Thermal-state transition pathways and spatial differentiation of permafrost in Northeast China

The evolution of the thermal state of permafrost in Northeast China exhibits significant phase transitions and multi-path characteristics. The dominant regional direction has shifted from an earlier adjustment toward lower thermal grades to a later adjustment toward higher thermal grades, accompanied by grade persistence, gradual transitions, directional reversals, and localized spatial reorganization. This paper constructs grade sequences from PTC-2 to PTC-15 based on four periods of 30 m permafrost temperature products from 2003 to 2021, strictly within the four-period common-valid pixels. By combining the transition matrices between adjacent periods, four-period trajectory classification, 10 km area-proportion fields, and spatial error models, the study analyzes the pathways of thermal state transitions, the spatial reorganization of PTC-13, and their environmental linkages. The results indicate that from 2003–2007 to 2008–2012, pixels that maintained their level or transitioned to an adjacent lower level accounted for 57.7% and 38.9%, respectively. In the subsequent two stages, the proportions of transitions to an adjacent higher level were 52.8% and 25.3%, respectively, while transitions spanning two or more levels did not exceed 1.72%. The area of PTC–13 initially increased by 32.1%, followed by decreases of 29.3% and 20.2%, respectively. Under the 30% reference threshold, the area of its high-proportion PTC–13 area expanded from 4.208×10 4 km 2 to 1.151×10 5 km 2 , then subsequently shrank to 2.378×10 4 km 2 and 3.506×10 3 km 2 . The entry-exit decomposition indicates that the initial stage is characterized by expansion based on high retention, while the subsequent two stages transition to an exit-dominated rapid contraction. The Jaccard index decreases to 0.20 and 0.12, while turnover intensity increases to 0.67 and 0.79. The centroid direction varies with the area-proportion threshold, suggesting that changes are primarily manifested as spatial reorganization rather than stable migration. Results from trajectory partitioning, based on the magnitude and frequency of thermal state changes, indicate that the persistent, transitional, and high-dynamic thermal-trajectory zones account for 57.20%, 40.18%, and 2.62%, respectively. The association between long-term climate means and thermal state trajectory differences is stronger than that with interannual climate fluctuations. After environmental correction, the mean NDVI of different thermal trajectory zones shows no stable differences; however, the interannual variability of NDVI increases with the degree of trajectory dynamics. The research findings reveal a pattern of thermal state change within the common effective permafrost in Northeast China, characterized by phase transitions, multi-path evolution, and heterogeneous spatial reorganization. This provides a repeatable analytical framework for regional thermal state comparison and the subsequent identification of monitoring targets.