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The response of terrestrial vegetation to a globally changing environment is central to predictions of future levels of atmospheric carbon dioxide (1,2). The role of tropical forests is critical because they are carbon-dense and highly productive (3,4). Inventory plots across Amazonia show that old-growth forests have increased in carbon storage over recent decades (5-7), but the response of one-third of the world's tropical forests in Africa (8) is largely unknown owing to an absence of spatially extensive observation networks (9,10). Here we report data from a ten-country network of long-term monitoring plots in African tropical forests. We find that across 79 plots (163 ha) above-ground carbon storage in live trees increased by 0.63 Mg C [ha.sup.-1] [yr.sup.-1] between 1968 and 2007 (95% confidence interval (CI), 0.22-0.94; mean interval, 1987-96). Extrapolation to unmeasured forest components (live roots, small trees, necromass) and scaling to the continent implies a total increase in carbon storage in African tropical forest trees of 0.34 Pg C [yr.sup.-1] (CI, 0.15-0.43). These reported changes in carbon storage are similar to those reported for Amazonian forests per unit area (6,7), providing evidence that increasing carbon storage in old-growth forests is a pan-tropical phenomenon. Indeed, combining all standardized inventory data from this study and from tropical America and Asia (5,6,11) together yields a comparable figure of 0.49 Mg C [ha.sup.-1] [yr.sup.-1] (n = 156; 562 ha; CI, 0.29-0.66; mean interval, 1987-97). This indicates a carbon sink of 1.3 Pg C [yr.sup.-1] (CI, 0.8-1.6) across all tropical forests during recent decades. Taxon-specific analyses of African inventory and other data (12) suggest that widespread changes in resource availability, such as increasing atmospheric carbon dioxide concentrations, may be the cause of the increase in carbon stocks (13), as some theory (14) and models (2,10,15) predict., Tropical forests cover 7-10% of the global land area, store 40-50% of carbon in terrestrial vegetation and annually process approximately six times as much carbon via photosynthesis and respiration as [...]