Synchronization theory for Pleistocene glacial-interglacial cycles

The dominant periodicity of glacial cycles changed from 41 kyr to roughly 100 kyr across the Mid-Pleistocene Transition (MPT) around 1 Myr ago. The mechanisms leading to these dominant periodicities and their changes during the MPT remain debated. We propose a synchronization theory explaining these features of glacial cycles and confirm it using an Earth system model that reproduces the MPT under gradual changes in volcanic CO2 outgassing rate and regolith cover. We show that the model exhibits self-sustained oscillations without astronomical forcing. Before the MPT, glacial cycles synchronize to the 41-kyr obliquity cycles because the self-sustained oscillations have periodicity relatively close to 41 kyr. After the MPT the time scale of internal oscillations becomes too long to follow every 41-kyr obliquity cycle, and the Earth's climate system synchronizes to the 100-kyr eccentricity cycles that modulate the amplitude of climatic precession. The latter synchronization is only possible with the help of the 41-kyr obliquity forcing through a mechanism that we term vibration-enhanced synchronization.