## 3 Results  
### Interannual Variation of Plant Growth and Air Temperature Indices in the Growing Season over the TP  
Figure 2a illustrated the spatial distribution of trends in the NDVI over the TP during the last four decades. As shown, NDVI has predominantly increased (94.9%, Figure 2a) over the TP, with 62.7% of the increase reaching statistical significance (\(p<0.05\)). This implies that vegetation growth over the TP region is significantly responsive (with an average of increasing rate of 0.001 year\({}^{-1}\)) to climate change, and this aligns with prior findings [33; 34]. Additionally, we found that the upward trend of NDVI declines from the southeastern to the northwestern regions, and this could be possibly due to the notable dissimilarities in climate conditions and elevations [33]. Figure 2b displayed the stability of NDVI on the TP region over the past 40 years. We observed a coexistence of high and low fluctuations, with noted regional variability over the TP region. The low fluctuation with a coefficient of variation (CV) between 0 and 0.02 occupied only 5.7% of the TP. In comparison, the high fluctuation area (0.06 \(<\) CV \(<0.1\)) characterized 36.3% of the area and exhibited a zonal distribution, mainly concentrated in the northern and western boundaries of the TP. It has been suggested that there is a greater variability of NDVI in seasonally frozen areas and permafrost compared to unfrozen areas, and it has been revealed that plant growth in regions with relatively lower altitudes and humid conditions has tended to be more stable. Figure 3 exhibited the spatial distribution of annual means of T\({}_{\rm min}\), T\({}_{\rm max}\), and T\({}_{\rm D}\) in the growing season over the TP. T\({}_{\rm min}\) displayed significant spatial heterogeneity, with over 71.41% of the regions experiencing a T\({}_{\rm min}\) below 0 \({}^{\circ}\)C, while the temperature in the southern edges was larger than 10 \({}^{\circ}\)C (Figure 3a). The T\({}_{\rm max}\) in 99.2% of the TP was above 0 \({}^{\circ}\)C, which confirms the presence of a significant diurnal temperature range over the region [35]. Figure 3c illustrated the spatial distribution of T\({}_{\rm D}\), revealing that the T\({}_{\rm D}\) in the unfrozen regions is the lowest. In contrast, the T\({}_{\rm D}\) in the seasonally frozen regions surpassed 10 \({}^{\circ}\)C and even exceeded 15 \({}^{\circ}\)C in certain parts of the permafrost grounds. As depicted in Figure 4a, there was a significant increasing trend in T\({}_{\rm min}\) during the growing season of the TP. Specifically, 96% of the regions displayed a significant increasing trend (\(p<0.05\)) in T\({}_{\rm min}\). It is interesting to note that in contrast to NDVI, the spatial distribution of T\({}_{\rm min}\) exhibited a gradual increase from southeast to northwest. Overall, the growth trend of T\({}_{\rm max}\) was found to be smaller than that of T\({}_{\rm min}\). A significant growth trend (\(p<0.05\)) was observed in 80.4% of the TP in T\({}_{\rm max}\). Under the combined effect of day and night temperatures, T\({}_{\rm D}\) exhibited a decreasing trend in 69.5% of the areas, of which 63.7% were statistically significant (\(p<0.05\)). A comparison of the findings with those of other studies confirms that the diurnal temperature over the past 60 years has declined, which can be primarily attributed to the pronounced increase in T\({}_{\rm min}\) in relation to T\({}_{\rm max}\)[36]. We also observe that 30.5% of TP displayed an increasing trend in T\({}_{\rm D}\), which is mainly distributed in unfrozen and seasonal frozen grounds. This suggests that T\({}_{\rm D}\) is symmetrically distributed on the TP, rather than exhibiting a unimodal distribution. We further investigated how the trends of the three temperature indices differ across different frozen grounds. Figure 5 demonstrates that the T\({}_{\rm max}\) and T\({}_{\rm min}\) of the unfrozen region have almost all exceeded 10 \({}^{\circ}\)C over the last 40 years.