA shift) has only significantly affected glaciers with the largest sizes (> 10 km2), presenting accumulation zones at higher elevations (r = 0.61, p < 0.001) and along the preferable south-north direction of the monsoons. Moreover, the largest glaciers present median upward shifts of the SLA (220 m) that are nearly double than that of the smallest (119 m); this finding leads to a hypothesis that Mt. Everest glaciers are shrinking, not only due to warming temperatures, but also as a result of weakening Asian monsoons registered over the last few decades. We conclude that the shrinkage of the glaciers in south of Mt. Everest is less than that of others in the western and eastern Himalaya and southern and eastern Tibetan Plateau. Their position in higher elevations have likely reduced the impact of warming on these glaciers, but have not been excluded from a relentlessly continuous and slow recession process over the past 50 years.