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.

从始新世到渐新世的温室到冰室的气候转变被西南太平洋和南极边缘的海面温度记录充分记录，这些记录显示出明显的长期降温的证据。然而，驱动机制的识别取决于更好地理解这种冷却是否也存在于陆地环境中。在这里，我们使用保存在澳大利亚东南部褐煤序列中的细菌分子化石（生物标志物），展示了从始新世中期到渐新世早期（约 41-3300 万年前）的半连续陆地温度记录。我们的研究结果表明，澳大利亚东南部的年平均气温从始新世中期的~27°C（±4.7°C）逐渐下降到始新世晚期的~22-24°C（±4.7°C），然后是~2 . 跨越始新世/渐新世边界的 4 °C 阶梯冷却。这一趋势与南半球的其他温度记录相当，表明存在共同的驱动机制，可能\(p{{\rm{CO}}_{2}}\)。我们用一套气候模型模拟证实了这些结果，证明只有包括\(p{{\rm{CO}}_{2}}\)下降的模拟会导致澳大利亚东南部的降温，这与我们的代理记录一致。我们的数据构成了一个重要的基准，用于测试南极主要冰川作用开始时的气候模型性能、海陆相互作用和气候强迫。