# 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  
To explore the difference between the changes in GPP in the near future (2021-2040) and the far future (2071-2100), the spatial distribution of the multi-year mean of GPP in each season under different climate scenarios in historical baseline, near, and far future is described in Figure 10 and Figure S2, respectively. Among them, the distributions under different scenarios in the future describe the change in the multi-year mean compared with that during baseline. The positive value represents the increase in GPP, while the negative one represents the decrease. Figure 10a1-a4 shows that the spatial distribution of the mean annual GPP in each season of the baseline GPP increased significantly in the four seasons compared with the seasonal average over the period 1850-2014 (Figure 8). In spring, the average annual GPP greater than 2 gCm\({}^{-2}\)d\({}^{-1}\) occupied 22.93% of the region, concentrated in the south of 30\({}^{\circ}\) N. In summer, the area with an annual average GPP of more than 2 gCm\({}^{-2}\)d\({}^{-1}\) doubled compared with spring, reaching 48.97%, with the maximum yearly peak at 14.19 gCm\({}^{-2}\)d\({}^{-1}\). There are similar peaks and spatial distribution in autumn and spring, while it remains almost the same in winter. There are significant seasonal differences in the variations in GPP based on the historical baseline under multiple scenarios in the near future. In spring, GPP increases in most eastern regions, and rises more with the enhancement of radiation forcing levels. However, GPP decreased significantly in the east of the plain of Henan Province under the high emission scenario. In summer, GPP increased substantially in nearly half of the regions of the country, including the Northeast, North, Southwest, and south of Northwest China. GPP accelerated with the increase in radiation forcing levels, with more than 20% of the regions having values greater than 0.4 gCm\({}^{-2}\)d\({}^{-1}\) under SSP5-8.5. In autumn, it increased significantly in Yunnan, while there was a slight weakening trend in the north of Henan. The spatial characteristics distribution maintains consistency under multiple climate scenarios. Winter is the only season when GPP decreases in a large area. Under the four scenarios, the GPP in 21% of regions is lower than the baseline, mainly distributed in the west, Inner Mongolia, and Heilongjiang. The reduction in GPP is the same under different scenarios. Except for the decrease in GPP in Fujian, it generally increases in the south. Figure 9: Spatial distribution of GPP variation trends in (**a**) MAM, (**b**) JJA, (**c**) SON, and (**d**) DJF over China during the historical period (1850–2014), with the dot indicating passing significance test at 5% level. The change in GPP becomes increasingly intense with the enhancement of radiative forcing levels in the far future, as seen from Figure S2. The variations in GPP are studied with the Heihe-Tengchong line as the boundary. In spring, it can be found that GPP increases more as radiative forcing levels are enhanced in the eastern part of the dividing line. The area with an increase of more than 0.8 gCm\({}^{-2}\)d\({}^{-1}\) rises from 0.62% to 23.99%. The change in the west of the boundary is gentle, with an average rise of 0.09 gCm\({}^{-2}\)d\({}^{-1}\). The area of GPP increasing by more than 0.8 gCm\({}^{-2}\)d\({}^{-1}\) expands from 0.62% to 23.99% of the regions. The change in the western part of the dividing line is relatively gentle, with an average increase of 0.09 gCm\({}^{-2}\)d\({}^{-1}\). Summer is the peak photosynthesis season for plants in northern ecosystems compared to other seasons, leading to a more extensive range and broader areas of increase. GPP increases in about 94% of the regions under SSP1-2.6 relative to the baseline.