Modelling the impact of future changes in climate, CO2 concentration and land use on natural ecosystems and the terrestrial carbon sink

We used a global vegetation model, ‘HyLand’, to simulate the effects of changes in climate, CO2 concentration and land use on natural ecosystems. Changes were prescribed by four SRES scenarios: A1f, A2, B1 and B2. Under all SRES scenarios simulated, the terrestrial biosphere is predicted to be a net sink for carbon over practically all of the 21st century. This sink peaks around 2050 and then diminishes rapidly towards the end of the century as a result of climate change. Averaged over the period 1990–2100, the net sink varies between scenarios, from B2 to 6 PgCyr . Differences are largely the result of differences in CO2 concentrations. Effects of climate change are substantially less, and counteract the effect of elevated CO2. Land use change results in a loss of carbon to the atmosphere in the B2B scenario, in which the increase in cropland area continues. In the other scenarios, there is a decrease in croplands and grassland, with a corresponding increase in natural vegetation, resulting in a net sink to the biosphere. The credibility of these results depends on the accuracy of the predictions of land use change in the SRES scenarios, and these are highly uncertain. As CO2 is the dominating influence on the vegetation, the scenarios with high fossil fuel emissions, and thus the highest CO2 concentrations (A1F & A2) generate the largest net terrestrial sink for carbon. This conclusion would change if these scenarios assumed continued deforestation and cropland expansion. Without the beneficial effects of elevated CO2, the effects of climate change are much more severe. This is of concern, as the long-term and large-scale effects of elevated CO2 are still open to question. Differences between scenarios in the predicted global spatial pattern of net biome productivity and vegetation type are relatively small, and there are not major shifts in the dominant types. The regions predicted to be at greatest risk from global environmental change are Amazonia, the Sahel, South Central USA and Central Australia. r 2003 Elsevier Ltd. All rights reserved.