## 4 Discussion  
### Potential Drivers of NDVI and NPP Trends  
The total global simulated NPP (mainly models ensemble) generally followed the temporal trend of NDVI for the study period 1982-2012. The spatial distribution of the modeled NPP and observed NDVI, however, widely differed for most of the globe, except for some regions e.g., Western Europe, African Sahel and Eurasia (Figures 1-3). Despite the regional differences, both NPP and NDVI showed increasing vegetation activity from 1982-2012, which is in line with other studies [3; 29]. Generally, we observed highest positive trends in both NPP and NDVI in the Northern Hemisphere (Tables 3 and 4). Comparatively, NDVI and NPP trends showed better consistency in Temperate regions (e.g., UK and Ireland) than in Boreal regions (e.g., central Russia). Furthermore, NPP showed larger trends (\(>\)0.1 Pg C \(\cdot\) y\({}^{-1}\cdot\) y\({}^{-1}\)) in Boreal regions, while NDVI showed larger trends (\(>\)0.05 y\({}^{-1}\)) in Temperate regions. Apart from the some inconsistency between NDVI and NPP in Northern Hemisphere, we observed a general trend of increased vegetation activity. Higher vegetation activity in Boreal regions (predominantly covered by forests) is likely due to earlier onset and longer growing seasons [30]. Similarly, higher vegetation activity in Temperate regions (predominantly covered by grasslands and croplands) may largely be explained by increased precipitation [31; 32], agricultural intensification [32; 33], nitrogen deposition and nitrogen fertilizer applications [34]. Decreasing trends of both NPP and NDVI were more prominent in the Southern Hemisphere. This NPP (\(<\)\(-\)0.1 Pg C \(\cdot\) y\({}^{-1}\cdot\) y\({}^{-1}\)) and NDVI (\(<\)\(-\)0.05 y\({}^{-1}\)) decreasing trend was most consistent in South American countries such as Argentina and Chile. This may be attributable to increasing temperatures that cause high evaporation and larger vapor pressure deficit, which in turn creates water deficit with negative impact on optimum plant growth [29]. Decreasing trends in Southeastern Asia may be driven by the expansion of palm oil and rubber trees at the cost of tropical forests [35]. This conversion has been reported to take place at a scale large enough to impact the overall NDVI and NPP patterns in this region. Decreasing trends in the Northern Hemisphere (e.g., Boreal regions of  
Figure 7: A latitudinal comparison between NDVI and simulated NPP trends for the five TRENDY models. North America and Northern China) can be attributed to regional droughts (_i.e._, reduced rainfall), pollution, wildfires and other disturbances [36; 37]. The regions with mixture of both increasing and decreasing trends (e.g., Sahel region) can be explained by the complex interactions between land use (grazing, fertilization) and climate conditions [38]. The inter-annual variability observed in both NPP and NDVI is mainly caused by different climate events (such as El-Nino) and volcanic eruptions. In this study, modeled NPP and NDVI showed an abrupt setback after 1991 in response to El-Nino, followed by resumption of plant growth in 1993 (Figures 4 and 5). Furthermore, the opposite trends between NDVI and NPP in 2000 can be attributed to the differential response of ecosystems to the cooling effect (leading to reduced length of growing season) caused by aerosols from Pinatubo eruption [1]. Both NPP (ranging from \(\sim\)\(-\)0.047 to \(-\)0.10 Pg C \(\cdot\) y\({}^{-1}\cdot\) y\({}^{-1}\)) and NDVI (ranging from \(\sim\)\(-\)0.016 to 0.035 y\({}^{-1}\)) trends also varied substantially with latitudes (Figure 6). The highest NPP and NDVI trends were observed in the Tropics (mainly in the Amazon forest and the Sahel region of Africa). In contrast, the lowest NPP and NDVI trends were observed in Extra Tropics (mainly in Southern part of South America, e.g., Argentina). This variation over tropical regions has been attributed to the El-Nino events due to their higher association with global atmospheric CO\({}_{2}\) growth rate [12].