## 1 Introduction
However, many uncertainties remain in the modeling projections of the terrestrial carbon cycle. A previous study showed that NPP enhancements in abrupt-4 \(\times\) CO\({}_{2}\) relative to piControl diverge among the eight CMIP5 ESMs, ranging between 10.8 PgC year\({}^{-1}\) and 121.6 PgC year\({}^{-1}\)[16]. The magnitudes of carbon-concentration feedback (mean \(\pm\) standard deviation = 0.97 \(\pm\) 0.40 PgC ppm\({}^{-1}\)) and carbon-climate feedback (\(-\)45.1 \(\pm\) 50.6 PgC \({}^{\circ}\)C\({}^{-1}\)) over land also show relatively large inter-model spread (one standard deviation) compared with the multi-model mean, and are more uncertain than those over ocean (0.79 \(\pm\) 0.07 PgC ppm\({}^{-1}\), \(-\)17.2 \(\pm\) 5.0 PgC \({}^{\circ}\)C\({}^{-1}\)); this illustrates divergent behaviors of the11 CMIP6 ESMs in projecting terrestrial carbon cycles [1]. Uncertainties in the performance of ESMs make it difficult to accurately project future climate change. These uncertainties result from many sources. Firstly, differences in model structures and parameterizations are a key source. Based on the 12 CMIP5 ESMs, a previous study reported that model structure is responsible for 80% of the uncertainty in projecting terrestrial carbon uptake, overwhelming the roles of emission scenarios and internal variability [17]. Models with nutrient limitations, particularly nitrogen and phosphorus, are generally characterized with weaker carbon-concentration feedback and carbon-climate feedback than models without [1, 18]. Great divergence in terrestrial carbon responses can also occur among models with nitrogen limitations as a result of differing parameterizations of key nitrogen cycle processes [19]. Secondly, different climate sensitivities of ESMs to CO\({}_{2}\) increases can lead to uncertainties in terrestrial carbon responses via carbon-climate feedbacks. ECS based on CMIP6 models range between 1.8 K and 5.6 K, higher than the values (1.5-4.5 K) based on CMIP5 [20]. This increase means persistent uncertainties remain in the projected response of global surface temperature to elevated CO\({}_{2}\), and consequently in that of the terrestrial carbon cycle. These sources of uncertainties in model projections of the terrestrial carbon cycle show there is an urgent need for systematic assessments and significant improvements for ESMs. The Chinese Academy of Sciences (CAS) Earth System Model version 2 (CAS-ESM2) has finished the proposed CMIP6 standard experiments [21]. This study used the results of the two experiments, abrupt-4 \(\times\) CO\({}_{2}\) and piControl, of the CAS-ESM2, and compared them with those of the CMIP6 models. We focused on assessing the NPP responses of the CAS-ESM2 to elevated CO\({}_{2}\), and on detecting the underlying causes of the differences with CMIP6 models. The assessments and detections are favorable for understanding the behavior of the CAS-ESM2 deeply, and for further improving the CAS-ESM2 in projecting the terrestrial carbon cycle. A detailed model description, datasets, and methods are in Section 2. Section 3 reports the comparisons of climatic anomalies, NPP anomalies, and the drivers between the CAS-ESM2 and CMIP6 models. A further discussion and perspective is given in Section 4, which is followed by Section 5 that summarizes this research.