## 3 Results  
### The Projected Terrestrial GPP Variations  
In terms of the total GPP during four representative months (January, April, July, and October), global climate change leads to declines in GPP of \(-\)39.69 TgC, \(-\)44.69 TgC, and \(-\)17.16 TgC under SSP126, SSP370, and SSP585 scenarios, respectively. However, a slight increase in GPP of 2.56 TgC is observed under the SSP585 scenario. The effect of global climate change exhibits distinct variations among different seasons and SSP scenarios as depicted in Figure 1 and Table S3. In January, GPP showed decreasing trends under most SSP scenarios, except for the SSP585 scenario under which a slight increase of 2.00% is observed. In April, increasing trends in GPP are observed under SSP245 and SSP585 scenarios, with increases of up to 6.35%. Conversely, decreasing trends occur under SSP126 and SSP370 scenarios by \(-\)0.60% and \(-\)2.96%, respectively. Decreasing trends in GPP are evident across all scenarios in July. Although the maximum relative reduction rate is only \(-\)4.62% under the SSP585 scenario, it represented the largest absolute reduction amount among the four months due to July having the highest baseline GPP value. In comparison, October showed increasing trends in GPP under most SSP scenarios by up to 8.00%, except for a decrease of 9.54% observed under the SSP245 scenario. Additionally, reductions caused by global climate change exceed \(-\)50.00 TgC as maximum amounts, which are relatively larger than increases that range around 40.00 TgC.  
On the contrary, the PER results in a substantial increase in GPP of 68.63 TgC by 2060, accounting for 2.88% of the baseline and effectively offsetting the decline in GPP caused by climate change up to fourfold depending on SSP scenarios. The impact of PER on GPP exhibits slight seasonal variation, with larger increases observed in July (20.96 TgC) and October (19.65 TgC) compared to April (14.21 TgC) and January (13.81 TgC). Considering the larger baseline GPP in July and April as shown in Table S3, the relative variations of GPP present more obvious seasonal differences. PER leads to a significant growth in GPP by 9.40% in January in terms of relative variations, followed by 3.90% in October, while relatively smaller increments are observed in April (2.44%) and July (1.81%). The magnitude of GPP increases induced by PER is considerable when compared to the impacts  
Figure 1: Changes in GPP under different scenarios. CN, CC, and CE represent the impact of PER only, global climate impacts only, and coupled effects of these two factors, respectively. Total represents the cumulative impact across four months. The blue, green, and black error bars represent the standard deviations of changes in GPP caused by CC, CN, and CE, respectively.  
of global climate change alone. For example, during the four representative months, the GPP enhancements resulting from PER can offset the reductions caused by global climate change at rates of 1.73, 1.54, and 4.00 times under SSP126, SSP245, and SSP370 scenarios, respectively.  
Resulting from the offset caused by PER, the GPP increases under most SSP scenarios considering the coupled effects of PER and global climate change. The maximum reduction in GPP is 43.53 TgC, occurring under the SSP126 scenario in January, accounting for 29.62% of the baseline GPP. Conversely, the maximum increase in GPP is significantly larger at 66.99 TgC and occurs under the SSP370 scenario in October, accounting for 13.31% of the baseline GPP. Notably, negative variations in GPP persist across all SSP scenarios due to positive contributions from PER in July.  
In summary, global climate change will lead to a reduction in GPP under most SSP scenarios by 2060 compared to that in 2016; however, PER can significantly enhance GPP in 2060 and offset the decline caused by global climate change up to fourfold. This positive feedback of PER on GPP highlights the effectiveness of the synergistic approach to air pollution control and carbon neutrality towards achieving China's carbon neutral target.