## 3 Results
### Characteristics of Temporal Variation in Carbon Fluxes  
Carbon fluxes have a tendency to fluctuate over time due to various factors such as climate change, vegetation conditions, and meteorological factors. These fluxes exhibit different patterns of change in different regions across the world. Figure 3 illustrates the trends in carbon fluxes based on the GCFD, showing the average annual change in each carbon flux variable from 1999 to 2020. The findings indicate that, among the three carbon flux variables, GPP and RECO demonstrated consistent trends over time, while NEE showed an opposite trend to GPP and RECO. The analysis primarily focused on the trend of GPP. Regarding regional variations, significant changes in carbon fluxes over time were observed in central Africa and near the tropics of South America, where GPP exhibited a clear upward trend. The northern regions of Asia and Europe also displayed a slight upward trend. Conversely, the central region of the USA demonstrated the most noticeable downward trend in GPP. Additionally, some areas on the edge of the Sahara desert and in certain parts of the Far East exhibited downward trends as well. A comparative analysis revealed that the extent of rising GPP in the tropics surpassed the areas where declining GPP was observed, indicating a more pronounced global trend of increasing GPP over time. Next, this study calculated the mean carbon fluxes over time on both the global scale and within seven climate regions, as depicted in Figure 4. Regarding the global time series, the annual sum of GPP and RECO exhibited a slight increase over time, while the value of  
Figure 3: Spatial distribution of annual average carbon flux trends from 1999 to 2020 in the GCFD dataset, with positive values indicating increases and negative values indicating decreases. NEE declined. Specifically, the global mean GPP rose from 1158.1 g C m\({}^{-2}\) yr\({}^{-1}\) in 1999 to 1217.8 g C m\({}^{-2}\) yr\({}^{-1}\) in 2019, and RECO increased from 995.0 g C m\({}^{-2}\) yr\({}^{-1}\) in 1999 to 1025.3 g C m\({}^{-2}\) yr\({}^{-1}\) in 2019. Conversely, NEE decreased from \(-\)215.6 g C m\({}^{-2}\) yr\({}^{-1}\) in 1999 to \(-\)244.6 g C m\({}^{-2}\) yr\({}^{-1}\) in 2019. It is plausible that the upward trend in global GPP is attributable to the recent increase in greenhouse gases and rising temperatures. Analyzing the time series of individual climate zones, a more pronounced trend of increasing GPP values can be observed in the tropical, Mediterranean, subtropical, continental, and polar climate zones. However, in the dry and oceanic climate zones, the regional averages of carbon fluxes fluctuated relatively widely over time. Overall, the general trend in GPP indicates a predominant increase over time, both globally and within individual regions. The global distribution patterns of carbon fluxes for the four seasons are presented in Figure 5. It is evident that regardless of the seasons, GPP exhibits its highest values near the equator and decreases with increasing latitude. In regions such as central Africa, the Amazon area, and the South Asian tropics, GPP values remain consistently high throughout the year. In certain temperate regions, there is a noticeable seasonality in GPP variations. For instance, in northern Asia and Europe, the eastern United States, and southeastern China, GPP values reach their peak during summer and hit their lowest point in winter. This disparity may be attributed to the contrasting vegetation types found in tropical and temperate areas. Vegetation in the tropics tends to remain evergreen due to favorable climatic conditions, while the climate in temperate regions undergoes substantial changes across seasons, resulting in phenomena like defoliation and a significant decline in productivity during the winter. Regarding the seasonal fluctuations of RECO, the spatial distribution maps for the four seasons demonstrate a relatively similar pattern to that of GPP.