Weak precipitation, warm winters and springs impact glaciers of south slopes of Mt. Everest (central Himalaya) in the last two decades (1994-2013)

Studies on recent climate trends from the Himalayanrange are limited, and even completely absent athigh elevation ( >5000ma.s.l.). This study specifically exploresthe southern slopes of Mt. Everest, analyzing thetime series of temperature and precipitation reconstructedfrom seven stations located between 2660 and 5600ma.s.l.during 1994-2013, complemented with the data from allexisting ground weather stations located on both sides ofthe mountain range (Koshi Basin) over the same period.Overall we find that the main and most significant increasein temperature is concentrated outside of the monsoon period.Above 5000ma.s.l. the increasing trend in the timeseries of minimum temperature (C0.072 C yr?1) is muchstronger than of maximum temperature (C0.009 C yr?1),while the mean temperature increased by C0.044 C yr?1.Moreover, we note a substantial liquid precipitation weakening(?9.3mmyr?1) during the monsoon season. The annualrate of decrease in precipitation at higher elevations issimilar to the one at lower elevations on the southern sideof the Koshi Basin, but the drier conditions of this remoteenvironment make the fractional loss much more consistent(?47% during the monsoon period). Our results challengethe assumptions on whether temperature or precipitation isthe main driver of recent glacier mass changes in the region.The main implications are the following: (1) the negaivemass balances of glaciers observed in this region can bemore ascribed to a decrease in accumulation (snowfall) thanto an increase in surface melting; (2) the melting has onlybeen favoured during winter and spring months and closeto the glaciers terminus; (3) a decrease in the probabilityof snowfall (?10 %) has made a significant impact only atglacier ablation zone, but the magnitude of this decrease isdistinctly lower than the observed decrease in precipitation;(4) the decrease in accumulation could have caused the observeddecrease in glacier flow velocity and the current stagnationof glacier termini, which in turn could have producedmore melting under the debris glacier cover, leading to theformation of numerous supraglacial and proglacial lakes thathave characterized the region in the last decades.