Milankovitch Theory Revisited to Explain the Mid-Pleistocene and Early Quaternary Transitions

The theory of orbital forcing as formulated by Milankovitch involves the mediation by the advance (retreat) of ice sheets and the resulting variations in terrestrial albedo. This approach poses a major problem, that of the period of glacial cycles which varies over time as happened during the Mid-Pleistocene Transition (MPT). Here, we show that various hypotheses are called into question because of the finding of a second transition, the Early Quaternary Transition (EQT) resulting from the million-year period eccentricity parameter. We propose to complement the orbital forcing theory to explain both the MPT and the EQT by invoking the mediation of western boundary currents (WBCs) and the resulting variations in heat transfer from the low to the high latitudes. From observational and theoretical considerations, it appears that very long period Rossby waves winding around subtropical gyres, the so-called “gyral” Rossby waves (GRWs), are resonantly forced in subharmonic modes from variations in solar irradiance resulting from the solar and orbital cycles. Two mutually reinforcing positive feedbacks of the climate response to orbital forcing have been evidenced, namely the change in the albedo resulting from the cyclic growth and retreat of ice sheets in accordance with the standard Milankovitch theory, and the modulation of the velocity of the WBCs of subtropical gyres. Due to the inherited resonance properties of GRWs, the response of the climate system to orbital forcing is sensitive to small changes in the forcing periods. For both the MPT and the EQT, the transition occurred when the forcing period merged with one of the natural periods of the climate system. The MPT occurred 1.25 Ma ago when the dominant period shifted from 41 ka to 98 ka, with both periods corresponding to changes in the Earth's obliquity and eccentricity. The EQT occurred 2.38 Ma ago when the dominant period shifted from 408 ka to 786 ka, with both periods corresponding to changes in the Earth's eccentricity. By providing new information, the aim of this article is essentially to spark new debates around a problem that has been pending since the discovery of glacial-interglacial cycles, where many hypotheses have been put forward without, however, fully answering all our questions.