## 4 Results
### Temporal Variation in GPP
This seasonal variation is primarily attributed to the synchronized occurrence of precipitation and warmth in the region, which provides favorable conditions for vegetation growth and development during summertime. The adequate temperature and moisture levels  
Figure 5: The historical changes in the annual GPP (blue line) and the projected future changes during the 21 st century under the ssp126 (green line), ssp245 (yellow line), ssp370 (red line), and ssp585 (purple line) scenarios on the Tibetan Plateau. The shaded regions depict the 95% confidence intervals for the predicted GPP during the future period. during this season improved the biomass compared to other seasons. In contrast, only vegetation at lower altitudes continues to grow during the winter months, while plants in higher-altitude regions show halted growth or undergo seasonal senescence due to the cold temperatures. During the historical period, the GPP showed an increasing trend, with the spring, summer, and autumn GPP trends passing the 99% significance test. However, the winter GPP trend did not pass the 99% significance test (Table 4). This can be attributed to the cold winter temperatures, weak solar radiation, and decreased precipitation in the form of snow, which leads to soil freezing on the Tibetan Plateau. As a result, vegetation growth is largely suspended, with most plants either showed paused growth or dying back during winter, leading to the lowest GPP values of the year. Although the temperature has increased, it remains insufficient to stimulate plant growth on the Tibetan Plateau, which does not result in a noticeable trend in the GPP during winter. The GPP in both spring and autumn showed a significant upward trend under the ssp245, ssp370, and ssp585 scenarios, particularly in spring, where the trend passed the 99% significance test for the period of 2061-2100. This trend is primarily driven by gradually rising temperatures, especially in high-altitude and cold regions like the Tibetan Plateau, which promote earlier vegetation growth and enhance photosynthesis. In autumn, the delayed occurrence of phenological events, particularly the postponement of plant senescence, extends the growing season and contributes to increased GPP. Moreover, rising atmospheric CO\({}_{2}\) concentrations further stimulate photosynthesis, particularly at the beginning of the growing season, leading to increases in GPP during both spring and autumn. Figure 6: The seasonal GPP dynamics during the historical (blue line) and 2021–2100 periods under the ssp126 (green line), ssp245 (yellow line), ssp370 (red line), and ssp585 (purple line) scenarios. The shaded regions depict the 95% confidence intervals for the predicted GPP during the future period. Under the ssp126 scenario, the seasonal GPP fluctuations are minimal. The GPP trends in all seasons fail to pass the 99% significance test, indicating that significant changes in GPP are unlikely under this scenario. In contrast, the ssp245 scenario shows a significant increase in the summer GPP, which passes the 99% significance test. This suggests that, under moderate emission levels, the summer GPP may benefit from the warmer temperatures and improved CO\({}_{2}\) concentrations. Under the ssp370 scenario, spring exhibits accelerated increases in GPP, with both the spring and autumn trends passing the 99% significance test in 2061-2100. This indicates the more robust response of vegetation growth to increased CO\({}_{2}\) levels and warming, particularly in these seasons. The ssp585 scenario presents the most pronounced seasonal variations in the GPP among all scenarios. Spring, summer, and autumn all show accelerated increases in GPP, with significant trends passing the 99% significance test in various time intervals. This scenario, characterized by high greenhouse gas emissions, suggests that vegetation growth on the Tibetan Plateau will be profoundly affected by this climatic scenario, inducing considerable seasonal shifts in the GPP.