Nonlinearity of atmospheric CO2 during the last 3 million years

No paleoclimate phenomenon has been more vexing to the climate modeling community than the Plio–Pleistocene glacial/interglacial cycles. We demonstrate that by using time-reversed forcings, model simulations do not generate the high CO2 levels and relatively ice-free conditions of the late Pliocene, as seen in the forward-in-time simulations of the same model. We also find that a transition toward depleted regolith and lowered atmospheric CO2 levels is required to produce the mid-Pleistocene Transition (MTP) toward large-amplitude ~100 ky sawtooth glacial/interglacial cycles. We show that these features originate from nonlinearities and initial state dependence in carbon cycle behavior