Regional Thinning and Local Thickening of the Seasonal Freezing Layer in Northeast China During 2005–2020

The seasonal freezing layer is an important indicator of frozen ground's responses to climatic warming and environmental changes. In Northeast China, the permafrost and taliks interweave with major spatial discontinuity and variability. The spatiotemporal variability of the seasonal freezing layer and the contribution of environmental factors to these changes are not fully understood. Based on 51 ground temperature monitoring sites in Northeast China from 2005 to 2020, the maximum depth of the seasonal freezing (MDSF) was calculated, and the environment and climate‐driven changes in the MDSF were analysed by classification and the regression trees method. The MDSF showed an overall decrease in the region. With local thickening, the MDSF varied between 1.2 and 3.1 m at a rate of −4.34 to 4.80 cm/year. Air temperature warming is the main driver (34%) of regional MDSF changes. Vegetation, rainfall and snow play a prominent role in the Da and Xiao Xing'anling region in Northeast China. The impact of other environmental factors locally outweighs the air temperature. The findings highlighted the importance of environmental factors beyond air temperature in influencing changes in the active layer, emphasising their contribution to spatial and temporal variations.