Investigating the roles of CO2 and paleogeography in shaping the Pliocene climate using an atmosphere-ocean-vegetation coupled model

The Pliocene is thought to have been warmer than present-day and has been the focus of many studies as a possible analogue of near-future warming. In the Pliocene, there were less tundra and more forests in the Arctic, the Sahara Desert was smaller than in present-day, and there were more grassland and forests elsewhere. Studies with multiple climate models in PlioMIP2 have indicated that, factors other than atmospheric CO₂, such as paleogeography, vegetation, and ice sheets, also play an important role in shaping the Pliocene climate. In many modelling studies, vegetation, which can change in response to the other forcings, is fixed. In this study, in order to investigate the role of CO₂ and paleogeography including the effect of vegetation feedback, we conducted experiments using an atmosphere-ocean-vegetation coupled model, MIROC4m-AOV. We set CO₂ concentration and paleogeography to those of pre-industrial and of the Pliocene as prescribed by the PlioMIP3 protocol. We also conducted experiments in which conditions at northern and southern high latitudes are set to Pliocene separately. While Pliocene CO₂ concentration contributes to a globally warmer climate than pre-industrial, paleogeography has a large effect, both seasonally and locally. With Pliocene paleogeography, the continents at northern high latitudes tend to be warmer in summer and colder in winter. The summer warming in these regions causes a reduction in tundra and an increase in forests, further enhancing the warming. The Pliocene paleogeography of the northern high latitudes also enhances the precipitation in North Africa via the summer monsoon.