## 3 Results
### NPP Anomalies  
In response to the quadrupled CO\({}_{2}\) and the CO\({}_{2}\)-induced climate change, terrestrial NPP showed an overall enhancement for all models at a global scale (Figure 3). The magnitude of the NPP enhancement was very different among the CAS-ESM2 and the 22 CMIP6 models, ranging between 7.56 PgC year\({}^{-1}\) for TaiESM1 and 83.95 PgC year\({}^{-1}\) for CanESM5. The CAS-ESM2 projected an enhanced NPP by a value of 37.72 PgC year\({}^{-1}\), which was slightly (7.06 PgC year\({}^{-1}\)) less than that for the MME. These positive anomalies in NPP for all models are because of the enhancement in GPP which overwhelms that in autotrophic respiration (Figure S1), reflecting the dominant effects of CO\({}_{2}\) fertilization. Figure 4 shows spatial distribution of NPP anomalies for the CAS-ESM2, MME, and their differences. The positive anomalies in NPP for the CAS-ESM2 (Figure 4a) occurred in 92% of land grids, and are mainly attributed to the highly productive regions, including boreal evergreen forests and tropical forests, except for the Amazon region which was characterized with large negative anomalies. For the MME (Figure 4b), the enhanced NPP occurred in almost all land grids, and the mentioned regions with high production, also made larger contributions to the positive anomalies in NPP. Being different from the CAS-ESM2, the MME projected positive NPP anomalies over Amazon forests, even though the enhancement was weaker than that over forests of tropical Africa and tropical Asia. The spatial distribution of differences between the CAS-ESM2 and MME (Figure 4c) shows that weaker NPP enhancement of the CAS-ESM2 relative to MME is seen over 53% of land grids, and was mainly contributed by tropical regions (Figure 4f), including the Amazon and Africa. In reverse, northern high latitudes (i.e., Europe) were characterized with stronger NPP enhancement for the CAS-ESM2 relative to the MME (Figure 4f). Figure 5 compares the time evolution of NPP anomalies for the CAS-ESM2 and MME over the globe and two regions colored in Figure 4f. The two regions, northern high latitudes and the tropics, represent regional characteristics with stronger and weaker enhanced NPP of the CAS-ESM2 relative to the MME, respectively. At a global scale, both the CAS-ESM2 and MME projected a decreased trend in NPP anomalies with modeling  
Figure 3: Global annual total NPP anomalies (PgC year\({}^{-1}\)) between simulations of abrupt-4 \(\times\) CO\({}_{2}\) and piControl for the CAS-ESM2 and the 22 CMIP6 models. The dashed line represents the multi-model ensemble value (MME). time, but the CAS-ESM2 showed a steeper one than MME. The difference between the two slopes is the main cause of the overall 7.06 \(\mathrm{PgC}\) year\({}^{-1}\), less the NPP enhancement of the CAS-ESM2 relative to the MME for the last 30 modeling years, since their difference in NPP anomalies was slight at the beginning time. The tropics show a similar characteristic as the globe, leading to the 9.24 \(\mathrm{PgC}\) year\({}^{-1}\), with less NPP enhancement of the CAS-ESM2 than the MME. Oppositely, northern high latitudes were characterized with an increased trend in NPP anomalies with modeling time, for both the CAS-ESM2 and the MME. Compared to the MME, the CAS-ESM2 simulated 3.03 \(\mathrm{PgC}\) year\({}^{-1}\) more NPP enhancement because of its steeper increasing trend in NPP anomaly. We further plotted the spatial pattern of the trends in NPP anomalies for the CAS-ESM2 and MME, as well as for their differences (Figure 6). Both the CAS-ESM2 and MME showed overall negative trends in NPP anomalies over tropical ecosystems, while northern mid-high latitudes were characterized with overall positive trends in NPP anomalies. The spatial differences in their trends in NPP anomalies show that the CAS-ESM2 simulated an overall stronger negative trend over tropical ecosystems, but an overall stronger positive trend over northern high latitudes, in comparison with the MME (Figure 6f).