Mid Pleistocene Transition caused by decline in atmospheric CO2

The Mid-Pleistocene Transition (MPT) marks a fundamental shift in Earth’s climate, with glacial cycles becoming longer and more intense despite unchanged orbital forcing periodicity and amplitude. Current hypotheses suggest that the critical drivers for the MPT were internal feedbacks involving atmospheric CO2 concentrations and ice sheet dynamics. However, limited data have hindered detailed analysis of CO₂ variability across the transition. Here, we present a highly resolved atmospheric CO2 reconstruction from 1.3 to 0.6 Ma based on foraminiferal boron isotopes from three sediment cores across the Indian, Pacific, and Atlantic Oceans, combined with ice sheet and global climate modelling. Our records show CO₂ and ice sheet dynamics remained tightly correlated throughout the MPT, with glacial CO₂ minima becoming lower and more persistent. NH ice sheet modelling indicates lower glacial CO2 enabled the Laurentide and Cordilleran ice sheets to merge and persist through insolation maxima, driving longer glacial cycles. Climate model simulations suggest long-term CO₂ decline induced Antarctic ice sheet expansion, enhanced Southern Ocean deep water formation, and promoted the reduction of glacial CO2. These findings highlight the central role of CO₂ and the Southern Ocean in initiating the MPT.