It is widely accepted that orbital variations are responsible for the generation of glacial cycles during the late Pleistocene. However, the relative contributions of the orbital forcing compared to CO₂ variations and other feedback mechanisms causing the waxing and waning of ice sheets have not been fully understood. Testing theories of ice ages beyond statistical inferences, requires numerical modeling experiments that capture key features of glacial transitions. Here, we focus on the glacial buildup from Marine Isotope Stage (MIS) 7 to 6 covering the period from 240 to 170 ka (ka: thousand years before present). This transition from interglacial to glacial conditions includes one of the fastest Pleistocene glaciation–deglaciation events, which occurred during MIS 7e–7d–7c (236–218 ka). Using a newly developed three-dimensional coupled atmosphere–ocean–vegetation–ice sheet model (LOVECLIP), we simulate the transient evolution of Northern Hemisphere and Southern Hemisphere ice sheets during the MIS 7–6 period in response to orbital and greenhouse gas forcing. For a range of model parameters, the simulations capture the evolution of global ice volume well within the range of reconstructions. Over the MIS 7–6 period, it is demonstrated that glacial inceptions are more sensitive to orbital variations, whereas terminations from deep glacial conditions need both orbital and greenhouse gas forcings to work in unison. For some parameter values, the coupled model also exhibits a critical North American ice sheet configuration, beyond which a stationary-wave–ice-sheet topography feedback can trigger an unabated and unrealistic ice sheet growth. The strong parameter sensitivity found in this study originates from the fact that delicate mass imbalances, as well as errors, are integrated during a transient simulation for thousands of years. This poses a general challenge for transient coupled climate–ice sheet modeling, with such coupled paleo-simulations providing opportunities to constrain such parameters.

中文翻译：

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用耦合的气候-冰盖模型模拟海洋同位素第7阶段

人们普遍接受轨道变化是导致晚更新世期间冰川周期产生的原因。但是，轨道强迫相对于CO ₂的相对贡献导致冰盖上蜡和褪色的各种变化和其他反馈机制尚未完全理解。要测试超出统计推断的冰期理论，需要进行数值模拟实验，以捕捉冰川转折的关键特征。在这里，我们重点研究海洋同位素阶段（MIS）7至6的冰川形成，涵盖了240至170 ka（ka：至今的一千年）的时期。从冰间期到冰期的过渡包括最快的更新世冰期－冰期事件之一，发生在MIS 7e-7d-7c（236-218 ka）期间。使用新开发的三维大气-海洋-植被-冰盖模型（LOVECLIP），我们模拟了MIS 7-6期间北半球和南半球冰盖在轨道和温室气体强迫作用下的瞬变演化。对于一定范围的模型参数，模拟可以很好地捕获重建范围内全球冰量的变化。在MIS 7–6期间，表明冰川开始对轨道变化更为敏感，而从深部冰川条件终止则需要轨道和温室气体强迫共同作用。对于某些参数值，耦合模型还显示出关键的北美冰原配置，超过该数值后，固定波-冰原的地形反馈会触发冰原的增长且不切实际。在这项研究中发现的强大的参数敏感性源于以下事实：在数千年的瞬态仿真过程中，微妙的质量失衡以及误差都得到了整合。这给瞬态耦合的气候-冰盖建模带来了一个普遍的挑战，这种耦合的古模拟提供了约束这些参数的机会。