Control of subpar interglacial CO2 levels on the emergence of 100-kyr glacial cycles during the Mid-Pleistocene Transition

The evolution of Earth’s climate during the Quaternary is characterized by glacial cycles with the periodic waxing and waning of large ice sheets most notably in the Northern Hemisphere. A fundamental transition in Earth climate occurred between 1250 and 650 kyr ago (ka) as the dominant glacial-interglacial variability shifted from ~40 to ~100-kyr cycles with more intense glacial climate. This is known as the Mid-Pleistocene Transition (MPT). The absence of appreciable change in the Milankovitch astronomical climate forcings during the MPT indicates that its occurrence might be a result of self-perpetuating climate feedback processes which would have been stimulated at ~900ka (i.e. Marine Isotope Stage 25-22) when global ice volume (e.g. Elderfield et al., 2012, Ford & Raymo, 2020), glacial ocean circulation (e.g. Pena &Goldstein 2014; Kim et al., 2021), deep ocean carbon reservoir (e.g. Lear et al., 2016; Farmer et al., 2019) and atmospheric CO2 levels (e.g. Hoenish et al., 2009; Chalk et al., 2017; Yamamoto et al., 2022) coherently experienced stepwise changes to post-MPT like glacial states from MIS22 onwards. Nevertheless, the triggering mechanism remains enigmatic because of the intertwined nature of these internal processes which precludes the disentanglement into their individual roles in the onset of post-MPT glacial ice volume and the pCO2 level at MIS22. In this study, applying a combined climate – ice sheet – marine biogeochemical modeling approach, we investigate unidirectional impacts of changes in either atmospheric CO2 levels or northern hemisphere ice sheet (NHIS) volume on the other in transient simulations spanning two successive obliquity cycles. Our results show that the emergence of a 100-kyr glacial cycle is controlled by the interglacial rather than glacial CO2 levels. A lower glacial CO2 levels does increase NHIS volume and hence intensify the glacial climate. But only when the interglacial CO2 levels are below a threshold (~250ppm in our model) at the first obliquity peak, the developed glacial NHISs can skip the summer insolation maximum and reach a larger volume in the following glaciation, heralding the onset of 100-kyr glacial cycles. Meanwhile, the increased glacial NHISs, as a positive climate feedback process, promote atmospheric CO2 absorption in the subpolar North Atlantic via the strengthened upper cell of Atlantic Meridional Overturning Circulation. This, coupled with the enhanced formation of Antarctic Bottom Water, eventually sequesters the absorbed carbon in the North Pacific, further lowering glacial CO2 levels. Consistent with available proxy records, our results thus reconcile previously competing hypotheses for the occurrence of the MPT, providing a new and coherent dynamic framework accounting for the emergence of 100-kyr glacial cycles.