[M-IS14] Paleoclimatology and paleoceanography

[MIS14-20] Investigating the roles of CO₂ and paleogeography in shaping the Pliocene climate using an atmosphere-ocean-vegetation coupled model

*Sachio Nakagawa¹, Ayako Abe-Ouchi¹, Wing-Le Chan¹, Ryouta O'ishi¹, Taro Higuchi¹ (1.University of Tokyo)

Keywords:Climate Model, Paleoclimate, Pliocene

The Pliocene is thought to have been warmer than present-day and has been the focus of many studies as a possible analog of near-future warming and as the period during which humans evolved. It is known that in the Pliocene, there were less tundra and more forests in the Arctic and the Sahara Desert was smaller than in present-day, and there were more grassland and forests elsewhere. In modeling studies about the climate change from the Late Pliocene to present-day, it is thought that the long-term reduction in atmospheric CO₂ caused the long-term cooling trend. On the other hand, in research on the Late Pliocene, which has led to the reconstruction of new paleogeography including land-sea mask and seaways, the role of paleogeography in addition to atmospheric CO₂ need to be considered. Studies with multi-climate models have indicated that factors other than atmospheric CO₂ such as paleogeography and vegetation play an important role in the differences between present day and Pliocene climates. However, the individual impacts of each factor other than CO₂ are not clear.
In this study, we investigate the role of atmospheric CO₂ and paleogeography in shaping the differences between present day and Pliocene climates and vegetation using an atmosphere-ocean-vegetation coupled model, MIROC4m(AOV). The result shows that 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 alone, the continents at northern high latitudes tend to be warmer in boreal summer and colder in boreal winter. The summer warming in these regions leads to a reduction in surface area covered by tundra and an increase in forests, further enhancing the warming. The effect of this paleogeography on the summer warming and vegetation change is as large as that due to CO₂ forcing. What is more, Pliocene atmospheric CO₂ and paleogeography cause the humid climate and enhancement of monsoon in North Africa. The humid climate in North Africa is caused by the increase in water vapor in the atmosphere due to Pliocene atmospheric CO₂ and, secondary, by the change in atmospheric circulation due to Pliocene paleogeography. Summer warming on the northern high latitude continent due to Pliocene paleogeography enhances the monsoon and contributes to an increase in precipitation over North Africa.