Accelerating thermokarst lake changes on the Qinghai-Tibetan Plateau

As significant evidence of ice-rich permafrost degradation due to climate warming, thermokarst lake was developing and undergoing substantial changes. Thermokarst lake was an essential ecosystem component, which significantly impacted the global carbon cycle, hydrology process and the stability of the Qinghai-Tibet Engineering Corridor. In this paper, based on Sentinel-2 (2021) and Landsat (1988-2020) images, thermokarst lakes within a 5000 m range along both sides of Qinghai-Tibet Highway were extracted to analyse the spatio-temporal variations. The results showed that the number and area of thermokarst lake in 2021 were 3965 and 4038.6 ha (1 ha = 10,000 m(2)), with an average size of 1.0186 ha. Small thermokarst lakes (<1 ha) accounted for 85.65% of the entire lake count, and large thermokarst lakes (>10 ha) occupied for 44.92% of the whole lake area. In all sub-regions, the number of small lake far exceeds 75% of the total lake number in each sub-region. R1 sub-region (around Wudaoliang region) had the maximum number density of thermokarst lakes with 0.0071, and R6 sub-region (around Anduo region) had the minimum number density with 0.0032. Thermokarst lakes were mainly distributed within elevation range of 4300 m-5000 m a.s.l. (94.27% and 97.13% of the total number and size), on flat terrain with slopes less than 3 degrees (99.17% and 98.47% of the total number and surface) and in the north, south, and southeast aspects (51.98% and 50.00% of the total number and area). Thermokarst lakes were significantly developed in warm permafrost region with mean annual ground temperature (MAGT) > - 1.5 degrees C, accounting for 47.39% and 54.38% of the total count and coverage, respectively. From 1988 to 2020, in spite of shrinkage or even drain of small portion of thermokarst lake, there was a general expansion trend of thermokarst lake with increase in number of 195 (8.58%) and area of 1160.19 ha (41.36%), which decreased during 1988-1995 (- 702 each year and - 706.27 ha/yr) and then increased during 1995-2020 (184.96-702 each year and 360.82 ha/yr). This significant expansion was attributed to ground ice melting as rising air temperature at a rate of 0.03-0.04 degrees C/yr. Followed by the increasing precipitation (1.76-3.07 mm/yr) that accelerated the injection of water into lake.