Relative contributions of CO₂ and ice sheets on the Mid-Pleistocene Transition

Abstract. The Mid-Pleistocene Transition (MPT) represents one of the most prominent and debated climate reorganizations within the long-term cooling trend of the last 3.6 million years. Although the role of ice sheets, greenhouse gases and their combined effects have all been implicated, their individual contributions to the MPT remain unresolved.

Here, we assess their respective roles using a unique set of four series of fully coupled equilibrium palaeoclimate model simulations spanning the last 3.6 Ma at 4-ka intervals, constrained by realistic boundary conditions. These simulations differ in their prescribed insolation, greenhouse gases and ice sheet forcings where a realistic baseline time evolving scenario is analysed against a series where each forcing is kept static throughout to constrain their individual impact on the MPT.

First, we evaluate the performance of the four sets of simulations including the combination of the three changing forcings against geological data. We find that the model captures the main characteristics of temperature variations identified by geological records, such as the global cooling trend over the past 3.6 Ma, acceleration of cooling at the MPT, with amplification of the glacial-interglacial cycles along with a change of pace from a 40-ka to a 100-ka cyclicity. Then, we compare the four simulated timeseries to untangle the individual role of each forcing through time and show that greenhouse gases exert a direct and dominant role on both the long-term global cooling trend and the shift in glacial-interglacial cyclicity associated with the MPT. Ice sheets primarily modulate the amplitude of glacial-interglacial variability through their influence on sea-ice formation and ocean circulation.