Geological evidence of active tropical glaciers reaching sea level during the Neoproterozoic (1,000–541 Ma), suggesting a global ocean completely covered in ice, was the key observation in the development of the hard Snowball Earth hypothesis. These conditions are hard to reconcile with the survival of complex marine life through Snowball Earth glaciations, which led to alternative waterbelt scenarios where a large-scale refugium was present in the form of a narrow ice-free strip in the tropical ocean. Here we assess whether a waterbelt scenario maintained by snow-free dark sea ice at low latitudes is plausible using simulations from two climate models run with a variety of cloud treatments in combination with an energy-balance model. Our simulations show that waterbelt states are not a robust and naturally emerging feature of Neoproterozoic climate. Intense shortwave reflection by mixed-phase clouds, in addition to a low albedo of bare sea ice, is needed for geologically relevant waterbelt states. Given the large uncertainty in mixed-phase clouds and their interaction with radiation, our results strongly question the idea that waterbelt scenarios can explain the Neoproterozoic geology. Hence, Neoproterozoic life has probably faced the harsh conditions of a hard Snowball Earth.

新元古代（1,000-541 Ma）期间活跃的热带冰川达到海平面的地质证据表明全球海洋完全被冰覆盖，这是硬雪球地球假说发展的关键观察结果。这些条件很难通过雪球地球冰川作用与复杂海洋生物的生存相协调，这导致了替代水带情景，在热带海洋中以狭窄的无冰带形式存在大规模的避难所。在这里，我们使用来自两个气候模型的模拟以及各种云处理结合能量平衡模型来评估由低纬度无雪的暗海冰维持的水带情景是否合理。我们的模拟表明，水带状态不是新元古代气候的一个强大且自然出现的特征。对于地质相关的水带状态，除了裸露海冰的低反照率外，还需要混合相云的强烈短波反射。鉴于混合相云及其与辐射的相互作用存在很大的不确定性，我们的结果强烈质疑水带情景可以解释新元古代地质学的观点。因此，新元古代生命很可能面临着坚硬的雪球地球的恶劣条件。我们的结果强烈质疑水带情景可以解释新元古代地质的观点。因此，新元古代生命很可能面临着坚硬的雪球地球的恶劣条件。我们的结果强烈质疑水带情景可以解释新元古代地质的观点。因此，新元古代生命很可能面临着坚硬的雪球地球的恶劣条件。