Simulation of the current and future dynamics of permafrost near the northern limit of permafrost on the Qinghai–Tibet Plateau

: Permafrost has been warming and thawing at a global scale with subsequent effects on the climate, hydrological, ecosystem and engineering system. However, the variation of permafrost 25 thermal state in the northern lower limit of the permafrost zone (Xidatan) on the Qinghai – Tibetan Plateau (QTP) is unclear. To evaluate and project the permafrost changes, this study simulated the spatiotemporal dynamics of this marginal permafrost historically (1970–2019) based on the detailed investigation and monitoring datasets from 1987 in this study region, improved remote sensing– based Land Surface Temperature product (LST) and climate projections from Global Climate Model 30 (GCM) outputs of Coupled Model Intercomparison Project Phase 5 and 6 (CMIP5, CMIP6). Our model takes into consideration of phase change processes of soil pore water, thermal property difference between frozen and thawed soil, geothermal flux flow, and ground ice effect. The results indicate that 1) our model can consistently reproduce the vertical ground temperature profiles and active layer thickness (ALT), is superior in recognizing permafrost boundaries, and would 35 realistically capture the evolution of the permafrost thermal regime, 2) spatial distribution of permafrost and its thermal conditions over the study area were controlled by elevational with a strong influence of slope aspects, 3) from 1970 to 2019, the regional averaged means annual ground temperature (MAGT) had warmed by 0.49 ℃ in the continuous permafrost zone and 0.40 ℃ in the discontinuous permafrost zone, and the lowest elevation of permafrost boundary (on north–facing 40 slopes) rose approximately 47 m, as well as the northern boundary of discontinuous permafrost has approximately retreated southwards 1~2 km, while the lowest elevation of permafrost boundary remains unchanged for continuous permafrost zone, 4) the warming rate in MAGT is projected to be slighter higher under Shared Socioeconomic Pathways (SSPs) than that of Representative Concentration Pathways (RCPs), but no distinct discrepancies in the areal extent of the continuous, 45 discontinuous permafrost and seasonally frozen ground among SSP and RCP scenarios. This study highlights the slow delaying process in the response of mountain permafrost to a warming climate, especially in terms of the areal extent of permafrost distribution. and anticipate possible changes by 2100 under different climate change scenarios, forced by improved remote sensing-based spatial product (LST), and CMIP5 RCP RCP4.5 RCP8.5) and CMIP6 SSP1–2.6, SSP2–4.5, and SSP5–8.5) climate projections. Our model gives full consideration to the thermal property difference between 115 frozen and thawed soil, the phase variations of the unfrozen water in frozen soil, the distribution of the ground ice, and geothermal heat flow. The model results were validated against considerable long–term continuous monitoring of soil temperatures at various depths, ALT, and observed permafrost distribution of boreholes as well as three existing permafrost distribution maps during model calibration. As suggested by Hipp et al. (2012), a manual, stepwise optimization procedure was used to adjust parameters. Specifically, calibration was performed firstly by systematically changing k over the given plausible ranges aiming for improving the agreement between the simulated and observed ground temperature at different depth 285 levels. Subsequently, minor adjustments were made to C to promote the model performance. by the calibrated model, yielding an MAE of 0.86–1.27 ℃ (RMSE of 1.15– 1.63 ℃) in the 0.5 and 1 m and 0.01–0.52 ℃ (RMSE of 0.08–0.80 ℃) in 3 and 15 m. In general, 360 very consistent daily fluctuations of the simulated and observed soil temperature at all observational depths for most calibration and validation sites, and indicate the daily variation of the ground and 18.21 (15.71) %, 505 respectively, by 2100, relative to 2019, under SSP1–26 (RCP2.6), SSP2–45 (RCP4.5), and SSP5– 8.5 (RCP8.5). The northern limit of the continuous permafrost zone is projected to retreat southwards around 1~2 km under SSP1–26 (RCP2.6) or SSP2–45 (RCP4.5) or RCP8.5, and about 1~3 km under SSP5–85. By comparison, the northern boundary of the discontinuous permafrost zone is anticipated to shift southward around 1 km under SSP1–26 (RCP2.6) or SSP2–45 (RCP4.5), 510 and around 1~2 km under SSP5–85 (RCP8.5). area, which means our model can well-describe the heat transfer process in permafrost and 700 reasonably capture the attenuation and time lag of heat in deep permafrost. Furthermore, we