Orbital-resolution Pliocene-Pleistocene simulations of CO2, sea level, global temperature and benthic δ¹⁸O, and d11B-based proxy-CO2 data@en

During the past five million yrs, benthic d18O records indicate a large range of climates, from warmer than today during the Pliocene Warm Period to considerably colder during glacials. Antarctic ice cores have revealed Pleistocene glacial-interglacial CO2 variability of 60-100 ppm, while sea level fluctuations of typically 125 m are documented by proxy data. However, in the pre-ice core period, CO2 and sea level proxy data are scarce and there is disagreement between different proxies and different records of the same proxy. This hampers comprehensive understanding of the long-term relations between CO2, sea level and climate. Here, we drive a coupled climate-ice sheet model over the past five million years, inversely forced by a stacked benthic d18O record. We obtain continuous simulations of benthic d18O, sea level and CO2 that are mutually consistent. Our model shows CO2 concentrations of 300 to 470 ppm during the Early Pliocene. Furthermore, we simulate strong CO2 variability during the Pliocene and Early Pleistocene. These features are broadly supported by existing and new d11B-based proxy CO2 data, but less by alkenone-based records. The simulated concentrations and variations therein are larger than expected from global mean temperature changes. Our findings thus suggest a smaller Earth System Sensitivity than previously thought. This is explained by a more restricted role of land ice variability in the Pliocene. The largest uncertainty in our simulation arises from the mass balance formulation of East Antarctica, which governs the variability in sea level, but only modestly affects the modeled CO2 concentrations.过去五百万年间，底栖δ18O（benthic d18O）记录显示气候跨度极大，从上新世暖期（Pliocene Warm Period）较现今更为温暖的状态，到冰期时显著寒冷的环境。南极冰芯（Antarctic ice cores）揭示了更新世（Pleistocene）冰期-间冰期旋回中二氧化碳浓度波动幅度达60~100 ppm，而代用资料（proxy data）记录的海平面波动幅度通常可达125米。然而，在冰芯记录出现之前的时期，二氧化碳与海平面的代用资料十分匮乏，且不同代用资料之间、同一代用资料的不同记录之间均存在分歧，这阻碍了学界对二氧化碳、海平面与气候三者间长期关联的全面理解。本研究利用堆叠底栖δ18O记录作为反演强迫条件，驱动过去五百万年的气候-冰盖耦合模式（coupled climate-ice sheet model），得到了底栖δ18O、海平面与二氧化碳三者相互一致的连续模拟结果。模式模拟显示，上新世早期的二氧化碳浓度介于300~470 ppm之间。此外，研究还模拟出上新世与早更新世时期显著的二氧化碳浓度波动，该特征得到了现有及新增的基于硼11（d11B）的代用二氧化碳数据的广泛支持，但与长链烯酮（alkenone）记录的吻合度相对较低。模拟得到的二氧化碳浓度及其变化幅度均大于全球平均温度变化所预期的结果，因此本研究表明地球系统敏感性（Earth System Sensitivity）较此前的认知更低，这可由上新世时期陆冰变化的受限作用予以解释。本模拟中最大的不确定性来自东南极洲（East Antarctica）的质量平衡方案（mass balance formulation），该方案主导了海平面的波动幅度，但对模拟的二氧化碳浓度影响有限。