THE ALBEDO OF TEMPERATE AND BOREAL FOREST AND THE NORTHERN-HEMISPHERECLIMATE - A SENSITIVITY EXPERIMENT USING THE LMD-GCM

A deforestation experiment is performed using the Laboratoire de Meteo rologie Dynamique Atmospheric General Circulation Model (LMD GCM) to d etermine the climatic role of the largest vegetation formation in the Northern Hemisphere, localized mostly north of latitude 45 degrees N, which is called the temperate and boreal forest. For this purpose, an iterative albedo scheme based on vegetation type, snow age, snowfall r ate and area of snow cover, is developed for snow-covered surfaces. Th e results show a cooling of Northern Hemisphere soil and an increase i n the snow cover when the forest is removed, as found by previous simi lar experiments. In our study this cooling is related to different cau ses, depending on the season. It is linked to modifications in the soi l radiative properties, like surface albedo, due to the disappearance of forest, and consequently, to a greater exposure of the snow-covered soil underneath. It is also related to alterations in the hydrologica l cycle, observed mainly in summer and autumn at middle latitudes. The model shows a strong sensitivity to the coupled surface albedo - soil temperature - fractional snow cover response in the spring. A later a nd longer snowmelt season is also detected. This study adds to our und erstanding of climatic variation on longer time scales, since it is wi dely accepted that the formation and disappearance of different vegeta tion formations is closely related to climatic evolution patterns, in particular on the time scale of the glacial oscillations.