Grassland/livestock systems differ in their wherewithal to enhance the magnitude of GHG fluxes (CO2, CH4, N2O) and their global warming potential (GWP). A simulation study was performed using the model PASIM to extrapolate potential emissions from grassland/livestock systems under alternative climate/soil/plant/management regimes at 12 French sites. Fluxes of CO2 associated with those of N2O indicate that both gases may be controlled by similar or concomitant environmental factors. In particular, CO2 and N2O fluxes were higher in the temporary grasslands, where grass management (mowing, N fertilization) promotes increased plant respiration and soil organic matter turnover. In these systems, high CO2 and N2O emissions were accompanied by absence of CH4 emissions. Similar responses were observed with intensive permanent grasslands, where the presence of grazing animals also determines CH4 emissions. High CH4, and low CO2 and N2O fluxes were generally estimated with extensive permanent pastures. While there are site-to-site and climate-to-climate variations, a general conclusion is that the GWP is expected to decrease in the future as result of increased soil dryness and C stocking. Also, temporary grasslands are indicated as the most desirable land use in reducing global warming. Strategies to enhance fertilizer use efficiency, animal feed and return of animal waste could be explored as adaptation/mitigation measures.