Projected changes of Active layer thickness over permafrost under 1.5~5.0 °C climate warming on the Qinghai-Tibet Plateau

Abstract. Permafrost degradation and active layer thickness (ALT) changes on the Qinghai-Tibetan Plateau (QTP) are caused by soil temperature increases under the continuous increase in global temperature. Therefore, the soil zero-degree layer can be used as an index to investigate the changes in permafrost and the ALT. The observed and projected permafrost and ALT were estimated by the summer soil zero-degree layer using soil temperature data from weather stations and the Coupled Model Intercomparison Project Phase 6 (CMIP6). The results revealed that the ALT is deeper in summer (e.g., July), indicating that the melting capacity increases. The CMIP6-simulated and observed soil temperatures are consistent in the vertical direction across the QTP, but the model results exhibit significant cold deviations. The average ALT on the Qinghai‒Tibet Plateau was approximately 3.75 m (range of 1.10~15.91 m) during 1961–1990 and increased to 5.77 m (range of 1.72~13.53 m) during 1991–2022, an increase of 53.9 %. The observed ALT were >6 m in the southeastern QTP area, such as east and south of Shigatse and Lhasa, where it was <3 m in the western Pamir Plateau, near Gaize, north of Qamdo in the northeast and east of Golmud, and other central and northern regions, where the values were 3–6 m. The ALT will continue to increase under the four Shared Socioeconomic Pathways (SSPs), especially when the radiative forcing level is high, and they will increase by 39.6 % under SSP5-8.5 by the end of the 21st century. The regional average of ALT will increase by 5.4 % for every 0.5℃ increase in global warming levels, increasing from 10.6 % to 47.1 %. A small change (e.g., <20 %) in the coverage area of the ALT will occur, decreasing from 1,424,735 km² to 13,682 km². However, the coverage area with a depth increase of more than 100 % will increase from 0 to 401,433 km². The regions where the ALT slightly increases are primarily distributed in the northeast, east and southeast regions of the QTP, and the region where the ALT dramatically increases is in the western region, where there is concentrated permafrost.