Alternative climatic steady states for the Permian-Triassic paleogeography

Abstract. Because of spatial scarcity and uncertainties in sedimentary data, initial and boundary conditions in deep-time climate simulations lack of constraints. On the other hand, climate is a nonlinear system with a multitude of feedback mechanisms, which compete and balance in a different way that depends on the initial and boundary conditions, opening the possibility, in numerical experiments, to obtain multiple steady states under the same forcing. Here, we use the MITgcm with a coupled atmosphere-ocean-sea ice-land configuration to explore the existence of such alternative steady states around the Permian-Triassic Boundary (PTB). We construct the corresponding bifurcation diagram accounting for processes on a timescale of thousands of years, in order to identify the stability range of the steady states and tipping points in regard to atmospheric CO₂ content. We find three alternative steady states with a difference in global mean surface air temperature of around 10 °C. We also investigate how these climatic steady states are modified when feedbacks acting over comparable or longer time scales are included, in particular vegetation dynamics and air-sea carbon exchange. Our findings for multistability provide a useful framework for explaining climatic variations observed in Early Triassic geological records, and some discrepancies between numerical simulations in the literature and geological data at the PTB and its aftermaths.