Glacier Variations In The Caucasus Mountains From 1960 To 2020

Glaciers are extremely sensitive to climate change,and as the climate warms,glaciers in all regions have been in a state of continuous retreat over the last hundred years,with more significant retreats since the 21st century.The Caucasus Mountains,one of the world’s most concentrated glacier areas,have experienced significant changes in glacier area over the last few decades.Glacial meltwater is an important freshwater resource in the Caucasus and is important for agricultural and industrial production such as irrigation and hydropower generation.However,due to the constraints of data sources,climate and topography,the existing glacier inventories in the Caucasus are not comprehensive in coverage,glacier boundaries are not fully accurate,and data sources are obtained over a large time span,which has led to controversies about the accuracy of the existing glacier inventories in the Caucasus,and the regional spatial and temporal trends need to be further explored It is important to clarify the response of glaciers to climate change.Therefore,it is important to improve and enhance the glacier inventory data in the region and to study glacier changes in recent decades on this basis in order to understand the recent spatial and temporal distribution of glaciers in the Caucasus mountains,and to understand the relationship between glacier distribution and climate and topography,and to provide a scientific basis for water resources assessment.In this study,43-view Landsat TM/ETM+/OLI optical remote sensing images and 6-view interferogram generated from Sentinel-1 image pairs were used to interpret the Caucasus mountain glacier boundaries in 2000 and 2020 by visual interpretation,the glacier inventory of the Caucasus Mountains in 1960 and 1986 provided by GLIMS were used to analyze and compare the characteristics of mountain glacier changes within different phases;The mass balance of glaciers in the Caucasus Mountains was estimated by SRTM DEM and ASTER DEM through geodesic methods;analysis of the mass balance changes in the long time series of representative glaciers Djankuat and Garabashi in the Caucasus Mountains using the mass balance communication provided by WGMS;the CRU grid point data for the period 1961-2020 were used to analyse the annual mean temperature and the trends in mean annual temperature and mean annual precipitation were analysed,and the reasons for the spatial and temporal distribution characteristics of glaciers in the Caucasus Mountains were analysed from a climatic perspective.The main conclusions are as follows:（1）In 2020,there were 1912 glaciers in the Caucasus Mountains,with a total glacier area of 1087.36±45.91 km².Under the influence of both topography and climate,the glaciers in the Caucasus Mountains are spatially dominated by small-scale glaciers in the east and west,with the majority of medium-to large-scale glaciers located in the central part of the mountains,and the number and area of glaciers developed on the northern slopes of the mountains is much higher than on the southern slopes.In time,the glacier area of the Caucasus mountains decreased by 587.36 km²from 1960 to 2020,with an average annual rate of glacier area change of-0.58%·a^-1,with the glacier shrinkage rate gradually accelerating from 1960 to 1986(-0.44%·a^-1),1986 to 2000(-0.66%·a^-1)and 2000 to 2020(-0.96%·a^-1).Spatially,the rate of glacier area shrinkage in the mountain ranges decreases from west to east and then increases;the rate of glacier area shrinkage in the mountain ranges tends to decrease with increasing glacier size class and elevation zone;in addition,the greatest rate of glacier area shrinkage is for glaciers facing due north.The largest glacier shrinkage is in the Terek River basin,and the largest relative shrinkage is in the Qusar District River basin.（2）The overall mountain glacier mass balance level in the Caucasus during 2000-2020 is-0.36±0.003 m w.e.·a^-1;in the four regions selected for this study,the mass balance levels from west to east are:-0.50±0.002 m w.e.·a^-1,-0.45±0.002 m w.e.·a^-1,-0.28±0.005 m w.e.·a^-1,-0.24±0.002 m w.e.·a^-1,overall,glaciers are in a state of mass deficit and the magnitude of glacier mass deficit decreases from west to east.The measured values of the mass balance of the monitored glaciers Djankuat and Garabashi during 2000-2017 were:-0.69 m w.e.·a^-1,-0.60 m w.e.·a^-1,respectively,and the mass balances obtained by geodesy were:-0.49±0.002 m w.e.·a^-1,-0.52±0.003 m w.e.·a^-1,which are 8%and 20%higher than the measured data.Djankuat and Garabashi glaciers were monitored for significant mass deficits,increasing fluctuations in the height of the equilibrium line and a gradual decrease in the area ratio of the accumulation zone during the study period.During the last 60 years,the mass balance of Djankuat glacier ranged from-2010 to 1540 mm w.e.,with an average annual mass balance of-276 mm w.e.and a cumulative mass balance of-13790 mm w.e.,a thinning of 15.3 m;between 1983 and 2020,the annual mass balance of Garabashi glacier varied from-1510 to 410 mm w.e.,with an average annual mass balance of-393 mm w.e.and a cumulative mass balance of-14148 mm w.e.,representing a 15.7 m thinning of the glacier;since the cumulative mass balance of Djankuat and Garabashi glaciers has declined sharply since the 1990s;the ELA of Djankuat and Garabashi glaciers has increased by 360 m and 300 m respectively during the study period;the AAR analysis shows that both Djankuat and Garabashi glaciers are in a state of retreat.（3）During the period 1961-2020,the temperature in the region where the Caucasus mountain glaciers are located continued to rise,while the annual precipitation was basically stable;in terms of spatial distribution,the rate of temperature change was low in the central part of the region and high in the eastern and western parts,and showed a high negative correlation with the spatial distribution of the shrinkage rate of the glacier area;Since the 1990s the rate of temperature increase in the region where the Caucasus mountain glaciers are located has accelerated and the accumulated mass balance of the glaciers has declined sharply,it is inferred that the main cause of the mass deficit in the Caucasus mountain glaciers is the continued increase in temperature in the region,while glaciers will continue to be in a state of material deficit even if the climate remains stable in the future.In addition,glacier mass loss is also related to the size of the glacier and the altitude of the glacier tongue et.al.