## Result
### Attributions of Long-Term Changes in GPP
Unlike the driving force of photosynthesis [62], our study did not show a strong positive correlation between SRAD and GPP, and some basins in the north, such as the CB and the HRB, even showed a negative correlation (R \(<-0.2\)). One explanation for this phenomenon is that strong solar radiation often reflects a lack of cloud cover, resulting in a reduction in precipitation and the enhancement of soil water evaporation [63]. As mentioned above, in arid regions such as northern China, vegetation is very sensitive to water changes, and the promotion of photosynthesis by radiation is not sufficient to offset the limitations brought by drought. Human activities can have substantial effects on vegetation productivity. Influenced by the policies of the Chinese government, for example, during 1999-2010, 16,000 square kilometers of farmland were turned into forest or grass by the Grain to Green Program [64]. LAI in most parts of China have shown an upward trend, which has a significant correlation with the trends of GPP. In the YERB, the northern HRB, the western SRB, and the eastern CB, the correlation was particularly evident (R \(>\) 0.3), implying that the government's efforts in improving the environmental quality of ecosystems have been effective. Figure 5: Annual mean of air temperature, downward shortwave radiation (SRAD), precipitation, leaf area index (LAI), and aerosol optical depth (AOD) from 1982 to 2015, and CO\({}_{2}\) emissions from 2010 to 2015. The definition of box and whiskers on the lower right of each panel is the same as Figure 1.  
Another factor regarding human activities is the increase in CO2 emissions. For example, in 2011, the use of fossil fuels related to human activities resulted in 33.2 billion tons of CO2 emissions worldwide [65]. The increase in atmospheric CO2 concentration can improve vegetation productivity. [PERSON] et al. [66] found that global primary productivity increases by about 2.3 Pg C each year. [PERSON] et al. [6] reported that about 60% of the carbon absorbed by land comes from CO2 emissions. Our results show that the rise in CO2 emissions in most parts of China can promote the growth of GPP. However, the correlation between CO2 and GPP is not significant in the SWB, the southern CB, the western YRB, or the northern SRB. Among them, the SWB was even negatively correlated (R = \(-\)0.101). This may have been due to the air temperature, which is low in these areas, and the GPP dynamic is more affected by hydrothermal conditions rather than CO2. Figure 6: Spatial distribution of the relevance of (**a**) air temperature, (**b**) SRAD, (**c**) precipitation, (**d**) LAI, (**e**) AOD, and (**f**) CO2 to GPP. The correlation map is based on the pixel-wise annual mean of GPP and each factor. Values with statistical significance at the 95% confidence level are displayed with cross symbols. The nan value is displayed in white. Furthermore, due to the human activities caused by rapid urbanization and industrial expansion, the air pollution level of more than 90% of China's cities exceeds the standards of the World Health Organization [67]. Since the amplitude of aerosols reflects the degree of air pollution to a certain extent, AOD is used to quantify aerosols [68; 69]. In this study, we analyzed the correlation between AOD and GPP. A negative correlation between AOD and GPP was observed on a national scale. This is acceptable, since the absorption of atmospheric aerosols reduces the photosynthetically active radiation in plants [70; 71], which affects vegetation productivity. However, this correlation was weak, regardless of whether it was positive or negative. For example, even where there was a positive correlation in some areas, such as the HRB, the correlation was still very weak. This is probably because factors such as air temperature and precipitation often have a more obvious impact on vegetation growth, which dampens the response of vegetation productivity to AOD. Another possible explanation is the complex climatic effects of aerosol species.