Late Miocene vegetation reconstruction with the CARAIB model

Climatic outputs from the atmospheric general circulation model ECHAM4 coupled to a mixed layer ocean model are used as inputs to the CARAIB global vegetation model to reconstruct the distribution of vegetation and the biosphere carbon stocks over the continents during the Late Miocene (Tortonian). The results show significant changes in the geographical distribution of vegetation during the Late Miocene compared to the present with a reduction of desert areas and an expansion of tropical seasonal forests, which reached temperate latitudes. These changes in vegetation distribution are accompanied by a moderate increase of the total biosphere carbon stock by 159 Gt. Sensitivity tests to atmospheric CO2 have also been performed with the vegetation model only, i.e., while keeping constant all climatic variables to their reference Tortonian state. These tests point out the potential importance of CO2 fertilization both regarding vegetation distribution and biosphere carbon storage. The impact of an atmospheric CO2 decrease (from 280 to 200 ppmv) or increase (from 280 to 560 ppmv) on the vegetation distribution appears to be at least as large as that of the climate change between the Tortonian and the present, while in terms of carbon storage the impact of atmospheric CO2 is far much larger than the climatic one. Although the actual response of vegetation to CO2 fertilization may be much smaller than its theoretical response in the model, these results emphasize the need to consider atmospheric CO2 as an important parameter for palaeovegetation reconstructions.