Quantifying thermokarst and alas lake changes and influencing factors during the last 70 years in Central Yakutia

The permafrost landscape of Central Yakutia is subject to rapid modifications due to intensive land use, extreme weather, and the current global warming. With regard to the predicted increase in precipitation and temperature as a result of climate change, quantitative knowledge of the small-scale variability of active thermokarst processes is required. Here, we mapped the change of thermokarst and alas lakes (i.e. residual lakes in alas basins) for 11 times covering periods of 2 to 18 years between 1944 and 2014 at the Yukechi study site (61.761289° N/130.470602° E). Historical airborne, current satellite as well as field data were utilized in analyzing lake-area changes and thaw subsidence on local scale. Additionally, a unique dataset of longterm climatic and ground-temperature data could be used in multivariate statistics to identify the climatological and/or general driving and inducing factors of thermokarst and alas-lake changes. On regional scale, size and distribution of lakes >0.1ha were analyzed on different ice-rich permafrost terraces in the Lena-Aldan-Amga interfluve region east of Yakutsk on the basis of Landsat 8 data from July 2013. Regionally, larger lakes distributed in higher frequency are dominating lower terraces. Smaller lakes dominate higher terraces. In particular, smaller lakes are distributed in less density on older and more ice-rich terraces while highest lake densities and larger lakes characterize younger and less ice-rich terraces. Remote sensing analyses at the Yukechi study site indicate that alas-lake levels are increasing strongly end of the 1960s and since the 1990s until present, but their area decrease in the 1940s, 1950s, 1970s, and 1980s. The mean rate of alas-lake-radius change for the 70 year time span account for 1.6 ± 2.9 m/yr. In the meanwhile, extensive agricultural use in the postwar period on the Yedoma ice-rich permafrost deposits led to a rapid and sustained growth of young thermokarst lakes over the entire time span. This is initiated by the strong disturbance of the thermal and hydrological balance of the permafrost. The mean rate of lake-radius change of all mapped thermokarst lakes is 1.2 ± 1.0 m/yr. The mean thaw subsidence below the thermokarst lakes account for 6.2 ± 1.4 cm/yr. Our statistical analyses indicate that climatic parameters (i.e. precipitation, air and ground temperature, and evaporation) show higher correlations with thermokarst-lake changes than with alas-lake changes. In particular, the influence of annual air temperature changes and evaporation is higher on thermokarst-lake level changes than on alas-lake level changes. However, the influence of precipitation, especially winter precipitation, is lower. Deeper ground temperature changes (3.2m depth) show higher correlation with thermokarst-lake changes, while the influence of ground temperatures in 1.6m depth is similar. Multiple regression analyses reveal more complex interrelations of climatic and ground thermal conditions with thermokarst and alas lake changes but further study is needed to validate these results. Our results show, however, that topography, geomorphology, and surficial cryolithology are important controlling factors on the regional distribution and size of the lakes. Furthermore, thermokarst activity is influenced by climatic parameters but it is accelerated and rapidly induced by disturbing factors such as land use. Climatic parameters are strongly affecting growing rates within certain time periods of thermokarst lakes but they do not lead to remarkable reductions or the disappearance of the lakes during the whole observation period. Alas lakes are increasing and decreasing. Distinguishing main controlling factors, however, are hampered probably due to larger catchment areas and subsurface hydrological conditions