Defining the importance of including transient ecosystem responses to simulate C-cycle dynamics in a global change model

The ability of plant species to migrate is one of the critical issues in as sessing accurately the future response of the terrestrial biosphere to clim ate change. This ability is confined by both natural and human-induced chan ges in land cover. In this paper we present land-cover and Carbon (C) cycle models designed to simulate the biospheric consequences of different types of land-cover changes. These models, imbedded in the larger integrated ass essment model IMAGE 2, were used to demonstrate the importance of consideri ng spatial aspects for global C-cycle modelling. A gradual-migration, an un limited-migration and a no-migration case were compared to show the range o f possible consequences. Major differences between these cases were simulat ed for land-cover patterns and the carbon budget. A large geographical vari ation in the biospheric response was also simulated. The strongest response was simulated in high-latitude regions, especially for the migration cases in which land-cover changes were permitted. In low-latitudes regions the d ifferences between the migration cases were smaller, mainly due to the effe cts of land-use changes. The geographical variation among, and the differen t responses, the migration cases clearly demonstrate how essential it is to assess biospheric responses to climate change and land use simultaneously. Moreover, it also shows the urgent need for enhanced understanding of spat ial and temporal dynamics of the biospheric responses.