Waterbelt solutions to avoid a hard Snowball Earth

During the Neoproterozoic, Earth experienced at least two extreme glaciations with ice extending to tropical latitudes. While the Snowball Earth hypothesis proposes a fully ice-covered planet, geological evidence and the persistence of life suggest that parts of the ocean may have remained ice-free. This has motivated the concept of Waterbelt states: alternative climate equilibria featuring open equatorial oceans that could act as refugia for early life and expand the range of habitable climates relevant to Earth-like exoplanets. Despite their appeal, Waterbelt states remain disputed due to uncertainties in the mechanisms required to halt the ice–albedo feedback at low latitudes, including the role of bare sea-ice albedo and cloud radiative effects.

Here, we investigate whether Waterbelt states are robust solutions of the coupled climate system and identify the processes controlling the stability of low-latitude ice margins. Using a hierarchy of models, this work combines mechanistic insights from a Budyko–Sellers energy balance model with a large ensemble of global climate simulations. In particular, we present results from a coordinated model intercomparison that includes three versions of the ICON model and five versions of the CAM model, all run in the same aquaplanet slab-ocean setup. The simulations are analyzed with respect to three key factors that have been proposed to influence Waterbelt stability: the area of exposed bare sea ice, cloud masking of the ice–albedo feedback, and shortwave cloud radiative feedbacks.

We demonstrate that stable Waterbelt states can be found in a wide variety of models. While ICON Waterbelt states depend on cloud tuning, all CAM models readily simulate stable Waterbelt states over a substantial range of CO2 radiative forcing. These differences are primarily due to cloud radiative effects: the CAM models exhibit stabilizing shortwave cloud feedbacks and stronger cloud masking than ICON. Overall, this suggests that clouds do not present a fundamental obstacle to Waterbelt climates, but instead play a modulatory role that varies across models. This implies that Waterbelt states may be more physically plausible than studies based on a single model have suggested, while at the same time emphasizing the importance of clouds for deep-time climate and exoplanet habitability.