3.2.1. Climate Variables

Figure 5 shows the variation trend of the meteorological variables (Ta, P, RH, and Rs) over the TRHR during 1982–2015. An increasing trend in Ta appeared over the TRHR with an average value of 0.597 ◦C/decade, which is much higher than the global warming average of 0.12 ◦C/decade [54], and 96.3% of the pixels showed a significant increasing trend (*p* < 0.05). The trend of Ta over Maduo, the north of Chengduo, and the east of Qumalai was relatively higher than in other areas and the maximum reached 1.47 ◦C/decade. As shown in Figure 5b, P also experienced a positive trend during the period of 1982–2015 over the TRHR. We found that the P substantially increased in arid areas, with a linear tendency of 41.1 mm/decade (*p* < 0.05). By contrast, a significant decreasing trend of RH over the majority of the region was evident, which corresponded to the warming tendency over the TRHR. A negative Rs trend occurred in the southeastern region, with an average of 3.05 <sup>W</sup>/m<sup>2</sup> per decade (*p* < 0.05). Some scientists suggested that the decline in Rs is consistent with solar dimming over the TP due to an increase in the amount of water vapor and the atmospheric concentrations of aerosols [55].

**Figure 5.** Spatial distributions of climate variable trends in the TRHR over the period 1982–2015: (**a**) temperature; (**b**) precipitation; (**c**) relative humidity; (**d**) downward shortwave radiation. The inset panels show the area where the climate variables trends were statistically significant (*p* < 0.05). Blue represents a significant increase and red represents a significant decrease.

Figure 6 shows the interannual and seasonal climate variables (Ta, P, RH, and Rs) of the TRHR during 1982–2015. Both annual and seasonal mean Ta and P showed a significant positive trend, with a linear trend of 0.6 ◦C/decade and 41.2 mm/decade, respectively. Figure 6a illustrates that a significantly increasing Ta has occurred since 1998, coincident with the last major El Nino event in 1998. During this period, the most significant increase in Ta occurred in the winter (0.901 ◦C/decade, *p* < 0.01), followed by autumn (0.57 ◦C/decade, *p* < 0.01), summer (0.475 ◦C/decade, *p* < 0.01), and spring (0.445 ◦C/decade, *p* < 0.01). Similarly, P also showed a significant positive trend in all four seasons (*p* < 0.01), with the largest P increases in summer (6.67 mm/decade, *p* < 0.01), and the rates for spring, autumn, and winter were 3.54, 2.74, and 0.806 mm per decade (*p* < 0.01), respectively. In addition, a severe drought was also detected in the summer of 2006, and the annual P decreased to 370 mm/year due to the abnormally high Ta and low P [14]. By contrast, the interannual RH and Rs of the TRHR showed a decreasing trend over the whole period. The largest decline in the regional mean surface RH occurred in winter (DJF) at 2.3%/decade (*p* < 0.01), which corresponded with the temperature rising in winter. A significant decrease of Rs occurred in summer (4.57 <sup>W</sup>/m<sup>2</sup> per decade, *p* < 0.01), while in other seasons, Rs presented a slight negative trend with no statistical significance.

**Figure 6.** Interannual and seasonal variability of climate variables trends over TRHR in the period 1982–2015 (**a**) temperature; (**b**) precipitation; (**c**) relative humidity; (**d**) downward shortwave radiation.

#### 3.2.2. Normalized Di fference Vegetation Index

Figure 7a shows the spatial distribution of the NDVI trend over the TRHR during 1982–2015. Our results showed that the vegetation in the TRHR experienced slight greening and over 77.6% of the area showed a slight increasing trend, of which 56.8% significantly increased at a rate of 0.0051/decade (*p* < 0.05). In particular, a significant increase in the annual NDVI occurred in the northern and western part of the TRHR, where the main land use type is alpine and subalpine meadows. Only a tiny portion of the region had a significant decreasing trend, which was mainly distributed in Chengduo and Yushu counties, and the majority of the midland region did not exhibit significant changes in vegetation cover. The increasing trend of NDVI was similar to the findings of Xu et al. [56], who found that the vegetation coverage of the TRHR showed a consistent and slight increase in the period of 1982–2006.

The annual and seasonal NDVI also presented a slightly enhanced trend, particularly after the implementation of the TRHR project (2005 to 2012) [57]. This indicated that the implementation of ecological projects also promotes vegetation growth and gradually reverses the degradation of grassland ecosystems. Specifically, the largest significant increase in the NDVI occurred in spring at the rate of 0.003/decade (*p* < 0.01), which contributed most to the interannual NDVI increase trend.

**Figure 7.** (**a**) Spatial distributions of the NDVI trends. (**b**) Interannual and seasonal variability of the NDVI trends. The inset panel shows the area where the NDVI trend was statistically significant (*p* < 0.05). Blue represents a significant increase and Red represents a significant decrease.
