3.2. MAGT and ALT during the reference period
Using the collected borehole data, we fitted the meteorological factors and geographical environmental factors to obtain the MAGT distribution of the permafrost regions on the QTP (Figure 4). We extracted the MAGT of the QTP below 0 °C as an average range of permafrost (Chen et al., 2015), which indicating suitable conditions for permafrost, with a total area of 1.04 × 106 km2 (excluding glaciers and lakes). Considering the heterogeneity and uncertainty of ground temperature on the QTP, the minimum permafrost extent is 0.8 × 106 km2 (the area within MAGT ≤ −0.5°C), and the maximum extent is 1.28 × 106km2 (the area within MAGT ≤ +0.5°C). Compared with the pan-Arctic permafrost, the permafrost temperature on the QTP is relatively high (Obu et al., 2019). Nearly half of the permafrost temperature area on the QTP exceed -1.0ºC and the average temperature is -1.35 ± 0.42 ºC. The permafrost temperature is not only affected by latitude, but also by altitude. As illustrated in Figure 4, the lower-temperature permafrost on the QTP generally occurs in high-altitude mountains, and the ground temperature gradually rises with decreasing altitude, with the lowest value distributes in the Kunlun Mountain and its surrounding regions. In general, the MAGT on the QTP was found to be higher in the southern region (GZIR) than that in the northern region (AEJIR), and higher in the eastern region (WQIR) than that in the western region (XKLIR).
Based on permafrost extent, the spatial distribution of the ALT for the entire QTP was obtained (Figure 5). The statistical results indicated that the average ALT is 2.3 ± 0.60 m on the QTP, and the ALT value of ~ 90% of the permafrost region ranged from 1.6 to 3.0 m. Geographically, the ALT in the eastern part of the QTP is relatively thinner (generally no more than 2 m) with slight variations. The ALT along the Qinghai-Tibet Highway and in the central and western plateau is highly heterogeneous. The overall ALT pattern is thin in the mountains, thick on the plains, thin in the hinterlands, and thick along the periphery of the permafrost. The maximum value appears along the southern boundary of the permafrost and the surrounding sporadic permafrost (generally ≥ 3.2 m). In general, MAGT and ALT exhibit a consistent trend in spatial distribution, with a correlation coefficient of 0.44. The smaller value of MAGT corresponds to thinner ALTs.