Exploring orbital-paced forcings impacts on the Mid-Pleistocene Transition using snapshot simulations

The change in ice age cyclicity from a 40-kyr to a 100-kyr pace between 750 kyr and 1,250 kyr called the Mid-Pleistocene Transition (MPT), has been widely observed from geological records.  While ice age cyclicity is commonly explained by insolation variations received at the top of the atmosphere, insolation forcing alone cannot explain the shift observed during the MPT, suggesting major impact played by the internal forcings like ice sheet variations and greenhouse gases concentrations. Previous works have highlighted the role of both these forcings along with a likely change of global ocean circulation. However, identifying the individual impacts of each forcing and whether ocean changes are a consequence of a driver of the MPT remains elusive.

Here, we use a recently updated version of HadCM3B paleoclimate model with realistic boundary conditions to explore orbital-scale drivers on global ocean circulation variations through the MPT. To this end, we take advantage of four recently extended sets of 919 snapshot simulations that cover the last 3.6 million years at ~4,000-year increments. They differ by their orbital timescale forcings: i) with changing insolation only (Pre-industrial ice sheet and GHG kept constant throughout), ii) with changing insolation and greenhouse gas variations only (Pre-industrial ice sheet kept constant throughout), iii) with changing insolation and ice sheet only (Pre-industrial GHG kept constant throughout) and iv) combining variations of insolation, greenhouse gas and ice sheet in unison.

After evaluating model’s results against geological reconstructions (with (Clark et al,. 2024) stacks in particular), we show the decisive impact of CO2 variations on global trends and ice age cyclicity shift during the MPT.  Ice sheet variations are mainly important to explain high latitudes changes and ice ages amplitude, driving then changes of ocean circulation strength.