# Evidence of Carbon Uptake Associated with Vegetation Greening Trends in Eastern China
## 3 Results
### Trends in Flux Estimates  
The corrected GM-XCO\({}_{2}\) trends of \(-0.070\) to \(-0.084\) ppm yr\({}^{-1}\) in EC suggest an increased biospheric uptake. In order to estimate the magnitude of the surface CO\({}_{2}\) fluxes that would produce that XCO\({}_{2}\) anomaly, we used the CT and MACC models to relate CO\({}_{2}\) fluxes to atmospheric CO\({}_{2}\) concentration anomalies. We explored correlation between model simulated XCO\({}_{2}\) changes daily or over 16 days and net CO\({}_{2}\) flux changes in the global or EC region shown in Figure 7. Significant and strong positive correlations were found in the global daily mean data both from CT (Figure 7a, R\({}^{2}\) = 0.84, \(p\) \(<\) 0.01) and MACC (Figure 7d, R\({}^{2}\) = 0.49, \(p\) \(<\) 0.01) with slopes approaching 0.26 ppm/(g C/m\({}^{2}\)/day) from CT and 0.21 ppm/(g C/m\({}^{2}\)/day) from MACC, where the former was a little larger. The correlation and slopes over EC were much lower than that in the global mean, but still significant (Figure 7b-d). This is because at the global scale, CO\({}_{2}\) is conserved during atmospheric transport within the global domain, whereas at the regional domain, transport can be a source or sink of CO\({}_{2}\). The correlation decreased as the domain considered got smaller from global to EC. The correlation from daily to 16 days (the time resolution of satellite observed XCO\({}_{2}\) and vegetation parameters) became stronger both for CT (Figure 7c) (R\({}^{2}\) = 0.26, \(p\) \(<\) 0.01) and MACC (Figure 7f) (R\({}^{2}\) = 0.23, \(p\) \(<\) 0.01). The flux-XCO\({}_{2}\) transform ratios of 16-day were similar for CT (0.0523 ppm/(g C/m\({}^{2}\)/16 days)) and MACC (0.0465 ppm/(g C/m\({}^{2}\)/16 days)). The correlation became stronger as the time interval increased (from daily to 16-day means), because the influence of persistent CO\({}_{2}\) fluxes built up within the domain. We estimated the CO\({}_{2}\) flux change from the corrected GM-XCO\({}_{2}\) trends and the model correlation between XCO\({}_{2}\) 16-day changes and CO\({}_{2}\) flux change shown in Table 2. The total XCO\({}_{2}\) change were  
Figure 7: Estimates of the relationship between XCO\({}_{2}\) drawdown and surface CO\({}_{2}\) flux magnitudes. Correlation between daily XCO\({}_{2}\) changes and daily CO\({}_{2}\) flux, from CarbonTracker (CT) and Monitoring Atmospheric Composition and Climate (MACC), for the global mean, (**a**) for CT and (**d**) for MACC, and Eastern China (EC), (**b**) for CT and (**e**) for MACC. Correlation of 16-day changes over EC is shown in (**c**) for CT and (**f**) for MACC. XCO\({}_{2}\) changes are calculated with the XCO\({}_{2}\) from adjacent two days (day 2 or day 16 minus day 1), and CO\({}_{2}\) flux is the mean value of CO\({}_{2}\) flux during day 1 or the whole of 16 days. Different colors in (**a**), (**b**), (**d**), and (**e**) show the CO\({}_{2}\) fluxes and concentrations from different seasons (green for March to May, blue for June to August, yellow for September to November, and black for December to February). In our inferred flux estimates, we adopt the 16-day relationship. estimated as \(-1.008\pm 1.080\), \(-0.972\pm 1.056\), and \(-0.840\pm 1.116\) ppm (standard deviation represents spatial variability) from 2003 to 2015 for different correction methods (CT-XCO\({}_{2}\) fitting, background GEOS-XCO\({}_{2}\) fitting, and nested GEOS-XCO\({}_{2}\) fitting). The inferred carbon uptake increase mean value over the region and its spatial variability were 9.57 \(\pm\) 10.94, 8.88 \(\pm\) 10.48, and 6.36 \(\pm\) 11.63 g C/m\({}^{2}\) using the CT ppm-to-flux transform ratio and 10.30 \(\pm\)11.85, 9.52 \(\pm\) 11.33, and 6.69 \(\pm\) 12.62 g C/m\({}^{2}\) using the MACC transform ratio. That is basically consistent with GPP increase of 9.24 g C m\({}^{-2}\) shown in Section 3.1. The total carbon uptake amount from 2003 to 2015 ranged from 28.41 to 46.04 Tg C with an EC region area of \(4.47\times 10^{12}\) m\({}^{2}\) shown in Figure S8.