## 4 Discussion
### CUE Extremes Were Mainly Controlled by GPP Rather Than Ra Extremes  
In this study, we found that the probabilities of GPP extremes resulted in CUE extremes being larger than Ra extremes, with the possibilities of GPP extremes resulting in CUE extremes being larger than 25% in most areas of global terrestrial ecosystems. These results indicated that CUE extremes were mainly controlled by GPP rather than Ra extremes. This is because the GPP is the foundation of the global carbon cycle, representing a natural component to start within any carbon budget [31]. The interannual variations of net carbon uptake in terrestrial are correlated larger with the peak of GPP than respiration [32]. Furthermore, as shown in Figure 1, a larger GPP is always correlated with a larger Ra [33]. Additionally, Ra is only a fraction of GPP, and there is evidence that Ra is closely linked to the supply rate through photosynthesis [34]. Therefore, increasing global plant photosynthesis is essential and beneficial for enhancing the proficiency of carbon sequestations in ecosystems from the atmosphere. We will investigate how to increase global plant photosynthesis in further work.  
Cooler and Wetter Than the Current Climate Conditions Are Beneficial for Enhancing CUE in Global Terrestrial Ecosystems  
This study indicated that CUE extremes are closely related to temperature and precipitation (Figure 7), consistent with the results that a global spatial and temporal pattern of global CUE are strongly associated with temperature and precipitation [35]. We also found that warmer and dryer climate conditions may lead to negative CUE extremes, consistent with the previous study that CUE typically decreases with slight increases in temperature [3]. In some regions of high temperature, Ra is more sensitive to temperature than GPP [8]; therefore, warmer temperatures increase Ra and result in lower CUE in specific areas, such as in some regions in the Northern Hemisphere at high latitude in JJA (Figure 7d). The previous study also indicated that global warming lengthens the growing season and increases ecosystem productivity and biome accumulation. Still, climate warming causes Ra proportionally higher than the increase in GPP [36]. Additionally, higher temperatures might accelerate the process of all biochemical reactions in the plant's kinetics, while up to high-temperature thresholds beyond which enzymes are inactivated, the CUE will decline [37]. Thus, global warming is not beneficial for enhancing CUE on a global scale.  
Precipitation directly influences ecosystem productivity [38]. CUE extremes correlate more with GPP extremes, and abundant precipitation is the foundation of positive GPP extremes [10]. Previous studies indicated that 40-50% of negative GPP extremes resulted from extremely low precipitation [39; 40], which agrees with the results of this study. Drought is harmful to vegetation growth [41]. Approximately 40% of GPP extreme events were controlled by precipitation in vegetated areas based on the diagnostic model and eddy covariance data [42]. Although higher CO\({}_{2}\) concentrations can increase GPP, climate change-induced drought, e.g., low soil water availability and heat stress, could reduce GPP at the same time [43]. Therefore, cooler and wetter conditions could enhance carbon absorption on a global scale.