Earth’s climate cooled markedly during the late Miocene from 12 to 5 million years ago, with far-reaching consequences for global ecosystems. However, the driving forces of these changes remain controversial. A major obstacle to progress is the uncertainty over the role played by greenhouse gas radiative forcing. Here we present boron isotope compositions for planktic foraminifera, which record carbon dioxide change for the interval of most rapid cooling, the late Miocene cooling event between 7 and 5 Ma. Our record suggests that CO₂ declined by some 100 ppm over this two-million-year-long interval to a minimum at approximately 5.9 Ma. Having accounted for non-CO₂ greenhouse gasses and slow climate feedbacks, we estimate global mean surface temperature change for a doubling of CO₂—equilibrium climate sensitivity—to be 3.9 °C (1.8–6.7 °C at 95% confidence) on the basis of comparison of our record of radiative forcing from CO₂ with a record of global mean surface temperature change. We conclude that changes in CO₂ and climate were closely coupled during the latest Miocene and that equilibrium climate sensitivity was within range of estimates for the late Pleistocene, other intervals of the Cenozoic and the twenty-first century as presented by the Intergovernmental Panel on Climate Change.

在 12 到 500 万年前的中新世晚期，地球气候显着降温，对全球生态系统产生了深远的影响。然而，这些变化的驱动力仍然存在争议。取得进展的一个主要障碍是温室气体辐射强迫所起的作用的不确定性。在这里，我们展示了浮游有孔虫的硼同位素组成，它记录了最快速冷却间隔的二氧化碳变化，即 7 到 5 Ma 之间的中新世晚期冷却事件。我们的记录表明，在这 200 万年的时间间隔内，CO _{2下降了约 100 ppm，最低约为 5.9 Ma。}考虑到非 CO ₂温室气体和缓慢的气候反馈后，我们估计全球平均地表温度变化导致 CO 增加一倍_{2 — 平衡气候敏感性 — 根据我们的 CO 2}辐射强迫记录与全球平均地表温度变化记录的比较，为 3.9 °C（95% 置信度下为 1.8–6.7 °C） 。我们得出结论，在最近的中新世期间，CO ₂和气候的变化密切相关，并且平衡气候敏感性在政府间气候专门委员会提出的晚更新世、新生代的其他间隔和 21 世纪的估计范围内改变。