## 3 Result
### Individual and Synergistic Effects of Climate Change, CO\({}_{2}\) and O\({}_{3}\)  
Daytime ozone concentrations have maintained a gradual increasing trend over the past two decades (we averaged the ozone concentrations for all periods with radiation greater than 50 W/m\({}^{2}\) per hour step per day for each grid to find the annual average time distribution of ozone concentrations) (Figure S3). The average ozone concentration in the study area increased from 62 ppb in the 2000s (2001-2010) to 65 ppb in the 2010s (2011-2020). The Qinghai-Tibet Plateau and northern regions showed the largest increase in ozone concentration, with the eastern coastal region increasing by approximately 6% and the southwestern region by approximately 2%. Changes in atmospheric CO\({}_{2}\) concentrations are derived from IPCC historical CO\({}_{2}\) concentration data, with mean CO\({}_{2}\) concentrations increasing from 371 ppm in 2001 to 412 ppm in 2020 (Figure S4). Under the historical climate change scenario, compared with the 2001s, the average daily temperature in the 2010s generally increased by 0.5-2 \({}^{\circ}\)C, and the radiation changed by \(-\)2.5-3%, relative humidity by \(-\)1-3.8%, and precipitation by \(-\)40-50% (Figure S5). In the historical climate change scenario, excluding the effects of [O\({}_{3}\)], climate change and [CO\({}_{2}\)] led to a 0-10% change in forest GPP (Figure 2a). The subtropical forests exhibited a higher growth rate of GPP, while the temperate forests showed a relatively lower growth rate of GPP. Under historical climate conditions, the GPP loss caused by single [O\({}_{3}\)] was approximately 0-22% (Figure 2c), and the synergistic effect of [O\({}_{3}\)] and [CO\({}_{2}\)] was relatively small (Figure 2b), approximately 0-18%, which is 2-4% smaller than the effect of single [O\({}_{3}\)] in the study area (Figure 2d). With the increase in ozone concentration, the GPP loss under the influence of single [O\({}_{3}\)] was 1.9-13.2% in the 2001s and 1.9-11.9% in the 2010s (Figure S6). The GPP loss under the synergistic action of [O\({}_{3}\)] and [CO\({}_{2}\)] was 3.1-17% and 3.2-15%, respectively. CO\({}_{2}\) mitigated the effect of O\({}_{3}\) to a certain extent. The effect of O\({}_{3}\) on the ecosystem may be greater than the GPP gain of the forest ecosystem caused by CO\({}_{2}\) fertilization (Figure S7). As the BEPS_O\({}_{3}\) model is a process model driven by remote sensing data, the leaf area index (LAI), as input data, includes the promoting effect of climate change and some carbon dioxide fertilization on vegetation [59; 60]. Therefore, we cannot obtain the impact of climate change on GPP by comparing the 2000s and 2010s GPP. However, it is still possible to calculate the synergistic effects of climate change and other factors on vegetation. The synergistic effect of [O\({}_{3}\)] and climate change will lead to more severe GPP reductions across the study area, with average annual GPP losses in the 2010s higher than those in the 2000s (0.9% and 1%). [CO\({}_{2}\)] and climate change significantly increased forest GPP by 1% and 5%, respectively. Under the synergistic effect of [O\({}_{3}\)], [CO\({}_{2}\)] and climate change, the effect of [O\({}_{3}\)] offset the enhancement of [CO\({}_{2}\)] on photosynthesis and still led to the decline in GPP, but the elevated [CO\({}_{2}\)] mitigated the loss of GPP caused by [O\({}_{3}\)] to a large extent (Figure 3). Overall, although the GPP loss rate caused by ozone pollution from 2001 to 2020 decreased, the GPP loss amount showed an increasing trend. This may be due to the overall increase in the total amount of GPP between 2001 and 2020. In terms of regional variation, GPP changes caused by [O\({}_{3}\)] and [CO\({}_{2}\)] are controlled by climate. Subtropical regions are rich in temperature and precipitation, and vegetation is mainly limited to [CO\({}_{2}\)] and [O\({}_{3}\)]. The single effect of [O\({}_{3}\)] and [CO\({}_{2}\)] has a great influence in regions with abundant precipitation and high temperature.