## 3 Results
### Drivers of the Differences in Trends of NPP Anomalies
The results attribute the differences to their different behaviors in projecting the associated climatic anomaly, and to different sensitivities to this climatic anomaly. However, in the abrupt-4 \(\times\) CO\({}_{2}\) simulation, the CO\({}_{2}\) fertilization effects change with temperature and water availability; thus, there are interactions with the climatic effects at each time step. To isolate these interactive feedbacks, we further used the CAS-ESM2 to conduct a simulation (biogeochemically coupled simulation), in which the quadrupled CO\({}_{2}\) affects the terrestrial carbon cycle but not the climate. The differences between the default fully coupled abrupt-4 \(\times\) CO\({}_{2}\) simulation and the biogeochemical coupled simulation reflect the climatic effects, as shown in Figure 8. \begin{table}
\begin{tabular}{l c c c c} \hline \hline  & \multicolumn{2}{c}{**Tropics**} & \multicolumn{2}{c}{**Northern High Latitudes**} \\ \cline{2-5}  & **CAS-ESM2** & **MME** & **CAS-ESM2** & **MME** \\ \hline NPP Anomalies & 9.76 & 19.00 & 14.23 & 11.20 \\ TAS Anomalies & 6.49 & 6.02 & 8.48 & 8.31 \\ Pre Anomalies & \(-\)0.10 & \(-\)0.01 & 0.53 & 0.40 \\ PCC\({}_{\text{NPP\&TAS\_Pre}}\) & \(-\)0.99 * & \(-\)0.98 * & 0.39 * & 0.64 * \\ PCC\({}_{\text{NPP\&Pre\_TAS}}\) & 0.72 * & 0.47 * & 0.48 * & 0.05 \\ Slope\({}_{\text{NPP\&TAS}}\) & \(-\)3.81 & \(-\)1.90 & 0.82 & 0.66 \\ Slope\({}_{\text{NPP\&Pre}}\) & 8.66 & 25.94 & 9.58 & 9.56 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Values of the analyzed variables for the CAS-ESM2 and multi-model ensemble (MME) over tropics and northern high latitudes. These variables include anomalies in NPP (units: PgC years\({}^{-1}\)); surface air temperature (TAS; units: K); precipitation (Pre; units: mm day\({}^{-1}\)); partial correlation coefficients between NPP and surface air temperature (\(\text{PCC}_{\text{NPP\&TAS\_Pre}}\)), and those between NPP and precipitation (\(\text{PCC}_{\text{NPP\&Pre\_TAS}}\)); slopes between NPP and surface air temperature (\(\text{Slope}_{\text{NPP\&TAS}}\); units: PgC year\({}^{-1}\)/K), and those between NPP and precipitation (\(\text{Slope}_{\text{NPP\&Pre}}\), PgC year\({}^{-1}\)/(mm day\({}^{-1}\))). Asterisks represent values that are statistically significant (\(p<0.001\)). The green and red lines in Figure 8a represent time evolutions of NPP for the biogeochemically coupled simulation and the fully coupled abrupt-4 \(\times\) CO\({}_{2}\) simulation, respectively. Clearly, there is no significant decrease in NPP for the biogeochemically coupled simulation, while the fully coupled abrupt-4 \(\times\) CO\({}_{2}\) simulation shows a significant decrease in NPP. This difference leads to an average of 16.7 PgC year\({}^{-1}\) less NPP for the fully coupled abrupt-4 \(\times\) CO\({}_{2}\) simulation relative to the biogeochemically coupled simulation for the last 20 modeling years. The corresponding spatial distribution of the averaged difference shows that the decreased NPP mainly occurs over the tropics, with larger contributions from the Amazon and Africa; meanwhile, northern high latitudes and the Tibet Plateau show slight increases in NPP (Figure 8b). This evidence isolates the climatic effects, and confirms that they are attributable to the NPP decrease for the CAS-ESM2 in the fully coupled abrupt-4 \(\times\) CO\({}_{2}\) simulation with modeling time.