## 4 Results and Analysis
### Interannual Variation of Carbon Sources and Sinks in the CGE  
Figure 6 shows the general trend of the interannual change of NEP of the CGE from 2010 to 2020. It can be seen from Figure 6 that the average annual NEP of the CGE from 2010 to 2020 increased year by year. However, the NEP decreased from 2014, and reached the maximum decline in 2015, which was 13.66 gC\(\cdot\)m\({}^{-2}\) lower than that in 2013. The main reason may be the decrease in precipitation from 2014 to 2016; the annual precipitation of the CGE was 416.19 mm in 2013, 387.49 mm in 2014 and 386.84 mm in 2015. The precipitation gradually recovered to the level of 2013 after 2016. The decrease in precipitation led to the reduction in area, the weakening of soil respiration, the reduction in primary productivity, the increase in grazing pressure and the increase in fire prevalence, which led to a decrease in NEP. From 2018 to 2020, the interannual mean value of NEP in the study area recovered and remained stable at the level of 2013. Figure 7 shows the grid-by-grid spatial distribution of the interannual change rate of NEP in the CGE. As can be seen from Figure 7, the interannual change rate of NEP in the CGE showed trend of obvious improvement for a large area, accounting for about 45% of the total area of the CGE. About 15% of the total area of the CGE had a trend of slight improvement. About 22% of the total area of the CGE had a trend of remaining unchanged. About 7% of the total area of the CGE had a trend of slight deterioration, and about 8% of the total area of the CGE was in significant deterioration. It can be seen that the annual average NEP change in the CGE tended to have good prospects, and the overall trend saw an increase. Affected by climate factors and policies related to environmental protection, the obvious improvement regions were mainly between the 200 mm isohyet and the 400 mm isohyet in China, and there was also more distribution in the northern part of Xinjiang. In addition to obvious improvement and slight improvement areas, significant deterioration areas could also be found in the southern grassland of China which is south of the 400 mm isohyet. In northwest China to the north of the 200 mm isohyet, most of the grasslands distributed on the Qinghai-Tibet Plateau and the western basin were in the range of slight deterioration and have remained unchanged under the influence of many factors, such as the climate factor. Figure 6: Interannual variation trend of NEP in the CGE from 2010 to 2020 in China. Figure 8 shows the proportions of different trends in the annual average NEP of the CGE in different provinces. It can be seen that the two provinces with the largest proportion of obvious improvement areas were Inner Mongolia and Sichuan, both accounting for 72% of their respective grassland area. The provinces and municipalities with a large proportion of obvious improvement were all located in northwest China. The province or municipality with the most areas of slight improvement was Xizang (32.08%), followed by Qinghai. Macao had the largest proportion of regions remaining unchanged (100%), followed by Hong Kong (72.06%) and Shanghai (69.62%). The larger the proportion of unchanged, slight improvement and obvious improvement regions, the higher the degree of environmental protection in the region to some extent. The proportion of slight deterioration in all regions of the study area was less than 20%, but the proportion of significant deterioration was not low. The proportion of significant deterioration regions exceeded that of obvious improvement regions in the follow provinces: Beijing, Fujian, Guizhou, Hainan, Hubei, Hunan, Jiangxi, Taiwan, Tianjin, Hong Kong, Zhejiang and Chongqing. If the number of significant deterioration regions exceeds that of obvious improvement regions for a long time, it will lead to the decline of NEP and the carbon sink capacity of the region, which is not conducive to sustainable development and even worse for the realization of the double carbon goal.