Oxygen isotopes from ocean sediments (δ¹⁸O) used to reconstruct past continental ice volumes additionally record deep water temperatures (DWTs). Traditionally, these are assumed to be coupled (ice-volume changes cause DWT changes). However, δ¹⁸O records during peak Middle Miocene warmth (~16–15 million years ago) document large rapid fluctuations (~1–1.5‰) difficult to explain as huge Antarctic ice sheet (AIS) volume changes. Here, using climate modelling and data comparisons, we show DWTs are coupled to AIS spatial extent, not volume, because Antarctic albedo changes modify the hydrological cycle, affecting Antarctic deep water production regions. We suggest the Middle Miocene AIS had retreated substantially from previous Oligocene maxima. The residual ice sheet varied spatially more rapidly on orbital timescales than previously thought, enabling large DWT swings (up to 4 °C). When Middle Miocene warmth terminated (~13 million years ago) and a continent-scale AIS had stabilized, further ice-volume changes were predominantly in height rather than extent, with little impact on DWT. Our findings imply a shift in ocean sensitivity to ice-sheet changes occurs when AIS retreat exposes previously ice-covered land; associated feedbacks could reduce the Earth system’s ability to maintain a large AIS. This demonstrates ice-sheet changes should be characterized not only by ice volume but also by spatial extent.

来自海洋沉积物的氧同位素 (δ ¹⁸ O) 用于重建过去的大陆冰量，还记录了深水温度 (DWT)。传统上，这些假设是耦合的（冰量变化导致 DWT 变化）。然而，δ ¹⁸中中新世高峰期（约 16-1500 万年前）的 O 记录记录了巨大的快速波动（约 1-1.5‰），难以解释为巨大的南极冰盖（AIS）体积变化。在这里，使用气候建模和数据比较，我们显示 DWT 与 AIS 空间范围而非体积相关，因为南极反照率变化会改变水文循环，影响南极深水生产区。我们认为中中新世 AIS 已从之前的渐新世最大值大幅回落。残余冰盖在轨道时间尺度上的空间变化比以前认为的要快，从而实现了大的 DWT 摆动（高达 4°C）。当中中新世温暖终止（约 1300 万年前）并且大陆尺度的 AIS 稳定时，进一步的冰量变化主要是高度而不是范围，对 DWT 几乎没有影响。我们的研究结果表明，当 AIS 撤退暴露先前被冰覆盖的土地时，海洋对冰盖变化的敏感性就会发生转变；相关的反馈可能会降低地球系统维持大型 AIS 的能力。这表明冰盖变化不仅应以冰量为特征，还应以空间范围为特征。