## 3 Results
### Temporal Trends of NDVI and Modeled NPP
Time series analysis of both global NPP and NDVI showed, in general, a corresponding pattern of increase (decrease) for the duration of this study except for a few years (Figure 4). For example, NDVI showed higher values around 1990, and over the period 1995-1998. There were also a few instances (e.g., 1993 and 2000), where global NDVI showed temporary declines while NPP did not. Among the five models, this pattern was more prominent in JULES and OCN as compared to other three models (Figure 5). On regional level, higher variations between NDVI and NPP were observed in the Tropics and in Temperate regions. In the Tropics, the NDVI peak values do not fully correspond with the NPP. This was prominent during 1990 and 1995-1998. The highest peak observed in NDVI was closer to the highest peak simulated NPP by JULES. In contrast, VEGAS and LPJ showed relatively smaller NPP increase for the entire period of this study. Broadly stated, global NPP and NDVI correspond more consistently in later years (_i.e._, the last decade of the study period), compared to earlier years; however, models showed more variability among themselves in later years than in earlier years (Figure 5). The correlation coefficients between NDVI and modeled NPP values are given in Table 5. Overall, all models showed moderate correlations with NDVI at global level. Models ensembles NPP showed the highest correlation with NDVI in Temperate and Boreal regions compared to Extra Tropics and Tropics, respectively. At global level, VEGAS showed the highest correlation with NDVI followed by OCN and JULES. Similarly, at regional level, OCN performed better in Temperate regions compared to other models. \begin{table}
\begin{tabular}{c c c c} \hline \multicolumn{4}{c}{**Global**} \\ \hline
**Models** & **NPP (Pg C \(\cdot\) y\({}^{-1}\))** & **Trend (Pg C \(\cdot\) y\({}^{-1}\) y\({}^{-1}\))** & **SE** \\ \hline \multicolumn{4}{c}{Temperature (25 N–50 N)} \\ MME & 13.55 & 0.034 ** & 0.05 \\ CLM4.5 & 14.04 & 0.032 ** & 0.04 \\ JULES & 14.16 & 0.037 ** & 0.08 \\ LPJ & 10.57 & 0.026 ** & 0.08 \\ OCN & 14.48 & 0.050 ** & 0.06 \\ VEGAS & 14.49 & 0.026 ** & 0.04 \\ \hline \multicolumn{4}{c}{Tropics (25 S–25 N)} \\ MME & 36.24 & 0.126 ** & 0.05 \\ CLM4.5 & 37.74 & 0.109 ** & 0.04 \\ JULES & 42.8 & 0.237 ** & 0.08 \\ LPJ & 29.89 & 0.081 ** & 0.05 \\ OCN & 33.97 & 0.133 ** & 0.04 \\ VEGAS & 36.78 & 0.072 ** & 0.04 \\ \hline \multicolumn{4}{c}{Extra Tropics (25 S–90 S)} \\ MME & 3.44 & 0.003 * & 0.13 \\ CLM4.5 & 3.58 & 0.006 * & 0.14 \\ JULES & 4.03 & 0.005 * & 0.23 \\ LPJ & 1.93 & \(-\)0.001 * & 0.23 \\ OCN & 3.15 & 0.005 * & 0.10 \\ VEGAS & 4.53 & 0.001 * & 0.09 \\ \hline \end{tabular}
\end{table} 
Table 4: _Cont._Figure 4: The modeled NPP and NDVI at global and regional (Boreal, Temperate, Tropics and Extra Tropics) levels, for the period of 1982–2012. Thick black line represents the MME, and shaded gray area captures the models’ spread. Figure 5: A comparison of five TRENDY models’ NPP and observed NDVI at global and regional (Boreal, Temperate, Tropics and Extra Tropics) levels, for the period of 1982–2012. Black line represents the NDVI trend.