## 3 Results  
### Spatiotemporal Patterns and Trends of Seasonal Frost Days from 2001 to 2018 in the Third Pole  
The seasonal trends of environmental factors (frost days, temperature, precipitation, and radiation) and vegetation changes (GOSIF and NDVI) for each grid cell in the Third Pole during the period 2001-2018 were calculated by using the Mann-Kendall method. To highlight the effects of interannual variation for comparison, the seasonal trends of environmental factors and vegetation changes were examined in the Third Pole before and after 2010. Frost days (FDs) decreased significantly before 2010 (slope \(=-1.0411\) days decade\(-1\), \(p=0.05\)) but had no significant trend after 2010 (slope \(=-0.2374\) days decade\(-1\), \(p=0.59\)), as shown in Figure 2a. Temp had no significant increasing trend before and after 2010, as shown in Figure 2b. Pre had no significant decreasing trend before 2010 and an increasing trend after 2010, as shown in Figure 2c. Srad had no significant decreasing trend before and after 2010, as shown in Figure 2d. Additionally, the vegetation index such as NDVI and GOSIF increased significantly (\(p<0.1\)) before 2010, while no significant trends occurred after 2010, as shown in Figure 2e.f. Figure 2: Seasonal trends of environmental factors and vegetation changes for each grid cell in the Third Pole during the period of 2001–2018: (**a**) frost days (FDs); (**b**) temperature (Temp); (**c**) precipitation (Pre); (**d**) short radiation (Srad); (**e**) NDVI; (**f**) GOSIF. The FDs at the growing season and seasonal scales for each grid cell on the Tibetan Plateau during the period of 2001-2018 were estimated from the CMFD datasets in Figure 3. To highlight the effects of the decadal differences in Tmin on FDs, we first separately analyzed the spatial patterns of FDs in the periods of 2001-2009 and 2010-2018 and then compared the differences between the two periods. From 2001 to 2009, the average SPR_FDs, SUM_FDs, FAL_FDs, and GS_FDs in the Third Pole were about 12.71 days, 18.41 days, 33.89 days, 43.89 days, respectively. From 2010 to 2018, the average SPR_FDs, SUM_FDs, FAL_FDs, and GS_FDs in the Third Pole were about 11.02 days, 15.17 days, 31.03 days, and 41.97 days, respectively. By comparison, there are some differences between before and after 2010 in the FDs at the growing season and seasonal scales. At length, the magnitudes of the FAL_FDs and GS_FDs before 2010 and after 2010 were close. Due to the relatively short summer on the Tibetan plateau, the number of frost days in summer and spring is more or less the same. Geographically, the spatial patterns of the FDs during the periods of 2001-2009 and 2010-2018 were consistent with the larger area of the Third Pole. The central Qinghai Plateau was more exposed to an increasing occurrence of FDs. The spatial variation patterns of FDs in different seasons (except in spring) were similar to those of the growing season FDs. Overall, the FDs in the Third Pole revealed divergent geographical differences among growing season and seasonal cycles between 2001-2009 and 2010-2018. Figure 3: Spatial distribution of the number of average frost days (FDs) in the Third Pole during the periods of 2001–2009 and 2010–2018: (**a,e,i**) spring: from the SOS to summer solstice; (**b,f,i**) summer: from 1 June to 31 August; (**c,g,k**) autumn: from the summer solstice to the EOS; (**d,h,l**) growing season: from the SOS to EOS. Examining different research periods led to different results that could not be directly verified and compared to each other; thus, comparative analysis could not be performed among multiple periods. Figure 4 shows the temporal trends in FDs during the growing season and seasonal scales in the Third Pole during periods of different lengths starting between 2000 and 2018. Overall, the FDs at the growing season and seasonal scales had no significant decreasing trends in the Third Pole from 2001 to 2018.