# Improving the Gross Primary Production Estimate by Merging and Downscaling Based on Deep Learning
## Results
### Long-Term Changes in Merged GPP in Projection Period under Multiple Climate Scenarios
With the enhancement of radiation forcing levels, the rise of GPP in Northeast, North, and the east of Southwest China becomes more and more intense, while that in Henan is turning from decrease to increase. The magnitude of the increase is further accelerated under SSP5-8.5. The reduction in GPP in Henan in autumn only occurs in the low-emission scenario, which differs from spring. In winter, GPP reduces in parts of the west and northeast regions, roughly similar to the scenario in the near future (Figure 10b1-e4). There is a considerable area reduction in GPP in the eastern part of the Qinghai-Tibet Plateau under the SSP1-2.6 scenario. With the increase in radiation forcing levels, the area of GPP reduction gradually decreases. GPP increased south of 30\({}^{\circ}\) N, with the maximum increase reaching 2.48 gCm\({}^{-2}\)d\({}^{-1}\) in Yunnan under the SSP5-8.5 scenario. Compared with the near future, GPP growth will be more dramatic in the far future. The multi-year variations in GPP in each pixel can better reflect the characteristics of spatial changes in addition to the regional average. Figure 11 describes the spatial distribution of GPP variation trends in China under four scenarios from 2015 to 2100. GPP generally shows an increasing trend in the projection period, with the trend becoming more and more intense as the radiation forcing levels increase. There is no significant change in GPP in the northwest arid regions under different scenarios, which maintains a low growth. A slight decreasing trend can be observed visually at the junction of Shandong, Henan, Jiangsu, and Anhui provinces under the SSP1-2.6 scenario, which is related to the environmental effects of aerosols [66]. The spatial pattern of GPP variations under  
Figure 10: Spatial distribution of the multi-annual mean CMIP\({}_{\rm 6 DL}\) GPP in **(a1–a4)** baseline (1995–2014) and near future (2021–2040) under **(b1–b4)** SSP1-2.6, **(c1–c4)** SSP2-4.5, **(d1–d4)** SSP3-7.0, and **(e1–e4)** SSP5-8.5 relative to baseline in **(a1–e1)** MAM, **(a2–e2)** JJA, **(a3–e3)** SON, and **(a4–e4)** DJF. the medium and high emission scenario is similar, with GPP increasing in each pixel. Likewise, the variation trends become more intense as radiative forcing levels increase. The most significant increase in GPP occurs in Yunnan and Guizhou under SSP2-4.5, SSP3-7.0, and SSP5-8.5. The regions where GPP is more sensitive to radiation forcing levels are concentrated in the southwest, the east of the northwest, and the northeast. GPP increases rapidly as radiative forcing levels increase. It is found that the increase in the high-emission scenario is even four times that of the low-emission scenario. Considering the characteristics of the future variations in GPP, it is divided into three periods for analysis, including 2021-2040 (near future), 2041-2070 (medium future), and 2071-2100 (far future) due to the long projection period of the merged data. As can be seen from Figure S3, the increasing trend of GPP turns into a decreasing trend as time goes on under the SSP1-2.6 scenario. In the near future, GPP shows a significant increasing trend except for the junction of Shandong, Jiangsu, Henan, and Anhui provinces. In the medium future, the changes in average regional GPP are relatively stable, though there are scattered pixels with increasing and decreasing trends of GPP across the country. In the far future, the regions with reduced trends of GPP nationwide account for about 93% of the country. Under the low emission scenario, the air temperature rises first and remains stable in the far future, while the carbon dioxide emissions begin to decrease in 2020 [67]. This shows that GPP is greatly affected by air temperature. The amount of carbon dioxide directly affects vegetation's carbon sequestration when the temperature is stable, leading to GPP decline. GPP increases in the near future under the SSP2-4.5 scenario; however, the variation trends slow down over time.