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 pCO₂. We corroborate these results with a suite of climate model simulations demonstrating that only simulations including a decline in pCO₂ 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.