Terrestrial ecosystems take up about 1/3 of anthropogenic emissions and serve as a critical carbon sink to mitigate climate change. The size of ecosystem carbon sink is dependent on the availability of exogenous (e.g. light, water and nutrient) and endogenous (e.g. carbohydrate, mycorrhizal) resources to ecosystems and how ecosystems use these resources. Ecosystems loose carbohydrate via respiration to provide metabolic energy to maintain the function and growth of living organisms in plants and soil. The carbon accumulated per unit carbon cost for an ecosystem (denoted as carbon cost efficiency, which is an extension of the commonly used carbon use efficiency by considering heterotrophic respiration) thus places a critical constraint on the potential carbon sink strength. In this study, we used a global network of eddy covariance measurements (~212 sites) to quantify the carbon cost efficiency of various ecosystems. We found strong relationships between carbon cost efficiency and environmental factors, soil and remotely-sensed vegetation status across flux sites, which allows for global inference of the potential of terrestrial ecosystems to offset future emissions.