  Due to spatial scarcity and uncertainties in sediment data, initial and
boundary conditions in deep-time climate simulations are not well constrained.
On the other hand, the climate is a nonlinear system with a multitude of
feedback mechanisms that compete and balance differently depending on the
initial and boundary conditions. This opens up the possibility to obtain
multiple steady states under the same forcing in numerical experiments. Here,
we use the MIT general circulation model with a coupled
atmosphere-ocean-thermodynamic 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, taking into
account processes on a timescale of thousands of years, in order to identify
the stability range of the steady states and tipping points as the atmospheric
CO$_2$ content is varied. We find three alternative steady states with a
difference in global mean surface air temperature of about 10 $^\circ$C. We
also examine how these climatic steady states are modified when feedbacks
operating on comparable or longer time scales are included, in particular
vegetation dynamics and air-sea carbon exchanges. Our findings on
multistability provide a useful framework for explaining the climatic
variations observed in the Early Triassic geological record, as well as some
discrepancies between numerical simulations in the literature and geological
data at PTB and its aftermath.
