Department of Earth Sciences, Faculty of Geoscience, Utrecht University, Utrecht, The Netherlands.
Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, USA.
Nature. 2018 Jul;559(7714):382-386. doi: 10.1038/s41586-018-0272-2. Epub 2018 Jul 2.
Palaeoclimate reconstructions of periods with warm climates and high atmospheric CO concentrations are crucial for developing better projections of future climate change. Deep-ocean and high-latitude palaeotemperature proxies demonstrate that the Eocene epoch (56 to 34 million years ago) encompasses the warmest interval of the past 66 million years, followed by cooling towards the eventual establishment of ice caps on Antarctica. Eocene polar warmth is well established, so the main obstacle in quantifying the evolution of key climate parameters, such as global average temperature change and its polar amplification, is the lack of continuous high-quality tropical temperature reconstructions. Here we present a continuous Eocene equatorial sea surface temperature record, based on biomarker palaeothermometry applied on Atlantic Ocean sediments. We combine this record with the sparse existing data to construct a 26-million-year multi-proxy, multi-site stack of Eocene tropical climate evolution. We find that tropical and deep-ocean temperatures changed in parallel, under the influence of both long-term climate trends and short-lived events. This is consistent with the hypothesis that greenhouse gas forcing, rather than changes in ocean circulation, was the main driver of Eocene climate. Moreover, we observe a strong linear relationship between tropical and deep-ocean temperatures, which implies a constant polar amplification factor throughout the generally ice-free Eocene. Quantitative comparison with fully coupled climate model simulations indicates that global average temperatures were about 29, 26, 23 and 19 degrees Celsius in the early, early middle, late middle and late Eocene, respectively, compared to the preindustrial temperature of 14.4 degrees Celsius. Finally, combining proxy- and model-based temperature estimates with available CO reconstructions yields estimates of an Eocene Earth system sensitivity of 0.9 to 2.3 kelvin per watt per square metre at 68 per cent probability, consistent with the high end of previous estimates.
古气候重建对于温暖气候和高大气 CO2 浓度时期至关重要，这对于更好地预测未来气候变化至关重要。深海和高纬度古温度代用指标表明，始新世（5600 万至 3400 万年前）包含了过去 6600 万年中最温暖的一段时期，随后逐渐冷却，最终导致南极洲冰盖的形成。始新世极地的温暖是明确的，因此量化关键气候参数（如全球平均温度变化及其极地放大）演变的主要障碍是缺乏连续的高质量热带温度重建。在这里，我们基于应用于大西洋沉积物的生物标志物古温度学，提出了一个连续的始新世赤道海面温度记录。我们将该记录与现有的稀疏数据相结合，构建了一个包含 2600 万年的始新世热带气候演变的多代理、多站点堆积记录。我们发现，在长期气候趋势和短期事件的影响下，热带和深海温度发生了变化。这与温室气体强迫而非海洋环流变化是始新世气候的主要驱动因素的假设是一致的。此外，我们观察到热带和深海温度之间存在强烈的线性关系，这意味着在通常无冰的始新世期间，极地放大因子保持不变。与完全耦合气候模型模拟的定量比较表明，与工业化前的 14.4 摄氏度相比，早始新世、早中始新世、中始新世和晚始新世的全球平均温度分别约为 29、26、23 和 19 摄氏度。最后，将代理和基于模型的温度估计与可用的 CO2 重建相结合，得出了始新世地球系统敏感性的估计值，在 68%的概率下为 0.9 至 2.3 开尔文/瓦特/平方米，与之前的估计值的高端一致。
