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The Mid-Pleistocene Transition, the shift from 41 kyr to 100 kyrglacial-interglacial cycles that occurred roughly 1 Myr ago, is oftenconsidered as a change in internal climate dynamics. Here we revisit the modelof Quaternary climate dynamics that was proposed by Saltzman and Maasch (1988).We show that it is quantitatively similar to a scalar equation for the icedynamics only when combining the remaining components into a single delayedfeedback term. The delay is the sum of the internal times scales of oceantransport and ice sheet dynamics, which is on the order of 10 kyr. We findthat, in the absence of astronomical forcing, the delayed feedback leads tobistable behaviour, where stable large-amplitude oscillations of ice volume andan equilibrium coexist over a large range of values for the delay. We thenapply astronomical forcing. We perform a systematic study to show how thesystem response depends on the forcing amplitude. We find that over a widerange of forcing amplitudes the forcing leads to a switch from small-scaleoscillations of 41 kyr to large-amplitude oscillations of roughly 100 kyrwithout any change of other parameters. The transition in the forced modelconsistently occurs near the time of the Mid-Pleistocene Transition as observedin data records. This provides evidence that the MPT could have been primarilya forcing-induced switch between attractors of the internal dynamics. Smalladditional random disturbances make the forcing-induced transition near 800 kyrBP even more robust. We also find that the forced system forgets its initialhistory during the small-scale oscillations, in particular, nearby initialconditions converge prior to transitioning. In contrast to this, in the regimeof large-amplitude oscillations, the oscillation phase is very sensitive torandom perturbations, which has a strong effect on the timing of thedeglaciation events.

The Mid-Pleistocene Transition induced by delayed feedback and bistability