## 3 Results and Discussion
### Influence of Future CO\({}_{2}\) Increase and Climate Change on NPP  
Our simulations indicated that continuous increase in CO\({}_{2}\) concentrations would significantly enhance terrestrial NPP in China (Figure 10). The average CO\({}_{2}\) concentration was 393.24 ppm during 2007-2016, and would increase to 817.15 ppm and 894.37 ppm under the A2 and RCP85 scenarios and to 547.67 ppm and 535.83 ppm under the B1 and RCP45 scenarios during 2091-2100. In the context of the above CO\({}_{2}\) concentrations without corresponding climate change, the total of simulated NPP in China would reach 5.08 Pg C yr\({}^{-1}\) and 5.23 Pg C yr\({}^{-1}\) under the A2 and RCP85 scenarios during 2091-2100, respectively. These values are 1.68 Pg C yr\({}^{-1}\) (49.49%) and 1.83 PgC yr\({}^{-1}\) (53.86%) higher than the average NPP during 2007-2016 (3.40 Pg C yr\({}^{-1}\)). Under the B1 and RCP45 scenarios, CO\({}_{2}\) concentration would increase at smaller magnitudes. As a consequence, the increases of NPP driven by sole CO\({}_{2}\) increase would be 0.82 PgC yr\({}^{-1}\) (24.02%) and 0.67 PgC yr\({}^{-1}\) (19.57%) by the end of the 21 st century, respectively, much smaller than the increases under A2 and RCP85 scenarios. The NPP simulated with only increase in CO\({}_{2}\) concentration was compared with the estimates that combined the effects of climate change and increasing CO\({}_{2}\). The total NPP (only CO\({}_{2}\) change) under the A2 and RCP85 scenarios would be 3.03% (0.15 Pg C yr\({}^{-1}\)) and 2.54% (0.13 Pg C yr\({}^{-1}\)) higher than couple scenarios during 2091-2100, respectively. The corresponding values would approximately 0.60% (0.03 Pg C yr\({}^{-1}\)) and 3.58% (0.14 Pg C yr\({}^{-1}\)) under B1 and RCP45 scenarios. The warmer climate in A2 and RCP85 scenarios induced slightly larger reductions of total NPP than B1 and RCP45 scenarios. As a whole, the warm climate in the future had a slightly negative influence on the NPP in China. Figure 9: Spatial distribution of annual NPP (g C m\({}^{-2}\) yr\({}^{-1}\)) in China simulated with CO\({}_{2}\) concentration and projected climate under different climate change scenarios in different periods. Values shown are averages of NPP simulated using climates projected by two different climate models. (**a–c**) are results in the A2 scenario; (**d–f**) are results in the B1 scenario; (**g–i**) are results in the RCP45 scenario; (**j–l**) are results in the RCP85 Scenario. There are no obvious trends in the total NPP of China simulated with projected future climate under the A2, RCP85 and B1, RCP45 scenarios and CO\({}_{2}\) concentration value in 2016 (Figure 10). Compared with the average NPP (3.4 PgC yr\({}^{-1}\)) in the past ten years (2007-2016), the solely future climate change would drive slight changes in total NPP with an increase of 0.78% (0.026 Pg C yr\({}^{-1}\)) and decreasing by 1.64% (0.056 Pg C yr\({}^{-1}\)) under the A2 and RCP85 scenarios respectively, and an increase of only 1.81% (0.062 Pg C yr\({}^{-1}\)) and decrease of 0.12% (-0.004 Pg C yr\({}^{-1}\)) under the B1 and RCP45 scenarios during 2051-2100, respectively. Previous studies have indicated that precipitation was the main limiting factor of increasing NPP, and that temperature was also an important assisting factor. In years of abundant (deficient) precipitation, the rise in temperature caused an increase (decrease) in NPP [19]. Interannual and spatial variations in precipitation in different regions of China led to trends and variability of NPP. Precipitation in Northwest China has increased with fluctuations since 1961, and the climate in Qinghai-Tibet Plateau area has become warmer and wetter [73]. The positive effect on NPP caused by increasing precipitation in the semi-arid ecosystems of China might offset the negative effect from warmer climates in other areas.