## 4 Discussion  
### Comparison with Historical Climate Change Effects  
Our study results show that Tianshan North Slope will become wet and warm in the future, and the ecosystem will benefit from this change under each RCP scenario with increasing NPP during 2006-2055. The NPP increasing rate during 2006-2055 is 1.16, 1.62, and 2.15 g C m\({}^{-2}\) year\({}^{-1}\) under RCP2.6, RCP4.5, and RCP8.5, respectively. Different from the climate change under the RCP scenarios, Central Asia (including our study area) has experienced a \"warm-dry\" climate with a CO\({}_{2}\) concentration increase of 18% from 1980 to 2014 [17]. In response to these changes, Central Asia presented as a carbon source with a declining rate of 0.82 g C m\({}^{-2}\) year\({}^{-1}\). However, they [17] also found that the NPP in Tianshan North Slope showed an increasing trend due to the increase of the annual precipitation. [PERSON] et al. [37] found that the regional NPP in Xinjiang province presented an increasing trend from 2008 to 2011, and decreasing trend during 2005-2006 and 2013-2014 by using three reanalysis data sets. Taking climate change, CO\({}_{2}\) fertilization effect, and grazing into consideration, [PERSON] et al. [12] found that the four vegetation types in Central Asia presented similar responses to the climate changes from 1979 to 2011, during which the NPP for GRS, forest, shrub land, and CRP decreased with a rate of 0.58 g C m\({}^{-2}\) year\({}^{-1}\), 1.58 g C m\({}^{-2}\) year\({}^{-1}\), 0.51 g C m\({}^{-2}\) year\({}^{-1}\), and 1.10 g C m\({}^{-2}\) year\({}^{-1}\), respectively. For the ET, our results show that the regional mean ET presents an increasing trend with a rate of 0.38 mm year\({}^{-1}\) under RCP2.6, 0.43 mm year\({}^{-1}\) under RCP4.5, and 0.52 mm year\({}^{-1}\) under RCP8.5. By using an ecosystem model, [PERSON] et al. [25] explored the dynamics of ET for different GRS types, and found that the Central Asia region experienced significant declines of 1.47-2.72 mm per decade from 1979 to 2011. Based on remotely sensed data, [PERSON] et al. [57] found that the ET of arid Central Asia presented a decline trend by 0.44 mm year\({}^{-1}\) from 2000 to 2014. During the same study period, [PERSON] et al. [58] found that the GLEAM ET data set showed that the ET for Tianshan North Slope presented a decreased trend, while the MODIS ET product showed an opposite trend. The WUE is influenced by climate factors, GPP, CO\({}_{2}\) concentration, and growing season length. The dominant factor for WUE varies in different ecosystems [31; 59]. For example, temperature plays an important role in the WUE dynamic of shrub ecosystems, while the precipitation is more important for grassland and forest ecosystems [32; 60]. Under the warm and wet trend and increasing CO\({}_{2}\), each vegetation type presents an increasing trend from 2006 to 2055 under all RCP scenarios. For the desert vegetation (NPS, GRS, and PS), the increasing WUE may be induced by the enhanced precipitation [61]. While, for the TNF, the increasing WUE could be enhanced by the increasing temperature. For the others, such as CRP, may benefit from all the changes in climate and CO\({}_{2}\) concentration [29]. Previous studies and this study have proved that the arid ecosystem is very sensitive to the changes of climate and CO\({}_{2}\) concentration, as well as to human activities, although there are lots of uncertainties caused by the forcing data sets and methods. The ecosystems in the arid area are fragile and the development of the economy is limited by the water resource. So, exploring the possible responses of the arid ecosystems to the changes in climate and CO\({}_{2}\) in the near future is very necessary for realizing the primary goal of the Sustainable Development Goals (SDGs). The climate action is one of the most important goals in SDGs, and it asks people to take urgent actions to deal with climate change and the effects of climate change.