Carbon use efficiency (CUE) indicates the fraction of carbon assimilated by photosynthesis that is fixed in plant biomass. Recent advances have demonstrated that CUE varies across space and time, however, the reason of which remains debated (e.g., the impact of temperature on CUE). The lack of knowledge on CUE variations is caused by the limited number of observations globally, as CUE measurements require strenuous field survey of biomass change and allometry. In this study, we use high quality land-atmosphere CO2 exchange records of ~1000 site-years from open access eddy covariance datasets (i.e., Fluxnet2015, ONEflux) to infer CUE and investigate its variation. The inferred CUE closely correlates with inventory-based CUE observations (R2 = 0.64, p<0.01), and distributes around 0.46 ± 0.12. Our results reaffirm that CUE has large spatial and temporal variations, and find CUE is negatively related to temperature, light and stand age. Importantly, we find there is a difference in the baseline CUE of deciduous and evergreen forests. Our study provides insights into the carbon fixation efficiency of plants, which can guide decision-makings on world reforestation efforts as a part of nature-based climate solutions.