## 5 Conclusions  
This study applied the CMIP6 standard pre-industrial experiment (piControl) and the abruptly quadruppled CO\({}_{2}\) experiment (abrupt-4 \(\times\) CO\({}_{2}\)) to investigate terrestrial NPP responses to elevated CO\({}_{2}\) and CO\({}_{2}\)-induced climate change. The research focused on projections of the CAS-ESM2, and on the underlying causes of its differences with the 22 CMIP6 models. We firstly showed climatic anomalies that were characterized with a warmer and wetter climate in the abrupt-4 \(\times\) CO\({}_{2}\) than in piControl for all models. The CAS-ESM2 projected a slightly (0.29 K) stronger warming than the MME, which was mainly contributed by northern Europe and the Amazon, while their precipitation anomalies were comparable. Then, we reported an overall enhanced NPP occurring for all models at a global scale, reflecting the dominant effects of CO\({}_{2}\) fertilization. The CAS-ESM2 projected a 37.72 \(\mathrm{P}\mathrm{G}\mathrm{c}\mathrm{y}\mathrm{a}^{-1}\) NPP enhancement, slightly (7.06 \(\mathrm{P}\mathrm{G}\mathrm{c}\mathrm{y}\mathrm{a}^{-1}\), 16%) less than that of the MME. This weaker NPP enhancement of the CAS-ESM2 relative to the MME was mainly attributed to tropical regions, including the Amazon and Africa, while northern high latitudes showed relatively strong NPP enhancement. Further analysis investigated the causes of these differences in NPP anomalies between the CAS-ESM2 and MME. This study highlights the role of their different CO\({}_{2}\)-induced climatic anomalies and different sensitivities to these climatic anomalies, while their sensitivities of photosynthetic capacity to CO\({}_{2}\) can also be crucial underlying causes. Our results reported the estimation of NPP responses to the quadrupled CO\({}_{2}\) in the CAS-ESM2, and emphasized the role of CO\({}_{2}\)-induced climatic effects in inducing uncertainties in projecting the terrestrial carbon cycle. The research is very important for the CAS-ESM2 group and other modeling groups. Firstly, the results provide valuable information about the overall performance of the CAS-ESM2 in projecting NPP responses to elevated CO\({}_{2}\), which can serve as a benchmark for modeling groups. Secondly, the vegetation was static and leaf area index also was not changed with climate in this study, which limited the interactions between vegetation and climate. Further research is needed  
Figure 10: Scatter plot of annual total NPP (\(\mathrm{P}\mathrm{G}\mathrm{c}\mathrm{y}\mathrm{a}^{-1}\)) in pre-industrial and abrupt-4 \(\times\) CO\({}_{2}\) simulations for the 23 models. The line represents their linear regression relationship. to re-assess these responses by including a dynamic global vegetation model. Thirdly, the results reveal the crucial effects of CO\({}_{2}\)-induced climate changes on terrestrial NPP. The climate sensitivities of models to elevated CO\({}_{2}\) can be reflected by equilibrium climate sensitivities. Therefore, quantitative assessment of the contributors to the differences in equilibrium climate sensitivities can lead to insights into the differences in NPP responses. Finally, the uncertainty analysis in this study reported hot spots with larger uncertainties in projecting NPP responses, such as the Amazon region and West Africa. This information is pivotal for modeling groups to further improve their ability to simulate the terrestrial carbon cycle.