## 4 Discussion: Uncertainties and Perspective  
This study compared NPP response to abruptly quadrupled CO\({}_{2}\) and the associated climate change between the CAS-ESM2 and the 22 CMIP6 models. The different NPP responses between the CAS-ESM2 and the 22 CMIP6 models reflect uncertainties in model projections of changes in the terrestrial carbon cycle. We used one standard deviation among the 23 models to represent the inter-model spread, referred to as the uncertainty in magnitudes of overall NPP responses, and standardized the inter-model spread by the ensemble mean NPP anomaly to reduce the effects of absolute NPP values (Figure 9a). Clearly, the standardized inter-model spread of overall NPP anomalies was generally larger over the tropics than over extratropical ecosystems, especially over the Amazon, West Africa, South Africa, and northern Australia. These regions were characterized with opposite NPP responses among the 23 models (Figure S2), which not only reduced the ensemble mean NPP anomaly, but also enhanced inter-model spread, leading to larger standardized inter-model spread. Furthermore, we defined a consistent degree as a percentage of models that showed the same signal in NPP response (positive or negative) as that of the ensemble NPP, and used it to represent uncertainties in the directions of overall NPP anomalies (Figure 9b). The results show lower consistency over the tropics, especially in regions with larger standardized inter-model spread, while extratropical ecosystems broadly show 100%  
Figure 8: (**a**) Time evolutions of NPP (units: PgC year\({}^{-1}\)) for the biogeochemically coupled simulation (green line) and the fully coupled abrupt-4 \(\times\) CO\({}_{2}\) simulation (red line). (**b**) Spatial distribution of the differences in NPP (TgC year\({}^{-1}\)) between the fully coupled abrupt-4 \(\times\) CO\({}_{2}\) simulation and the biogeochemically coupled simulation for the last 20 years. consistency. Overall, these features reveal that the NPP responses were relatively more uncertain over tropical ecosystems than over extratropical ecosystems, in both magnitudes and directions. These uncertain behaviors come from many sources. Our results emphasized differences in the CO\({}_{2}\)-induced climatic effects, including different associated climatic anomalies and different climate sensitivities. Many previous studies also reported on the crucial effects of model structures and parameterizations on the terrestrial carbon cycle [1; 17; 18]. For example, [PERSON] et al. (2020) showed that models with or without a representation of nitrogen limitation are different in magnitudes of terrestrial carbon responses to the elevated CO\({}_{2}\) and the associated climate changes [1]. Similarly, this study reports that models with carbon-nitrogen interactions showed relatively weak NPP increases. The three smallest magnitudes of NPP increase among the 23 models, 15.42 PgC year\({}^{-1}\), 9.75 PgC year\({}^{-1}\), and 7.76 PgC year\({}^{-1}\), were simulated by the ACCESS-ESMI-5, SAM0-UNICON, and TaiEGNI models, respectively. All of them are interactive with nitrogen limitations on the carbon cycle [1]. In contrast, models (CanESM5, CNRM-CM6-1, and GFDL-ESM4) that lacked nitrogen limitations projected the three greatest magnitudes of NPP increase among the 23 models, 82.36 PgC year\({}^{-1}\), 80.55 PgC year\({}^{-1}\), and 82.43 PgC year\({}^{-1}\), respectively. These differences indicate that more efforts are needed to understand and model terrestrial carbon cycle processes accurately. Even though many uncertainties remain, the multi-model ensemble results provide a valuable opportunity to estimate possible changes in the terrestrial carbon cycle in response to quadrupled CO\({}_{2}\) and the associated climate change. Figure 10 shows the relationship between the simulated global NPP in picontrol (NPP\({}_{\rm PI}\)) and that in the abrupt-4 \(\times\) CO\({}_{2}\) simulation (NPP\({}_{\rm 4C02}\)) among the 23 CMIP6 models.