CO ₂ and summer insolation as drivers for the Mid-Pleistocene Transition

Abstract. During the Mid-Pleistocene Transition (MPT; ∼ 1.2–0.8 Myr ago) the dominant periodicity of glacial cycles increased from 41 kyr to an average of 100 kyr, without any appreciable change in the orbital pacing. As the MPT is not a linear response to orbital forcing, it must have resulted from feedback processes in the Earth system. However, the precise mechanisms underlying the transition are still under debate. In this study, we investigate the MPT by simulating the Northern Hemisphere ice-sheet evolution over the past 1.5 Myr. The transient climate forcing of the ice-sheet model was obtained using a matrix method, by interpolating between two snapshots of global climate model simulations. Changes in climate forcing are caused by variations in CO2 and insolation, as well as implicit climate–ice-sheet feedbacks. Using this method, we were able to capture glacial–interglacial periodicity during the past 1.5 Myr and thereby reproduce the shift from 41 to 100 kyr cycles without any additional drivers. Instead, the modelled frequency change results from the prescribed CO2 combined with orbital forcing and ice-sheet feedbacks. Early Pleistocene terminations are initiated by insolation maxima. After the MPT, low interstadial CO2 levels may compensate insolation maxima which would otherwise favour deglaciation, leading to a longer duration of the glacial cycle. Terminations are also affected by ice volume. If the North American ice sheet is small or very large, it becomes sensitive to small temperature increases. A medium-sized ice sheet is less sensitive through its location and the merger of the Laurentide and Cordilleran ice sheets. Therefore, Late Pleistocene terminations are also facilitated by the large ice-sheet volume, where small changes in temperature lead to self-sustained melt. Additionally, we carried out experiments with constant CO2, where we can capture the 41 kyr cycles and some Late Pleistocene cycles. However, no persistent 100 kyr periodicity is established. Experiments with constant (or evolving) CO2 concentrations did not generate a substantial precession signal in the ice volume. Instead, the frequency is dominated by successful terminations, which are initiated by strong (generally obliquity) insolation maxima. Our results therefore indicate that the glacial cycle periodicity of the past 1.5 Myr can be described by changes in insolation, CO2, and ice-sheet feedback processes and that maintaining low CO2 throughout insolation maxima may prolong glacial cycles.