## 3 Results
### Drivers of the Differences in Trends of NPP Anomalies  
Many published studies and our previous research reported on the crucial role of CO\({}_{2}\)-induced climate change in regulating the terrestrial carbon cycle [27; 28; 29]. The CO\({}_{2}\)-induced warmer and drier climatic conditions can lead to a decreasing trend in terrestrial NPP for the tropics, while northern high latitudes are characterized with an increasing trend in terrestrial NPP because of the CO\({}_{2}\)-induced warmer and wetter climate change. Thus, we next focused on how the anomalies in surface air temperature (TAS) and precipitation (Pre) drive the differences in trends of NPP anomalies between the CAS-ESM2 and MME, for both tropical ecosystems and northern high-latitude ecosystems. Figure 7 and Table 1 compare the relationships between NPP anomalies, TAS and Pre over the two ecosystems for the CAS-ESM2 and MME. Over tropical ecosystems, both the CAS-ESM2 and MME showed significant and negative correlations between NPP anomalies and TAS anomalies, with partial correlation coefficients \(-0.99\) and \(-0.98\), respectively. Meanwhile, the slope of the CAS-ESM2 (\(-3.81\) PgC K\({}^{-1}\)) is steeper than that of MME (\(-1.90\) PgC K\({}^{-1}\)). In other words, a stronger  
Figure 7: Scatter plots of NPP anomalies (PgC year\({}^{-1}\)) with (**a**) surface air temperature anomalies (TAS; K) and (**b**) precipitation anomalies (Pre; mm day\({}^{-1}\)) over tropics for the CAS-ESM2 (circled dots) and multi-model ensemble (MME; solid dots). Each dot represents a value in each year of the 149-year simulation, and their colors change from cool to warm ones with modeling time. The lines represent their linear regression relationships. (**c,d**) are the same as (**a,b**) but for northern high latitudes. decrease in tropical NPP will occur for the CAS-ESM2 relative to MME, even if the same increase occurs in TAS. Moreover, the anomaly in tropical TAS for the CAS-ESM2 (6.49 K) is larger than that for MME (6.02 K). These differences in TAS anomalies and the slopes contribute to the stronger negative trend in tropical NPP and the consequently weaker NPP enhancement for the CAS-ESM2, in comparison with the MME. On the other hand, tropical NPP anomalies were significantly and positively correlated with precipitation anomalies, and the partial correlation coefficients were 0.72 and 0.47 for the CAS-ESM2 and MME, respectively. Compared to the MME, even though the CAS-ESM2 shows the gentler slope, the CAS-ESM2 projects a stronger decrease in precipitation, resulting in a stronger decrease in NPP for the CAS-ESM2. Overall, the CAS-ESM2, relative to MME, projects a warmer and drier tropical climate, and its tropical NPP is more sensitive to changes in surface air temperature, which together lead to the relatively weak NPP enhancement. Over northern high-latitude ecosystems, both the CAS-ESM2 and MME showed significant and positive correlations between NPP anomalies and TAS anomalies (Table 1). Correspondingly, the slopes are 0.82 PgC K\({}^{-1}\) and 0.66 PgC K\({}^{-1}\), and the TAS anomalies are 8.48 K and 8.31 K, respectively. This slightly steeper slope and warmer climate of the CAS-ESM2 than those of the MME contribute to its stronger NPP enhancement. On the other hand, the CAS-ESM2 also showed significant and positive correlations between NPP anomalies and precipitation anomalies, while there were no significant correlations for the MME. Compared to the MME, the CAS-ESM2 is also characterized with a slightly steeper slope between NPP anomalies and precipitation anomalies, and with a wetter climate, which further contribute to its relatively strong NPP enhancement. Therefore, for northern high-latitude ecosystems, the 3.03 PgC year\({}^{-1}\) additional NPP enhancement of the CAS-ESM2 is caused by its warmer and wetter climate and the correspondingly larger sensitivity to these climate changes, in comparison with the MME.