ALBERT

Description: Studies on recent climate trends from the Himalayan range are limited, and even completely absent at high elevation. This contribution specifically explores the southern slopes of Mt. Everest (central Himalaya), analyzing the minimum, maximum, and mean temperature and precipitation time series reconstructed from seven stations located between 2660 and 5600m a.s.l. over the last twenty years (1994–2013). We complete this analysis with data from all the existing ground weather stations located on both sides of the mountain range (Koshi Basin) over the same period. Overall we observe that the main and more significant increase in temperature is concentrated outside of the monsoon period. At higher elevations minimum temperature (0.072 ± 0.011 °C a−1, p 〈 0.001) increased far more than maximum temperature (0.009 ± 0.012 °C a−1, p 〉 0.1), while mean temperature increased by 0.044 ± 0.008 °C a−1, p 〈 0.05. Moreover, we note a substantial precipitation weakening (9.3 ± 1.8mm a−1, p 〈 0.01 during the monsoon season). The annual rate of decrease at higher elevation is similar to the one at lower altitudes on the southern side of the Koshi Basin, but here the drier conditions of this remote environment make the fractional loss much more consistent (47% during the monsoon period). This study contributes to change the perspective on which climatic driver (temperature vs. precipitation) led mainly the glacier responses in the last twenty years. The main implications are the following: (1) the negative mass balances of glaciers observed in this region can be more ascribed to less accumulation due to weaker precipitation than to an increase of melting processes. (2) The melting processes have only been favored during winter and spring months and close to the glaciers terminus. (3) A decreasing of the probability of snowfall has significantly interested only the glaciers ablation zones (10%, p 〈 0.05), but the magnitude of this phenomenon is decidedly lower than the observed decrease of precipitation. (4) The lesser accumulation could be the cause behind the observed lower glacier flow velocity and the current stagnation condition of tongues, which in turn could have trigged melting processes under the debris glacier coverage, leading to the formation of numerous supraglacial and proglacial lakes that have characterized the region in the last decades. Without demonstrating the causes that could have led to the climate change pattern observed at high elevation, we conclude by listing the recent literature on hypotheses that accord with our observations.

Description: Studies on recent climate trends from the Himalayan range are limited, and even completely absent at high elevation (〉 5000 m a.s.l.). This study specifically explores the southern slopes of Mt. Everest, analyzing the time series of temperature and precipitation reconstructed from seven stations located between 2660 and 5600 m a.s.l. during 1994–2013, complemented with the data from all existing ground weather stations located on both sides of the mountain range (Koshi Basin) over the same period. Overall we find that the main and most significant increase in temperature is concentrated outside of the monsoon period. Above 5000 m a.s.l. the increasing trend in the time series of minimum temperature (+0.072 °C yr−1) is much stronger than of maximum temperature (+0.009 °C yr−1), while the mean temperature increased by +0.044 °C yr−1. Moreover, we note a substantial liquid precipitation weakening (−9.3 mm yr−1) during the monsoon season. The annual rate of decrease in precipitation at higher elevations is similar to the one at lower elevations on the southern side of the Koshi Basin, but the drier conditions of this remote environment make the fractional loss much more consistent (−47% during the monsoon period). Our results challenge the assumptions on whether temperature or precipitation is the main driver of recent glacier mass changes in the region. The main implications are the following: (1) the negative mass balances of glaciers observed in this region can be more ascribed to a decrease in accumulation (snowfall) than to an increase in surface melting; (2) the melting has only been favoured during winter and spring months and close to the glaciers terminus; (3) a decrease in the probability of snowfall (−10%) has made a significant impact only at glacier ablation zone, but the magnitude of this decrease is distinctly lower than the observed decrease in precipitation; (4) the decrease in accumulation could have caused the observed decrease in glacier flow velocity and the current stagnation of glacier termini, which in turn could have produced more melting under the debris glacier cover, leading to the formation of numerous supraglacial and proglacial lakes that have characterized the region in the last decades.

Description: 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.

Description: We contribute to the debate on glacial shrinkage in the Himalaya by analyzing glaciers in southern slopes of Mt. Everest that are characterized by extensive debris coverage and the highest elevation in the world. In this paper, we make a complete analysis from 1962 to 2011, considering five intermediate periods using optical satellite imagery. We found an overall surface area shrinkage of 13.0 ± 3%, an upward shift of 182 ± 9 m in snow-line altitude (SLA), a terminus retreat of 403 ± 9 m, and an increase of 17.6 ± 3% in debris coverage. The recession process of glaciers has been relentlessly continuous over the past fifty years. Furthermore, since early 1990s, we have observed an acceleration of the surface area shrinkage, which resulted in a median annual rate double that of the previous three decades (an increase from 0.27% a−1 to 0.46% a−1). Comparing the SLA over the same periods, it shifts upward with a velocity almost three times greater (from 2.2 ± 0.5 m a−1 to 6.1 ± 0.9 m a−1), which points to a worsening of the already negative mass balance of these glaciers. However, the increased recession velocity has only significantly affected glaciers with the largest sizes, which are located at higher altitudes and along the preferable south-oriented direction of the monsoons. Moreover, these glaciers present median upward shifts of the SLA that are double others; 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 decades. We conclude that the shrinkage of these glaciers is less than that of others in the Himalayan range. Their position in higher elevations have surely reduced the impact of warming on these glaciers, but have not been excluded from a relentlessly continuous and slow recession process over the past fifty years.