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Understanding the net ecosystem CO 2 exchange (NEE) between terrestrial ecosystems and the atmosphere is crucial for accurate estimation of carbon budgets, which remains unclear for grassland ecosystems. Here we upscaled site-level NEE from 44 grassland flux towers (1,457 site-months) to the global scale by using a model tree ensemble approach that considers the management activities (grazing and cutting) (MTE-GM). Cross-validation showed that MTE-GM performs reasonably well in terms of among-site variability and seasonal variation, with a Nash-Sutcliffe efficiency of 0.90 and 0.86 and an R 2 of 0.91 and 0.86, respectively. Radiation (shortwave and longwave), temperature, leaf area index, and fraction of absorbed photosynthetic active radiation had the highest relative explanatory power in predicting NEE. Based on MTE-GM, mean annual NEE of global grassland was 72 ± 4 g C m −2 year −1 (1.9 ± 0.11 Pg C year −1 ) during 1982-2011, suggesting that the grassland ecosystems have been acting as a small carbon source during the past three decades. However, grasslands in temperate and continental regions had the largest carbon sink of −61.9 ± 5.7 and −51.8 ± 7.9 g C m −2 year −1 , respectively. Moreover, we found that elimination of grassland management effect resulted in an extra emission of 1.7% CO 2 to the atmosphere (CO 2 sink from the management is 0.03 Pg C year −1 ). From the 1980s to the 2000s, 38% (22%) and 17% (18%) of pixels showed an increased (decreased) carbon uptake and decreased (increased) carbon release, respectively. Uncertainty assessment suggested that there would be higher confidence in NEE estimates in most parts of middle-to high-latitude regions in the Northern Hemisphere.