  A set of idealized experiments are performed to analyze the competing effects
of declining atmospheric CO2 concentrations, the opening of an ocean gateway,
and varying orbital parameters. These forcing mechanisms, which influence the
global mean climate state, may have played a role for triggering climate
transitions of the past (for example during the Eocene-Oligocene climate
transition and the build-up of the Antarctic Ice Sheet). Sensitivity
simulations with a coupled atmosphere-ocean general circulation model are
conducted to test these three forcings and their roles for the global climate.
The simulations are carried out under idealized conditions to focus on the
essentials. The combination of all three forcings triggers a climate transition
which resembles the onset of the Antarctic glaciation. In particular, the
temperatures in the southern high latitudes decrease and snow accumulates
constantly. Moreover, the relative importance of each possible forcing is
explored. All three of the mechanisms (atmospheric CO2 decrease, opening of the
ocean gateways, and changing orbital parameters) cool the climate of the
southern polar continent. The change induced by the orbital parameters depends
strongly on the magnitude of the change in obliquity.
