Permafrost on the Tibetan Plateau is degrading: Historical and projected trends

Permafrost degradation on the Tibetan Plateau (TP) will significantly affect local water cycle processes, downstream water ecology, and water security. In this study, we evaluate the long-term interannual dynamics of permafrost distribution and active layer thickness (ALT) on the TP based on historical data from Climatic Research Unit gridded Time Series (CRU TS) downscaling and projected data under four shared socio-economic pathways (SSPs) in Scenario Model Intercomparison Project (ScenarioMIP) of the Coupled Model Intercomparison Project Phase 6 (CMIP 6). To achieve this, we employ a data-driven scheme at 1 km resolution for both historical and future periods (1901-2100) that compares the performance of four machine learning algorithms to select the optimal algorithm for permafrost distribution and ALT simulations. Our results indicate that the permafrost on the TP has been undergoing degradation in both historical and future periods, with a decrease in permafrost area and an increase in ALT. The changing rates of permafrost area and regionally averaged ALT during the historical period (1901-2020) are -1.05 x 104 km2 decade-1 and 0.012 m decade-1, while an accelerated degradation is observed after the 1970 s (with changing rates of permafrost area and regionally average ALT of -3.62 x 104 km2 decade-1 and 0.055 m decade-1). Our results also suggested that permafrost degradation on the TP will continue in the future under the four SSP scenarios. The individual global climate models (GCMs) exhibit a consistent degradation trend but great uncertainty in degradation speed. The ensemble mean of simulations across 15 selected GCMs showed that the degradation percentage of permafrost area on the TP under scenarios SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5 was 26.0 +/- 6.8 %, 50.4 +/- 5.6 %, 79.2 +/- 4.5 %, and 89.0 +/- 4.0 % by 2100, and the regionally average ALT increased by 0.301 +/- 0.112 m, 0.628 +/- 0.113 m, 1.204 +/- 0.119 m, and 1.486 +/- 0.125 m, respectively. We also analyze permafrost stability and elevationdependent changes of ALT on the TP. The permafrost stability increases with elevation and latitude, and ALT changes more intensely with increasing elevation. This study will provide valuable data for hydrological and ecological studies related to permafrost on the TP.