## 3 Results and Discussion
### Climate Attribution
\begin{table}
\begin{tabular}{c c c c c} \hline
**GPP vs. P** & **TC** & **TM** & **STM** & **TP** \\ \hline BCC & 0.68 *** & 0.74 *** & 0.62 *** & 0.1 \\ \hline CMCC & 0.72 *** & 0.58 *** & 0.42 ** & \(-\)0.01 \\ \hline MPI & 0.81 *** & 0.36 ** & \(-\)0.26 & 0.23 \\ \hline GPP vs. T & & & & \\ \hline BCC & \(-\)0.09 & \(-\)0.35 ** & \(-\)0.25 & 0.35 ** \\ \hline CMCC & 0.17 & 0.42 ** & 0.34 * & 0.74 *** \\ \hline MPI & \(-\)0.42 ** & 0.13 & 0.33 * & 0.62 *** \\ \hline \end{tabular}
\end{table} 
Table 1: Correlation parameter \(r\) between GPP and climate variables during the historical period of 1980–2013. * denotes correlation that is significant at the 0.1 level; ** denotes correlation that is significant at the 0.05 level; *** denotes correlation that is significant at the 0.01 level. \begin{table}
\begin{tabular}{c c c c c} \hline  & **TC** & **TM** & **STM** & **TP** \\ \hline BCC & 1.37(2.21) & 1.66(2.37) & 1.16(1.86) & 1.37(2.10) \\ \hline CMCC & 1.28(1.94) & 1.12(1.78) & 0.81(1.27) & 1.19(1.71) \\ \hline MPI & 1.18(1.89) & 1.1(1.79) & 0.83(1.69) & 1.14(2.02) \\ \hline \end{tabular}
\end{table} 
Table 2: Regional temperature change (\({}^{\circ}\)C) in comparison with that of the reference period of 1986–2005 under 1.5 and 2 \({}^{\circ}\)C (in parentheses) of global warming. \begin{table}
\begin{tabular}{c c c c c} \hline  & **TC** & **TM** & **STM** & **TP** \\ \hline BCC & 12.47(28.22) & 8.11(47.13) & \(-\)6.18(57.36) & 10.37(32.23) \\ \hline CMCC & 11.31(45.68) & \(-\)20.08(16.22) & \(-\)51.89(20.50) & 30.24(94.89) \\ \hline MPI & 2.65(8.66) & 6.91(19.23) & 32.83(46.13) & 11.5(9.42) \\ \hline \end{tabular}
\end{table} 
Table 3: Regional precipitation change (mm yr\({}^{-1}\)) in comparison with that of the reference period of 1986–2005 under 1.5 and 2 \({}^{\circ}\)C (in parentheses) of global warming. Based on the ecosystem-climate relationship (Table 4) and the known climate changes (Tables 2 and 3), the GPP change over the climate zones with the different ESMs is attributed quantitatively in Figure 6. As mentioned before, some contributions from the climate factors are negative, for which there are two major reasons. First, the climate change is negative--e.g., the negative precipitation change leads to a negative contribution to GPP. Second, the correlation between the climate factor and GPP is negative--e.g., temperature and GPP in the temperate continental zone in MPI-ESM1-2-HR. A positive anomaly in temperature could also lead to GPP reduction. The climate contribution to the variation in GPP changes among the studied ESMs. Even with the same model--e.g., MPI-ESM1-2-HR--it can be \(-12.8\%\) in the temperate continental zone and \(61.1\%\) on the Tibetan Plateau. Generally, under the effects of global warming, the influence of temperature on the ecosystem is larger than that of precipitation. This is mainly because the relative change in temperature is much larger than that in precipitation. On the Tibetan Plateau, where temperature is the most constraining factor (Tables 1 and 4), temperature plays a more dominant role than precipitation in the increase in GPP (Figure 6). In addition, it is observed that the relative GPP increase in the Tibetan Plateau is much larger than that in other regions. On the one hand, it is related to the low baseline value of GPP in the Tibetan Plateau (Figure 1d). A light increase in the GPP of the Tibetan Plateau is salient in relative values compared to the respective change in the subtropical-tropical monsoonal region. On the other hand, this may be related to the vegetation structure on the Plateau. Demonym plants can be divided into three types based on their photosynthesis patterns (i.e., C3, C4, and corassulacean acid metabolism). C3 photosynthesis produces a three-carbon compound during the Calvin cycle, while C4 photosynthesis makes an intermediate four-carbon compound that splits into a three-carbon compound for the Calvin cycle. They favor different conditions of nature.