ESTIMATES of the global carbon dioxide balance have identified a substantial ’missing sink’ of 0.4-4.3 Gt per year(1). It has been suggested that much of this may reside in the terrestrial biosphere(2). Here we present an analysis of the carbon stored by pastures based on deep-rooted grasses which have been introduced in the South American savannas. Although the deep-rooted grasses were chosen principally for agricultural reasons(3), we find that they also sequester significant amounts of organic carbon deep in the soil. If our study sites are representative of similar pastures throughout South America, this process could account for the sequestration of 100-507 Mt carbon per year-a substantial part of the ’missing sink’. Thus, although some land-use changes(4) (such as burning tropical rainforests) contribute to the atmospheric CO2 burden, we conclude that the introduced pastures studied here help to offset the effect of anthropogenic CO2 emissions.