## 3 Results and Discussion
### Attribution of Detected Changes in NDVI and GPP  
In boreal Eurasia regions (the Boreal Eurasia North West, North East, South West and South East regions), the modeled increases in GPP are explained by variations in temperature (Biome-BGC and LPJ models), precipitation (Biome-BGC, LPJ, and TRIFFID models), and atmospheric CO\({}_{2}\) concentration (LPJ and TRIFFID models) (Figure 5a-d). Changes in temperature and precipitation provide similar contributions to the GPP increase. The Biome-BGC model showed that increases in the atmospheric CO\({}_{2}\) concentration had a small effect on the GPP increase. However, the LPJ and TRIFFID models showed that changes in atmospheric CO\({}_{2}\) concentration, temperature, and precipitation had similar effects on GPP.  
In the temperate Asia regions (the Temperate Asia North West, North East and South East regions), the causes of the GPP increases were not consistent among the models (Figure 5e-g). The GPP increases were primarily due to changes in temperature in the Biome-BGC model. According to the LPJ and TRIFFID models, atmospheric CO\({}_{2}\) concentration change is the main driver of the GPP increases.  
In tropical Asia (the Tropical Asia North and South regions), the GPP increases were mostly explained by changes in atmospheric CO\({}_{2}\) concentration in the LPJ and TRIFFID models. In the Biome-BGC model, changes in precipitation and the atmospheric CO\({}_{2}\) concentration equally contributed to the increases in GPP. Changes in incoming solar radiation were another important factor in the GPP increases in the Tropical Asia South region in the Biome-BGC and LPJ models. However, quantifying the solar radiation contribution requires better estimates of solar radiation (see Section 3.4).  
Although the estimated GPP trends were similar for the different models and were generally consistent with the satellite-based NDVI trends, the contribution of CO\({}_{2}\) fertilization to the increase in GPP differed among the models and regions (Figure 5). The CO\({}_{2}\) fertilization effect on the GPP trends was lower in the Biome-BGC model compared with the LPJ and TRIFFID models. Therefore, further analysis using data from field observations [71] is required to elucidate the effects of CO\({}_{2}\) fertilization on GPP.  
Figure 5: Contribution of the modeled GPP trend of each individual factor (_i.e._, temperature, precipitation, solar radiation, and atmospheric CO\({}_{2}\) concentration) as stacked bar graphs in each region (**(a)** Boreal Eurasia North West, **(b)** Boreal Eurasia North East, **(c)** Boreal Eurasia South West, **(d)** Boreal Eurasia South East, **(e)** Temperate Asia North West, **(f)** Temperate Asia North East, **(g)** Temperate Asia South East, **(h)** Tropical Asia North, and **(i)** Tropical Asia South). Cumulative sums of GPP trends for the simulations with each input data temporally changing are approximately consistent with the GPP trends in the simulation with all input data temporally changing (y = 1.21 x + 0.09, R\({}^{2}\) = 0.92; x is the sum of GPP trends for the simulations with each input data changing in each region and for every model, and y represents the GPP trends for the simulation with all input data changing in each region and for each model; 27 data points (3 models \(\times\) 9 sub-regions)).