Ground thermal regime changes caused by integrated warming, wetting, and greening in permafrost regions of the Qinghai-Tibetan Plateau

The Qinghai-Tibetan Plateau (QTP) has experienced significant warming, wetting, and greening since the 1980s, altering the thermal regime of the frozen ground and associated geomorphological and ecological processes. Previous studies focused mainly on the long-term warming effects on permafrost, but the synergistic effects of seasonal variations in temperature, precipitation, and vegetation remain unclear. This study, focusing on the source regions of the Yangtze River in the QTP, used a distributed process-based model integrated with the fuzzy k-means method to explore the impact of temperature, precipitation, and leaf area index (LAI) on frozen ground. Findings reveal that cold-season (October-April) warming affects permafrost degradation 1.66 times more than warm-season (May-September) warming. A unit increase in LAI during the warm-season results in a 2.99 cm rise in ALT, whereas MFD increases by 30.28 cm. The wetting climate, particularly in the warm-season, mitigates permafrost degradation. A 100 mm increase in warm-season precipitation decreases ALT by an average of 0.135 m, and the same increase during the cold-season decreases MFD by 0.095 m. The study also reveals that transition permafrost is most sensitive to temperature changes, while unstable permafrost is less affected but more influenced by precipitation. Greening vegetation predominantly affects the thermal regimes of frozen ground in high-altitude regions. Overall, air temperature remains the primary driver of changes in the thermal regimes of frozen ground. Plain language summary: Permafrost is the subsurface material that stays frozen for at least two consecutive years. The degradation of permafrost will affect geomorphological and ecological processes. Here, we employ a processbased, hydro-thermal coupled hydrological model to evaluate how changes in air temperature, precipitation, and leaf area index affect frozen ground thermal regimes. Results show that permafrost degradation is more sensitive to cold-season warming than warm-season warming. Greening vegetation has a greater impact on seasonally frozen ground than on permafrost. Increased precipitation mitigates permafrost degradation but accelerates the degradation of seasonally frozen ground. Rising air temperatures remain the primary driver of permafrost degradation.