## 4 Discussion
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Ecosystem Type** & **Study Area** & **Methodology** & \(\beta\)**-Factor (1 SE)** & **Source** \\ \hline \multirow{4}{*}{Grassland} & Phoenix city, AZ, USA & Model simulation & 0.85 (0.26) & This study \\ \cline{2-5}  & central Colorado, USA & Open Top Chamber & 0.92–0.95 & [PERSON] et al., 2005 [54] \\ \cline{2-5}  & northwestern Switzerland & Screen aided CO\({}_{2}\) control & 0.45 & [PERSON] et al., 2001 [55] \\ \cline{2-5}  & Minnesota, USA & FACE experiments & 0.52 & [PERSON] et al., 2001 [56] \\ \cline{2-5}  & Belgium & Open Top Chamber & 0.50 & [PERSON] et al., 2012 [57] \\ \hline \multirow{4}{*}{Shrubland or desert} & Phoenix city, AZ, USA & Model simulation & 0.79 (0.27) & This study \\ \cline{2-5}  & Nevada Desert FACE facility, & & 1.68 (0.92) & [PERSON] et al., 2006 [58] \\ \cline{2-5}  & southern Nevada, USA & Open Top Chamber & 1.31 & [PERSON] et al., 2000 [59] \\ \cline{2-5}  & Global desert & Model simulation & 0.76–0.77 & [PERSON] et al., 2003 [51] \\ \hline \multirow{4}{*}{Forest} & Phoenix city, AZ, USA & Model simulation & 0.83 (0.31) & This study \\ \cline{2-5}  & Florida, USA & Open Top Chamber & 1.02–1.95 & [PERSON] et al., 2002 [60] \\ \cline{2-5}  & Florida, USA & Open Top Chamber & 0.88–1.18 & [PERSON] et al., 1999 [61] \\ \cline{2-5}  & Belgium & Open Top Chamber & 0.72 & [PERSON] et al., 2000 [62] \\ \cline{2-5}  & Edinburgh, UK & Open Top Chamber & 0.84 & [PERSON] et al., 1997 [63] \\ \cline{2-5}  & Global forest & FACE experiments & 0.6 & [PERSON] et al., 2005 [49] \\ \hline \multirow{4}{*}{Cropland} & Phoenix city, AZ, USA & Model simulation & 0.15–0.16 & This study \\ \cline{2-5}  & Netherlands & Open Top Chamber & 0.28 & [PERSON] et al., 1999 [64] \\ \cline{2-5}  & Switzerland & FACE experiments & 0.1 (0.29) & [PERSON] et al., 1997 [65] \\ \cline{1-1} \cline{2-5}  & Germany & FACE experiments & 0.37 & [PERSON] et al., 2010 [66] \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Comparison of NPP trend derived from CO\({}_{2}\) change between HPM-UEM model and FACE/OTC experiment (FACE: Free-air CO\({}_{2}\) enrichment). RCP8.5 scenarios imply the climate and CO\({}_{2}\) changes may enhance the urban ecosystem services in the 21 st century. In particular, the simulated results indicate the cropland yield may increase rapidly in the 21 st century (Figure 4). Other studies also showed future climate change may benefit food provision [69; 70]. Improved plant growth and leaf area will also enhance other urban ecosystem services such as heat regulation [71], buffering capacity or the absorption capacity for wastes and emissions [72; 73], and carbon sequestration [74]. These ecosystem services would partially moderate the worsen heat stress and air pollution in urban areas under the future scenarios [75; 76; 77; 78; 79]. It is noteworthy that the wealthy population living in the mesic residential areas may enjoy more benefit from the enhanced ecosystem services than the poorer population living in the xeric residential areas, because the NPP of the mesic ecosystems (lawns and urban forests) would increase much more quickly than the xeric ecosystems (dry shrubs and grassland) (Figures 4 and 5). Although our model projections indicate that higher atmospheric CO\({}_{2}\) may stimulate ecosystem NPP in the urban area, it does not mean that a high CO\({}_{2}\) emission scenario is preferable, nor does it imply that the enhanced ecosystem productivity could offset the elevated urban CO\({}_{2}\) emission. Even if not taking into account the higher soil CO\({}_{2}\) emission due to enhanced heterotrophic respiration under a warmer climate [80], the magnitude of ecosystem NPP in the Phoenix city (629 k ton C/year by the late-21 st century, under the RCP8.5 scenario) is too small compared with the anthropogenic CO\({}_{2}\) emission (71,356 K ton C/year according to [PERSON] and [PERSON], 2002) [81].