Can the Jormungand hypothesis be an alternative explanation for the Neoproterozoic pan-glaciations?

新元古代（距今1000- 5.41亿年前）的泛冰期是地球历史上最极端、最激动人心的气候之一。新元古代的地质为长达200万年的冰期提供了充足的证据，在此期间，热带大陆被广泛覆盖的冰川覆盖，冰川一直延伸到海平面。经典的雪球地球理论认为，只有假设海洋完全被海冰覆盖，才能解释这一现象。然而，2011年，PI与芝加哥大学的同事们提出了另一种Jormungand假说。与雪球地球理论一样，Jormungand假说可以解释新元古代的地质，并为泛冰期的整个生命周期提供了一个场景。然而，与雪球地球理论相比，Jormungand假说考虑到一小片开阔的赤道海洋，因此它更容易解释生命是如何在极端气候下生存下来的。Jormungand假说和与之相关的滞后是由大气的基本动力学和哈德利环流对亚热带水文循环和海冰反照率的影响产生的。Jormungand假说已经成功地在能量平衡模型和全球气候模型模拟中得到了证明，这些模拟是在没有海洋和海冰动力学的水行星环境下进行的。然而，为了评估Jormungand假说是否实际上是雪球地球理论的可行替代方案，需要在实际模型设置中进行气候模型模拟。因此，该项目旨在通过全球气候模式ICON的模拟来研究Jormungand假说。模拟将采用分层方法来调查大陆、亚热带风驱动的海洋细胞以及海洋和海冰动力学的影响。将解决四个问题：1)Jormungand假说是理想水行星设置的稳健模型特征吗？2)。大陆和它们的几何形状如何影响Jormungand假说？3)。海洋能量输送如何影响Jormungand假说，这是否取决于大陆？4)。我们能否在一个具有现实新元古代大陆、海洋动力学和海冰动力学的耦合大气-海洋模型中证明Jormungand假说？如果可以，是什么决定了Jormungand滞后的大小？新元古代泛冰期是基础气候动力学中最令人兴奋的课题之一。此外，该项目承诺在一个与今天截然不同的状态下测试新的气候模型ICON，并得出可能与太阳系外行星的可居住性相关的见解。

The pan-glaciations of the Neoproterozoic era (1000-541 Million years before present) are among the most extreme and most exciting climates in the history of Earth. The Neoproterozoic geology provides ample evidence for two million-year long glaciations during which tropical continents were covered by widespread glaciers that reached down to sea level. The classical Snowball Earth theory posits that this can only be explained if one assumes that the oceans were completely covered by sea ice. In 2011, however, the PI together with colleagues from the University of Chicago proposed the alternative Jormungand hypothesis. As the Snowball Earth theory the Jormungand hypothesis can explain the Neoproterozoic geology and offers a scenario for an entire lifecycle of the pan-glaciations. In contrast to the Snowball Earth theory, however, the Jormungand hypothesis allows for a small strip of open equatorial ocean, because of which it can much more easily explain how life survived these extreme climates. The Jormungand hypothesis, and the hysteresis associated with it are created by fundamental dynamics of the atmosphere and the impact of the Hadley circulation on the subtropical hydrological cycle and sea-ice albedo. The Jormungand hypothesis has been successfully demonstrated in energy balance models as well as global climate model simulations in aquaplanet setup without ocean and sea-ice dynamics. Yet, in order to assess whether the Jormungand hypothesis can in fact be a viable alternative to the Snowball Earth theory, climate model simulations in realistic model setups are needed. The project therefore aims to study the Jormungand hypothesis through simulations with the global climate model ICON. The simulations will take a hierarchical approach to investigate the impact of continents, subtropical wind-driven ocean cells, and ocean and sea-ice dynamics in general. Four questions will be addressed: 1) Is the Jormungand hypothesis a robust model feature of idealized aquaplanet setups? 2.) How do continents and their geometry affect the Jormungand hypothesis? 3.) How does ocean energy transport affect the Jormungand hypothesis, and does this depend on continents? 4.) Can we demonstrate the Jormungand hypothesis in a coupled atmosphere-ocean model with realistic Neoproterozoic continents, ocean dynamics, and sea-ice dynamics, and if so what determines the magnitude of Jormungand hysteresis? The Neoproterozoic pan-glaciations are among the most exciting topics in fundamental climate dynamics. In addition, the project promises to test the new climate model ICON in a state very different from today, and to lead to insights that could be relevant also to the habitability of extrasolar planets.