A challenge for Beyond EPICA Oldest Ice: Predictions of greenhouse gases across the MPT

Understanding the drivers of the Mid-Pleistocene Transition (MPT) remains one of the most challenging problems in palaeoclimate. One unfulfilled prerequisite for tackling this problem is a comprehensive view of greenhouses gases (GHGs) across the MPT. The Beyond EPICA Oldest Ice project and other ice core efforts are now focused on extending the ice core record of GHGs through the MPT. As the new data emerges, it is useful to define a set of testable hypotheses - in this case, using predictions of GHGs across the MPT.

Most work on extending GHGs beyond the current ice core record have focused solely on predicting atmospheric CO₂, although it is recognized that the combined radiative impact of CH₄ and N₂O could be of overlooked importance. The methods vary in complexity from statistical approaches using ocean sediment data - to box modelling efforts with prescribed forcings - to inversion methods targeting proxy data with a hierarchy of models - to earth system modelling with minimal (but none-the-less important) assumptions about external forcings.

Here we will build up an objective overview of these predictions. First, we review previous work from the literature. Second, we explore some new statistical models for all three GHGs, with a particular focus on utilizing high-resolution sediment records that capture millennial- and orbital-scale variability (Hodell et al., 2023) as well as highlighting the implications of new estimates of global surface temperature and ice volume (Clark et al., 2024). Finally, we provide novel histories using a combination of box model and published climate model data (Yun et al., 2023) that also go beyond predicting just CO₂ and allow us to discuss coeval changes in CH₄, N₂O and δ¹³C-CO₂.

This synthesis will provide one possible template for interpreting the new datasets that will be presented elsewhere. In particular, we will breakdown the various hypotheses in terms of changes in the mean and range of variability over the past 1.5 million years (i.e. the changes in overall mean, the glacial minima, and interglacial maxima). Furthermore, we will investigate how covariations of greenhouse gases concentration and isotopic composition can constrain the nature of biogeochemical feedbacks operating in the earth system across the MPT.

References

Clark, P.U, et al. (2025) Global mean sea level over the past 4.5 million years. Science 390, eadv8389, DOI:10.1126/science.adv8389

Hodell, D. A.,et al. (2023) A 1.5-million-year record of orbital and millennial climate variability in the North Atlantic, Clim. Past, 19, 607–636, https://doi.org/10.5194/cp-19-607-2023.

Yun, K.-S.,et al. (2023) A transient coupled general circulation model (CGCM) simulation of the past 3 million years, Clim. Past, 19, 1951–1974, https://doi.org/10.5194/cp-19-1951-2023