A methodology combining ecological modelling with geographical information analysis and remote sensing was employed to determine the effects of sea-level rise in estuarine salt marshes, using the Tagus estuary (Portugal) as a case study. The development of salt marsh vegetation was simulated separately for C3 and C4 plants, using a combined biogeochemical and demographic model. This simulation, which provided small-scale (m 2) results of annual above-ground primary production, was upscaled to the whole salt marsh area, using bathymetry data, remote sensing and Geographic Information System (GIS) for assessing vegetation cover and determining areal distribution of C3 and C4 vegetation. Based on IPCC data, several sea-level rise scenarios were considered, and the coupled ecological model-GIS were applied to these in order to determine changes in global salt marsh productivity. The results indicate that the salt marshes of the mesotidal estuaries such as the Tagus are susceptible to sea-level rise only in a worst case scenario, which is more likely to occur if the terms set out by the Kyoto protocol are not met by several industrialised nations. The low vulnerability of salt marshes supports the suggestion that areas with high tidal ranges are less vulnerable to sea level change, due to greater sediment transport and accretion. Nevertheless, the precautionary principle should always be applied by coastal planners, due to the great uncertainty surrounding forecasts of sea-level rise. © 2001 Elsevier Science B.V. All rights reserved.