## 4 Discussion
Due to the lack of spatially explicit CO\({}_{2}\) concentrations over Europe, we utilized daily CO\({}_{2}\) concentrations from the Mauna Loa Observatory (MLO) instead. In this study, CO\({}_{2}\) fertilization was identified as the second key factor contributing to the GPP increase (Figure 8a). In response solely to CO\({}_{2}\) fertilization, the GPP increased by 8.6% (i.e., 0.78 Pg C yr\({}^{-1}\)) per 100 ppm (Figure 8a), which was slightly smaller than the CO\({}_{2}\) fertilization effect on total net primary productivity according to free air CO\({}_{2}\) enrichment (FACE) experiments (10 to 13%) [30; 50]. However, if all the factors were included, the GPP would increase by 27.8% per 100 ppm (i.e., 2.53 Pg C yr\({}^{-1}\)) (Figure 8a) across Europe, which was much higher than the sensitivity of vegetation productivity to elevated CO\({}_{2}\) at the global scale (ranging from 10% to 18.6%) [6; 51; 52; 53]. This could be explained by land surface greening across Europe, which could further enhance the GPP. From 2001 to 2016, we found widespread LAI increases (i.e., greening) across Europe (Figure 2b,d) according to the GLASS LAI product, in accordance with the widely reported global greening phenomenon [7; 8; 21]. The mean annual LAI increased by 0.0046 m\({}^{2}\) m\({}^{-2}\) yr\({}^{-1}\) (\(p\) < 0.01) over the study area (Figure 3). FGM factorial simulations suggested that increases in LAI was the dominant factor (62%, 35.5 Pg C yr\({}^{-2}\)) contributing to the interannual GPP increasing trend in Europe (Figures 5b and 8). On the global-scale, GPP showed no proportional increase from land surface greening [43]. However, the FGM simulations indicated proportional increases in GPP (0.55% yr\({}^{-1}\)) from the greening terrestrial ecosystem across Europe (Figure 3a). Although substantial discrepancies exist in different LAIproducts [54; 55], the magnitude and annual anomaly of GLASS LAI has been proven to be within a valid range [7; 54]. The annual total GPP calculated by BEPS (0.62% yr\({}^{-1}\)), MODIS C6 (0.67% yr\({}^{-1}\)), GLASS V6 (0.47% yr\({}^{-1}\)), and GOSIF (0.58% yr\({}^{-1}\)) also showed significant increasing trends (Figure 3a), which were close to our estimates (0.55% yr\({}^{-1}\)). Although these products were produced based on various LAI products, their higher consistent trends (Figure S2a) indicated that employing GLASS LAI as input could reasonably simulate the interannual variations in European GPP trends. In addition, the annual GPP dynamics estimated by FGM correlated well with those estimated by BEPS, GOSIF, VPM, and GLASS V6 GPP products (Figure S2b). While general agreement exists between FGM estimations and other GPP products, the current version of the FGM does not consider nitrogen deposition [56], soil moisture or drought [57], and ozone pollution [58], thus inevitably introducing some uncertainty into European GPP estimations; this topic warrants future research. Without any land cover transitions from 2001 to 2016, the stable areas still displayed a significant greening trend with an increasing LAI rate of 0.008 m\({}^{2}\) m\({}^{-2}\) yr\({}^{-1}\) (Figure 9), suggesting that most of the European greening was largely due to climate change. In addition, the mean LAI increasing trend for change areas (0.01 m\({}^{2}\) m\({}^{-2}\) yr\({}^{-1}\)) was approximately 30% larger than that for stable areas, suggesting that LULCC contributed to approximately 20% of the observed greening in Europe. This result could possibly be explained by the abandonment of agricultural land in Eastern Europe and mountain areas in recent decades [59]. The return of woody vegetation at the expense of the loss of cropland enhanced land surface greening corresponded well to the regions where the annual LAI dynamics showed significant greening trends (Figure 2). LAI dynamics are a complex interplay of climate change and land cover change.