## 3 Results  
### Spatial Variation in Global Vegetation Phenology  
Vegetation phenological metrics modeled from the remote sensing (RS) imagery are generally in agreement with the flux-based observations in terms of RMSE, ME, and the scatter point plots (Figure 3), though disparity exists among the vegetation cover types and phenological events (SOS and EOS) (Figure 3). The RMSE for the modeled SOS and EOS ranges from 10.0 to 37.3 days and 10.4 to 38.1 days, respectively (Figure 3a). The modeled metrics for forest, grass, and croplands show a larger difference from the field observations than those for shrubland and savannas. The statistics of ME indicate that cropland contains the largest overall error in the modeled metrics (Figure 3b). The site-year scatter points between the flux-based observations and remote-sensing-modeled metrics in an annual basis show that they are closely correlated for both SOS and EOS (Figure 3c,d).  
Figure 3: Validation of the remote-sensing-derived phenological metrics, on a yearly basis, against the FLUXNET observations through indicators of (**a**) root mean square error (RMSE), (**b**) mean error (ME), (**c**) scatter of SOS, and (**d**) scatter of EOS.  
The spatial distributions of vegetation phenological events, including SOS, EOS, and LOS, for the temporally averaged mean and standard deviation (std) over the years are shown in Figure 4. The phenological metrics are highly heterogeneous across the study area and vary among the ecosystems. The critical latitudes at 30\({}^{\circ}\) in both the Northern Hemisphere and Southern Hemisphere divide the SOS and EOS patterns of the global vegetation into three distinct parts, namely the Northern Hemisphere over 30\({}^{\circ}\)N (North), Southern Hemisphere over 30\({}^{\circ}\)S (South), and the middle part located between the North and South (Middle). Because of the reversed seasons in the North and the South, SOS in the North starts from March to no later than May (145.72 \(\pm\) 7.65), while EOS ends after August (243.42 \(\pm\) 23.95, Figure 4a), but vegetation in the South presents EOS in March, April, or May (126.39 \(\pm\) 81.22) and SOS after September (269.60 \(\pm\) 31.02, Figure 4c). Furthermore, SOS varies spatially in the North and the South. Vegetation turns green earlier in a great part of North America (#1) and Southern Europe (#2) compared with that in the rest of the North, and vegetation in Australia (#3) presents earlier green-up than that in the rest of the South (Figure 4a). Both SOS and EOS differ along the latitude belts (Figure 4a,c). For instance, in the North, SOS turns green later along with the northward increase in latitude, while in the South SOS, it is delayed with the southward increase in latitude. EOS advances along the increase in latitude belts in the South, but stays rather stable in the North. The variations in SOS and EOS are reflected by the standard deviation (std) over the years (Figure 4b,d). Both SOS and EOS in the Middle highlight the most sensitive response to the changed latitudes, as shown from the rapid changes in std among the latitude belts. Furthermore, the variation in SOS and EOS is much smaller in the North than that in the South. LOS and its standard deviation (std) computed from the gap between SOS and EOS are shown in Figure 4e,f. Generally, vegetation demonstrates a longer growing season in the South, as shown by the higher LOS in the South (129.86 \(\pm\) 25.37) than in the North (87.82 \(\pm\) 45.27). Similarly, LOS near the equator is highly variable along the latitude belts (Figure 4f). In some regions of the Northern Hemisphere, for example, the southeast of the United States (#1), the west of Europe (#2), the south of China (#3), and Japan (#4), where forests are prominent, LOS is rather higher than that in the rest of the North (Figure 4e).