Eocene to Oligocene terrestrial Southern Hemisphere cooling caused by declining pCO2

The greenhouse-to-icehouse climate transition from the Eocene into the Oligocene is well documented by sea surface temperature records from the southwest Pacific and Antarctic margin, which show evidence of pronounced long-term cooling. However, identification of a driving mechanism depends on a better understanding of whether this cooling was also present in terrestrial settings. Here, we present a semi-continuous terrestrial temperature record spanning from the middle Eocene to the early Oligocene (~41–33 million years ago), using bacterial molecular fossils (biomarkers) preserved in a sequence of southeast Australian lignites. Our results show that mean annual temperatures in southeast Australia gradually declined from ~27 °C (±4.7 °C) during the middle Eocene to ~22–24 °C (±4.7 °C) during the late Eocene, followed by a ~2.4 °C-step cooling across the Eocene/Oligocene boundary. This trend is comparable to other temperature records in the Southern Hemisphere, suggesting a common driving mechanism, likely $$p{{\rm{CO}}_{2}}$$ . We corroborate these results with a suite of climate model simulations demonstrating that only simulations including a decline in $$p{{\rm{CO}}_{2}}$$ lead to a cooling in southeast Australia consistent with our proxy record. Our data form an important benchmark for testing climate model performance, sea–land interaction and climatic forcings at the onset of a major Antarctic glaciation. Terrestrial Southern Hemisphere cooling through the Eocene–Oligocene transition points to decreasing atmospheric CO2 dominantly driving global change, according to biomarker records from southeast Australian coals and palaeoclimate modelling.