Under a recent SCOPE collaborative project, longterm data from eleven tropical and temperate grassland sites were used (a) to validate the CENTURY model of plant-soil ecosystems and (b) to model climate change and CO2 effects for thirty-one temperate and tropical grassland sites, representing seven ecoregions of the world. Model calibration and testing showed that soil carbon and nitrogen dynamics can be well simulated for the grassland biome worldwide, over a wide range of climate and soil types. The interannual response of above ground biomass and plant residue to variation in climate resulted in a good correspondence between simulated and observed dynamics on a monthly basis. These results are useful for analysis and description of grassland carbon dynamics, and as a reference point for testing predictions of net primary production (NPP) and biomass dynamics from levels of more physiologically based models. Prediction of plant and soil organic matter C and N dynamics requires knowledge of climate, soil texture, N inputs and fire and grazing patterns. CENTURY simulations of climate change and CO2 effects showed increased NPP for climate change alone, except in cold desert steppe regions, and CO2 increased production everywhere. Climate changes, predominantly a warming of these ecosystems, caused soil carbon to decrease overall, especially in cold desert and temperate steppes. Increased production due to elevated CO2 tended to ameliorate soil carbon losses and tropical savannas were actually soil carbon sinks. Climate change alone projected a carbon loss of 3-4 Pg after 50 years, and 1-2 Pg for the combined climate change and CO2 simulated effects. We analysed the dynamic response of some of the major CENTURY output parameters (e.g. NPP, soil organic matter, N mineralization and decomposition) for their sensitivity to climate change and increasing CO2 for one of the two general circulation models (GFHI scenario). This analysis was limited to a subset of five well-known study sites, representing five of the seven ecoregions.