Carbon use efficiency (CUE) of plants, the ratio of net primary productivity (NPP) to gross primary productivity (GPP) is critical as it determines how much carbon (C) fixed by photosynthesis becomes plant biomass. Whether global climate change, such as increased temperature and elevated carbon dioxide (eCO2), affect carbon use efficiency of plants is not well understood. In this study, we conducted a meta-analysis investigating the effects of increased temperature and eCO2 on plant CUE. Overall, neither warming nor eCO2 was found to significantly affect CUE due to a dual increase in both productivity and autotrophic respiration. We did find some significant changes of CUE by warming or eCO2 under certain environmental and experimental conditions. These results support that CUE is more variable than previously assumed, which can greatly impact both regional and global C sequestration estimates and therefore, should be considered in Earth system models.