Holocene land cover change on the Tibetan Plateau

The Tibetan Plateau is expected to be one of the most climatic sensitive regions of the earth. Due to its large horizontal and vertical extend it has a large influence on the regional as well as global climate. As a heat source for the atmosphere in spring and summer it plays an important role for the onset and maintenance of the Asian summer monsoon and therewith influences one of the strongest monsoon circulations of the world. On the other hand, monsoon related convection above the Plateau leads to large-scale subsiding air-masses and dry climate in the regions north and west of the Plateau. These processes depend to a large part on the land cover as it controls the energy balance at the surface and the strength of the diabatic heat fluxes. Land cover changes on the Tibetan Plateau may thus exert strong influence on the regional and northern hemispheric climate. Therefore it is very important to represent past and future land cover changes correctly in global climate models, not only to understand the mechanism behind these land cover changes, but also to understand past and future climate change in Asia. To assess the performance of the comprehensive Earth system model ECHAM5-JSBACH/MPIOM with respect to the land cover on the Tibetan Plateau, we compare results of a transient numerical experiment with pollen-based vegetation reconstructions from four representative sites on the Plateau, covering the last 6000 years. Generally, the reconstructed and simulated trends are similar for most sites. Data and model show a strong decrease of forests on the Plateau. According to the model results, the averaged forest fraction has been reduced by almost one-third from mid-Holocene (41.4%) to present-day (28.3%) and is replaced by shrub and grass. The model mainly identifies differences in near-surface air temperatures due to an orbital induced insolation change as the reason for the vegetation change. Reconstructions rather indicate decreasing summer monsoon precipitation causing the shift in vegetation.