The Eocene-Oligocene Transition is marked by a sudden δ18O excursion occurring in two distinct phases: a precursor event at 34.15±0.04 Ma and the Earliest Oligocene oxygen Isotope Step at 33.65±0.04 Ma. These events signal a shift from the warm Late-Eocene greenhouse climate to cooler conditions, with temperature decreases of 3-5 °C, and the emergence of the first continent-wide Antarctic Ice Sheet (AIS). Despite clear evidence from proxy data, general circulation models (GCMs) struggle to replicate this Antarctic transition accurately, failing to capture the shift from warm, ice-free to cold, glaciated conditions. Even with unrealistically low pCO2 levels, Late-Eocene Antarctic summers in GCMs remain too warm and moist for snow or ice to survive. This study evaluates CESM1.0.5 simulations conducted by Baatsen et al. (2020), using a 38 Ma geo- and topographical reconstruction, considering different radiative (4 pre-industrial carbon levels (PIC) and 2 PIC) and orbital (present-day insolation and low Antarctic summer insolation) forcings. The climate is found to be highly seasonal, characterised by hot and wet summers and cold and dry winters. While reduced radiative and summer insolation forcing weaken this seasonality, the persistent atmospheric circulation still impedes ice sheet growth by limiting summer snow survival. For that reason, a new simulation is conducted with regional, moderately-sized ice sheets imposed on the continent, in order to investigate their stability and their influence on the atmospheric circulation. These ice sheets demonstrate self-sustaining and even expansion potential under 2 PIC and low summer insolation conditions. However, correlating resulting temperature and precipitation patterns with proxy data proves challenging, given the absence of terrestrial proxies. Extended simulations with coupled GCM-ISM models are therefore recommended, allowing for more dynamic atmosphere-ice-ocean-vegetation feedback mechanisms and dynamic radiative and orbital forcing.@en

Studies on the palaeoclimate and palaeoceanography using numerical model simulations may be considerably dependent on the implemented geographical reconstruction. Because building the palaeogeographic datasets for these models is often a time-consuming and elaborate exercise, palaeoclimate models frequently use reconstructions in which the latest state-of-the-art plate tectonic reconstructions, palaeotopography and -bathymetry, or vegetation have not yet been incorporated. In this paper, we therefore provide a new method to efficiently generate a global geographical reconstruction for the middle-late Eocene. The generalised procedure is also reusable to create reconstructions for other time slices within the Cenozoic, suitable for palaeoclimate modelling. We use a plate-tectonic model to make global masks containing the distribution of land, continental shelves, shallow basins and deep ocean. The use of depth-age relationships for oceanic crust together with adjusted present-day topography gives a first estimate of the global geography at a chosen time frame. This estimate subsequently needs manual editing of areas where existing geological data indicate that the altimetry has changed significantly over time. Certain generic changes (e.g. lowering mountain ranges) can be made relatively easily by defining a set of masks while other features may require a more specific treatment. Since the discussion regarding many of these regions is still ongoing, it is crucial to make it easy for changes to be incorporated without having to redo the entire procedure. In this manner, a complete reconstruction can be made that suffices as a boundary condition for numerical models with a limited effort. This facilitates the interaction between experts in geology and palaeoclimate modelling, keeping reconstructions up to date and improving the consistency between different studies. Moreover, it facilitates model inter-comparison studies and sensitivity tests regarding certain geographical features as newly generated boundary conditions can more easily be incorporated in different model simulations. The workflow is presented covering a middle-late Eocene reconstruction (38ĝ€Ma), using a MatLab script and a complete set of source files that are provided in the supplementary material.@en

The Eocene-Oligocene Transition (EOT) is marked by a sudden δ18O excursion occurring in two distinct phases, ~500 ky apart. These phases signal a shift from the warm Middle- to Late-Eocene greenhouse climate to cooler conditions, with global surface air temperatures decreasing by 3–5 °C and the emergence of the first continent-wide Antarctic Ice Sheet (AIS). While ice-sheet modelling suggests that ice sheet growth can be triggered by declining pCO2, it still remains unclear how this transition has been initiated, in particular the first growth phase that seems to be related to oceanic and atmospheric cooling rather than ice sheet growth. Recent climate model simulations of the Late-Eocene show improved accuracy but depict climatic conditions that are not conducive to the survival of incipient ice sheets throughout the summer season. This study therefore examines whether it is plausible to develop ice sheets of sufficient scale to trigger the feedback mechanism(s) required to disrupt the atmospheric regime above the Antarctic continent during warm Late-Eocene summers and establish more favourable conditions for ice expansion. We thereby aim to assess the stability of an incipient AIS under varying radiative, orbital and cryospheric forcing. To do so, we evaluate Community Earth System Model 1.0.5 simulations, using a 38 Ma geo- and topographical reconstruction, considering different radiative (4 and 2 pre-industrial carbon) and orbital (present-day and low summer insolation) forcings. The climatic conditions prevailing during (the lead-up to) the EOT can be characterised as extremely seasonal and monsoonal, featuring a short yet intense summer period and contrasting cold winters — highly inhospitable to ice sheet growth for most of the continent, as limited snow accumulation is expected to survive the summer season. A narrow convergence zone with moist convection around the region where sub-cloud equivalent potential temperature is high is shown to exhibit a ring-like structure, advecting moist surface air advected from the Southern Ocean. This advection leads to high values of moist static energy and subsequent precipitation in these regions. To assess the influence of cryospheric forcing, we conducted another simulation, with regional, moderately-sized ice sheets imposed on the continent, to investigate their stability and influence on the atmospheric circulation. Regionally, these relatively small ice sheets respond strongly to radiative and orbital forcing, and demonstrate remarkably favourable self-sustaining and even expansion potential under 2 PIC and low summer insolation conditions. This emphasises a significant hysteresis effect for local and/or regional ice sheets on the Antarctic continent, suggesting the potential for a significant volume of ice on the Antarctic continent without an imminent full glaciation prior to the EOT.@en