ИСТИНА

Three consequent debris thickness surveys, undertaken in 1983, 1994 and 2010 all over the surface of the Djankuat Glacier in the Caucasus, the best explored in Russia, revealed that superficial moraine became thicker by 70 cm near the terminus during 1983–2010 whereas total lithogenic matter volume experienced 141% increment. Currently ongoing deglaciation due to protracted rise of ablation season air temperature (almost 10C since 1968), coupled with concomitant freezing-thaw shattering, provokes intensive and frequent rockfalls (particularly tremendous in 2001-2003). They promote hereby colluvium delivery onto the glacier snout - mainly at the expense of belts with the least ice thickness, i.e. on the steep slopes of rocky framework over the firn basin which are highly unstable due to stress relaxation. Covering only 2% of the entire glacier area in 1968, debris share grew up to 13% by 2010, resulting thereby in 6.5-fold enlargement during 42 years. This process influenced mass balance values and distorted its spatial pattern. In general, debris cover renders a screening effect upon melt-rate, and thereby glacier-derived liquid bulk run-off turns out to be reduced annually by ca. 20% on average. Debris surface differs from clean ice by its thermo-physical properties - it is characterized by its peculiar heat balance structure. After studying influence of debris cover on ablation and considering its anticipated evolution, glacier topography by 2025 was predicted based on HadCM3 climate model (A2 scenario). Downscaling air temperature and precipitation led to a forecast of Djankuat mass balance patterns and glacier morphometry for every year till 2025. Debris-covered parts would experience lowering 7-15 meters less than clean ice, and lowering rate would be 42-45% smaller. Hence, at the current stage debris cover evolution seems to have significance for glacier behaviour comparative with that of climate change factor.