## 4 Conclusions and Discussions
### The Increase of Carbon Sequestration Would Reach a Peak During the Late 2020s  
Although NEP shows an increasing trend at both two warming targets, the abnormity of NEP displays more fluctuation in spatial and temporal patterns. Our results suggest that NEP has higher sensitivity and heterogeneity in response to climate change and other driven factors. Model results show obvious differences in impacts on NEP of Chinese terrestrial ecosystems between 1.5 \({}^{\circ}\)C and 2 \({}^{\circ}\)C warming targets. Most CMIP5 model simulations show that terrestrial ecosystems continue to uptake carbon under all scenarios [16]. However, increases of decomposition could offset or reverse the NEP increase, and the terrestrial ecosystem might become carbon sources [16; 58]. Our estimations of NEP responding to global warming suggest a similar trend by [PERSON] et al. [31] and [PERSON] et al. [59]. Some previous studies illustrated various changing trends of NEP under different scenarios with and without CO\({}_{2}\) fertilization. [PERSON] et al. (2000) suggested that global NEP would reach a peak in the 2030s and then decline to zero by 2100 [60]. [PERSON] et al. (2007) suggested that Chinese forest ecosystems would range from carbon sink to carbon source in the late 21\({}^{\text{st}}\) century under different scenarios with or without CO\({}_{2}\) fertilization [46]. However, our projections show that carbon sequestration would reach the peak during the late 2020s and then decline, which is consistent with the simulation of [PERSON] et al. [60]. All of the differentiations illustrated great uncertainties in estimations of future NEP compared with NPP. More models and regional studies are needed to explore the responses of terrestrial ecosystems to climate change and global warming in the next steps [20]. Figure 7: Spatial pattern of changes in annual mean temperature at 1.5 \({}^{\circ}\)C and 2 \({}^{\circ}\)C global warming relative to the reference period 1986 to 2005 under RCP4.5 and RCP8.5 scenarios. (**a**) 1.5 \({}^{\circ}\)C warming under RCP4.5; (**b**) 2 \({}^{\circ}\)C warming under RCP4.5; (**c**) 1.5 \({}^{\circ}\)C warming under RCP8.5; (**d**) 2 \({}^{\circ}\)C warming under RCP8.5. Potential Impacts of Land Use and Land Cover Change on Carbon Sequestration of Terrestrial Ecosystems  
Land Use and Land Cover (LULC) changes bring large uncertainties to risk analysis of future climate impacts on the terrestrial ecosystem. Human-induced land use change has a significant influence on carbon storage and fluxes in terrestrial ecosystems [61; 62]. However, the estimation of carbon sequestration due to land use change still has large uncertainties [15]. Previous studies have controversial conclusions on the effects of land use change on China's terrestrial ecosystem productivities. [PERSON] et al. (2018) reported that land cover change has caused over 63% of total carbon losses in terrestrial ecosystems since the 1980s [63]. Some other studies evidenced that the increases in NPP due to climate change were not enough to offset the NPP losses due to LULC change [64; 65]. However, the satellite data shows the earth is greening in recent 20 years, which is mostly attributed to the greenness of forests in China and intensive agriculture in India [66]. In addition, recommended agriculture management practices in China during recent 20 years, such as application of organic fertilizer, straw return, non-tillage, etc., enhanced soil carbon sequestration in cropland greatly [67], which would be expected to reduce the risk of carbon emissions of the terrestrial ecosystem. It is important to consider the effects of human-induced land use changes into the projections of the future terrestrial ecosystem. However, only three out of the five models (BNU-ESM, MPI-ESM-LR and NorESM1-ME) coupled with the dynamic vegetation process, and all models did not consider the effects of human-induced land use change. Considering the importance of this issue, we analyzed China's current policies and their possible impacts on future ecosystems.