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 Euro pe and the North Atlantic are used to assess the sensitivity of the Europea n climate with respect to continental-scale anthropogenic vegetation change s. An ensemble of six control integrations with present-day vegetation para meters (April until August, 1987-1992) is compared against an ensemble of s ix sensitivity experiments (for the same time period and years) using a pot ential 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 sensiti vity experiment leads to a moister and cooler spring, followed by a drier a nd warmer summer. The following key physical processes were identified: Eva potranspiration is increased in the simulation with potential vegetation du ring 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 experimen t with potential vegetation drop locally below a critical value (as a conse quence of the larger springtime evapotranspiration rates), whereupon transp iration is almost completely inhibited during the following 2 months. This leads to a sign reversal of the sensitivity to vegetation cover. This contr asting behavior during spring and summer is consistently simulated for all 6 years considered. The results thus demonstrate that vegetation substantia lly impacts upon the summertime climate in the Mediterranean region.