The Eocene–Oligocene transition (EOT) was a climate shift from a largely ice-free greenhouse world to an icehouse climate, involving the first major glaciation of Antarctica and global cooling occurring ∼34 million years ago (Ma) and lasting ∼790 kyr. The change is marked by a global shift in deep-sea δ¹⁸O representing a combination of deep-ocean cooling and growth in land ice volume. At the same time, multiple independent proxies for ocean temperature indicate sea surface cooling, and major changes in global fauna and flora record a shift toward more cold-climate-adapted species. The two principal suggested explanations of this transition are a decline in atmospheric CO₂ and changes to ocean gateways, while orbital forcing likely influenced the precise timing of the glaciation. Here we review and synthesise proxy evidence of palaeogeography, temperature, ice sheets, ocean circulation and CO₂ change from the marine and terrestrial realms. Furthermore, we quantitatively compare proxy records of change to an ensemble of climate model simulations of temperature change across the EOT. The simulations compare three forcing mechanisms across the EOT: CO₂ decrease, palaeogeographic changes and ice sheet growth. Our model ensemble results demonstrate the need for a global cooling mechanism beyond the imposition of an ice sheet or palaeogeographic changes. We find that CO₂ forcing involving a large decrease in CO₂ of ca. 40 % (∼325 ppm drop) provides the best fit to the available proxy evidence, with ice sheet and palaeogeographic changes playing a secondary role. While this large decrease is consistent with some CO₂ proxy records (the extreme endmember of decrease), the positive feedback mechanisms on ice growth are so strong that a modest CO₂ decrease beyond a critical threshold for ice sheet initiation is well capable of triggering rapid ice sheet growth. Thus, the amplitude of CO₂ decrease signalled by our data–model comparison should be considered an upper estimate and perhaps artificially large, not least because the current generation of climate models do not include dynamic ice sheets and in some cases may be under-sensitive to CO₂ forcing. The model ensemble also cannot exclude the possibility that palaeogeographic changes could have triggered a reduction in CO₂.

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始新世-渐新世过渡：海洋和陆地代理数据，模型以及模型-数据比较的回顾

始新世-渐新世过渡（EOT）是从一个基本上无冰温室世界气候转变到冰室气候，包括南极洲的第一大冰期和全球变冷发生〜34万年前（MA）和持久~790  KYR。的改变是通过在深海一个全球移标记δ ¹⁸ ö表示深海冷却和在陆地冰量的增长的组合。同时，海洋温度的多个独立指标表明海面正在降温，全球动植物群的重大变化表明向更适应气候的物种转变。关于这种转变的两个主要建议解释是大气中CO ₂的下降以及海洋通道的变化，而轨道强迫可能会影响冰川的精确计时。在这里，我们回顾并综合了来自海洋和陆地领域的古地理，温度，冰盖，海洋环流和CO ₂变化的代理证据 。此外，我们定量地将变化的代理记录与整个EOT上的温度变化气候模型模拟集合进行比较。该模拟比较了整个EOT的三种强迫机制：CO ₂ 减少，古地理变化和冰盖生长。我们的模型综合结果表明，除了施加冰盖或古地理变化外，还需要一种整体冷却机制。我们发现CO ₂强迫涉及到CO ₂的大幅度下降。40％（约325  ppm的下降）最适合现有的代理证据，而冰盖和古地理的变化则起次要作用。尽管这一大幅减少与某些CO ₂代理记录（减少），那么关于冰块生长的积极反馈机制是如此强大，以至于适度的CO ₂下降超过冰块起始的临界阈值，就足以触发冰块快速生长。因此，我们的数据模型比较表明，CO ₂降低幅度应该被认为是一个较高的估计值，可能是人为的，这不仅是因为当前的气候模型不包括动态冰盖，而且在某些情况下可能不太敏感到CO ₂强迫。模型集合也不能排除古地理变化可能触发CO ₂减少的可能性。