## 3 Results  
### Spatial Distribution of Averaged SOS and EOS from 1982 to 2015  
Spatial patterns of the SOS and EOS were shown in Figure 2. These results showed that the mean SOS in the 1982-2015 period in the northern hemisphere mainly occurred from March to May (from the 80 th to 150 th day of the year (DOY)), which accounted for 93.8% of the pixels (Figure 2c). Earlier SOS (\(<\)80 th DOY) regions were located in the southeast regions of North America, the southwest regions of Eurasia, and the middle-east regions of China, whereas the later SOS (\(>\)150 th DOY) regions were concentrated mainly in high-latitude areas (Figure 2a). Additionally, in more than 96.2% of the study area, the EOS occurred from September to November (from the 255 th to 325 th DOY), and 76.2% occurred from September to October (Figure 2d). Most late EOS (\(>\)315 th DOY) regions were located in the southeastern and northeastern parts of North America and western Eurasia, whereas most early EOS (\(<\)275 th DOY) regions were located in inner Asia (Figure 2b).  
Temporal and Spatial Patterns of Climate Factors and SPEI within Vegetation Growing Seasons in 1982-1998 and 1999-2015  
A piecewise linear regression method was used to quantify the change in annual averaged climate factors (mean value during the growing period), including precipitation and temperature before and after 1998. Results showed no obvious changing trend in precipitation in 1982-1998, but precipitation increased significantly (\(p<0.05\)), by 7.8 mm per decade, in 1999-2015 (Figure 3a). With respect to temperature, no apparent warming hiatus was observed after 1998. On the contrary, the temperature increased by 0.3 \({}^{\circ}\)C per decade both before and after 1998, and this increasing trend was statistically significant (\(p<0.05\)) in 1999-2015 (Figure 3b). In addition, it is worth noting that the standard deviation of precipitation and temperature were both higher in 1982-1998 than in 1999-2015. As for the SPEL, a significant (\(p<0.01\)) increasing trend was observed over the entire study period, which indicated that the vegetation zone of the northern hemisphere had undergone a considerable wetting trend in the growing season of 1982-2015. When the period was divided into two parts, neither increasing trend in the SPEI was statistically significant, but the trend increased more rapidly in 1999-2015 (Figure 3c).  
Spatial patterns of the SPEI during the two study periods were reflected by their mean values and the standard deviation. Significantly different spatial patterns in the mean SPEI were observed both before and after 1998 (Figure 4). In 1982-1998, the regions with a low SPEI (\(<\)-0.1) accounted for 27.7% of the study area and were mainly located in the high and mid-latitudes, including the middle and northeastern part of Russia, northwestern Canada, and northern Kazakhstan (Figure 4a). However,  
Figure 2: The spatial distribution of average SOS **(a)** and EOS **(b)** of vegetation area in the Northern Hemisphere during 1982–2015. (**c,d**) shows the frequency distribution of 34-year averaged SOS and EOS, respectively.  
after 1998, the regions with a low SPEI decreased to 13.6% and were mainly concentrated in low-latitude areas, including northern Mongolia and northeastern China (Figure 4b). Overall, the high-latitude areas underwent a considerable increase in SPEI (wetting trend) in 1999-2015. Compared with the SPEI, the standard deviation also showed a high spatial heterogeneity for the study area, but no apparent difference was observed between the two study periods. Most standard deviations were in the range of 0.5 to 0.9. Regions with a high standard deviation were observed mainly in the northeastern and north-central parts of Russia and northwestern Canada, whereas areas with low standard deviations were widely concentrated in the middle- and low-latitude regions (Figure 4c,d).