A range of future climate scenarios are projected for high atmospheric CO₂ concentrations, given uncertainties over future human actions as well as potential environmental and climatic feedbacks. The geological record offers an opportunity to understand climate system response to a range of forcings and feedbacks which operate over multiple temporal and spatial scales. Here, we examine a single interglacial during the late Pliocene (KM5c, ca. 3.205±0.01 Ma) when atmospheric CO₂ exceeded pre-industrial concentrations, but were similar to today and to the lowest emission scenarios for this century. As orbital forcing and continental configurations were almost identical to today, we are able to focus on equilibrium climate system response to modern and near-future CO₂. Using proxy data from 32 sites, we demonstrate that global mean sea-surface temperatures were warmer than pre-industrial values, by ∼2.3 ^∘C for the combined proxy data (foraminifera Mg∕Ca and alkenones), or by ∼3.2–3.4 ^∘C (alkenones only). Compared to the pre-industrial period, reduced meridional gradients and enhanced warming in the North Atlantic are consistently reconstructed. There is broad agreement between data and models at the global scale, with regional differences reflecting ocean circulation and/or proxy signals. An uneven distribution of proxy data in time and space does, however, add uncertainty to our anomaly calculations. The reconstructed global mean sea-surface temperature anomaly for KM5c is warmer than all but three of the PlioMIP2 model outputs, and the reconstructed North Atlantic data tend to align with the warmest KM5c model values. Our results demonstrate that even under low-CO₂ emission scenarios, surface ocean warming may be expected to exceed model projections and will be accentuated in the higher latitudes.

中文翻译：

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高CO ₂世界的经验教训：三百万年前的海景

鉴于未来人类行为的不确定性以及潜在的环境和气候反馈，预计大气CO ₂浓度高的一系列未来气候情景。地质记录为了解气候系统对一系列在多个时空尺度上作用的强迫和反馈的响应提供了机会。在这里，我们研究了上新世晚期（KM5c，约3.205±0.01  Ma）时大气CO ₂的单一间冰期超过了工业化前的浓度，但与今天和本世纪的最低排放情景相似。由于轨道强迫和大陆构造几乎与今天相同，因此我们能够专注于平衡气候系统对现代和近乎未来的CO _2的响应 。从32个站点使用代理数据，我们表明，全球平均海面温度比前工业值较暖，通过~2.3  ^∘用于组合代理数据（有孔虫ç镁/钙和烯酮），或通过~3.2 -3.4  ^∘C（仅限烯酮）。与工业化前的时期相比，北大西洋的经向梯度减小和变暖现象得以持续重建。在全球范围内，数据和模型之间存在广泛的共识，区域差异反映了海洋环流和/或代理信号。但是，代理数据在时间和空间上的不均匀分布的确为我们的异常计算增加了不确定性。KM5c的重构全球平均海表温度异常比PlioMIP2模型输出中的所有三个要高，但北大西洋重构数据倾向于与最暖的KM5c模型值一致。我们的结果表明，即使在低CO _2下 在排放情景下，预计海洋表面变暖将超过模型预测，并且在高纬度地区将会加剧。