## 3 Results
### Responses of NPP to 1.5 \({}^{\circ}\)C and 2 \({}^{\circ}\)C Warming  
At the national scale, the mean annual NEP was estimated at approximately 15.9 \(\pm\) 10.8 g C m\({}^{-2}\) yr\({}^{-1}\) in the reference period, and was projected to increase to 25.2 \(\pm\) 11.7 g C m\({}^{-2}\) yr\({}^{-1}\) and 23.3 \(\pm\) 11.8 g C m\({}^{-2}\) yr\({}^{-1}\) for 1.5 \({}^{\circ}\)C, 24.3 \(\pm\) 13.8 g C m\({}^{-2}\) yr\({}^{-1}\) and 32.6 \(\pm\) 14.1 g C m\({}^{-2}\) yr\({}^{-1}\) for 2 \({}^{\circ}\)C under the different RCP scenarios, respectively. The total NEP was projected to be increased by 53% at 1.5 \({}^{\circ}\)C warming, and by 79% at 2 \({}^{\circ}\)C warming (averages of the two scenarios) compared with that in the reference period. NEP was projected to increase approximately by 28% with the additional 0.5 \({}^{\circ}\)C warming, which suggests that the terrestrial ecosystems in China were projected to be an increasing carbon sink with the enhanced warming target, and warming together with the elevated CO\({}_{2}\) concentration would promote the net carbon uptake.  
However, the total NEP shows a declined growth rate with the additional 0.5 \({}^{\circ}\)C under the two scenarios. For the RCP4.5 scenario, both linear trend and mean value of NEP show a slight decrease in 2 \({}^{\circ}\)C period compared with those in 1.5 \({}^{\circ}\)C period. The linear slope of NEP depresses rapidly at 2 \({}^{\circ}\)C warming under the RCP8.5 scenario though the mean value is higher than that at 1.5 \({}^{\circ}\)C (Figure 4). During the 2 \({}^{\circ}\)C warming period, NEP shows enhanced fluctuations but a declining growth, especially under the RCP8.5 scenario.  
Figure 3: Spatial variation of the linear slope of NPP changing in 1.5 \({}^{\circ}\)C and 2 \({}^{\circ}\)C warming period under RCP4.5 (**a**) and RCP8.5 (**b**) scenarios.  
The projections suggest that most Chinese terrestrial ecosystems will act as carbon sinks, and the southern China and mountain areas of northeastern China have the highest NEP (Figure 5). Also, NEP was projected to be higher for the RCP8.5 scenario and at 2 \({}^{\circ}\)C warming.  
Spatially, the responses of NEP to warming are different from that of NPP substantially. Although Chinese terrestrial ecosystems were estimated to act as carbon sinks overall and NEP was simulated to  
Figure 4: Linear fitting of annual NEP during the period of the 1.5 \({}^{\circ}\)C and 2 \({}^{\circ}\)C global warming for different RCP scenarios.  
Figure 5: Spatial variations of annual NEP in China averaged over 1.5 \({}^{\circ}\)C and 2 \({}^{\circ}\)C global warming periods under RCP4.5 and RCP8.5 scenarios. (**a**) _NEP1\({}_{RCP45}\);_ (**b**) _NEP2\({}_{RCP45}\);_ (**c**) _NEP1\({}_{RCP85}\);_ (**d**) _NEP2\({}_{RCP85}\)Subst._  
increase continually during 1961-2099 under two scenarios, some areas, especially in northeastern China, would experience a decline of carbon uptake capacity. Under the RCP4.5 scenario, the NEP increases in most China's areas at 1.5 \({}^{\circ}\)C, especially in the central, southwestern China and mountain areas of northeastern China (Figure 6a). However, the response of NEP to a further 0.5 \({}^{\circ}\)C warming has large spatial discrepancies. Warming promotes the increase of NEP further in the regions where NEP increases at 1.5 \({}^{\circ}\)C. In contrast, warming accelerates the decrease of NEP in the regions where NEP decreased originally (Figure 6b). Under the RCP8.5 scenario, the response of NEP to 1.5 \({}^{\circ}\)C warming would be similar to 2 \({}^{\circ}\)C warming under the RCP4.5 scenario. The increase of NEP would occur in central and southwestern China, and northeastern China would experience high decreases in NEP (Figure 6c). However, the additional 0.5 \({}^{\circ}\)C warming would induce further increases in NEP over most parts of China, and areas with decreasing NEP would decrease (Figure 6d).