Climate impacts of European-scale anthropogenic vegetation changes:: A sensitivity study using a regional climate model

Summertime seasonal simulations with a regional climate model covering Europe and the North Atlantic are used to assess the sensitivity of the European climate with respect to continental-scale anthropogenic vegetation changes. An ensemble of six control integrations with present-day vegetation parameters (April until August, 1987-1992) is compared against an ensemble of six sensitivity experiments (for the same time period and years) using a potential distribution as obtained from a vegetation model. The two vegetation distributions differ strongly in the Mediterranean region, reflecting the anthropogenic deforestation that took place during the last 2000 years. The numerical simulations reveal that the additional vegetation in the sensitivity experiment leads to a moister and cooler spring, followed by a drier and warmer summer. The following key physical processes were identified: Evapotranspiration is increased in the simulation with potential vegetation during the period from April until mid-July, thus cooling the Earth surface, moistening the boundary layer, reducing vertical stability, and eventually enhancing precipitation. In mid-July, soil moisture values in the experiment with potential vegetation drop locally below a critical value (as a consequence of the larger springtime evapotranspiration rates), whereupon transpiration is almost completely inhibited during the following 2 months. This leads to a sign reversal of the sensitivity to vegetation cover. This contrasting behavior during spring and summer is consistently simulated for all 6 years considered. The results thus demonstrate that vegetation substantially impacts upon the summertime climate in the Mediterranean region.