Waterbelt states controlled by sea-ice thermodynamics

Snowball Earth refers to multiple periods in the Neoproterozoic during which geological evidence indicates that Earth was largely covered in ice. A Snowball Earth results from a runaway ice-albedo feedback, but it is still under debate how the feedback stopped: with fully ice-covered oceans or with a strip of open water around the equator. 

The latter are called waterbelt states and are an attractive explanation for the Snowball Earth events because they provide a refugium for the survival of photosynthetic aquatic life, while still explaining Neoproterozoic geology. Waterbelt states can be stabilised by bare sea ice in the subtropical desert regions with lower surface albedo stopping the ice-albedo feedback. However, the sea-ice model used in climate simulations can have a significant impact on the snow cover of ice and hence the surface albedo. 

Here we investigate the robustness of waterbelt states with respect to the thermodynamical representation of sea ice. We compare two thermodynamical sea-ice models, an idealised 0-layer Semtner model and a 3-layer Winton model that takes into account the heat capacity of ice. We deploy the atmospheric part of the ICON-ESM model (ICOsahedral Nonhydrostatic - Earth System Model) in a comprehensive set of simulations to determine the extent of the waterbelt hysteresis. 

The thermodynamic representation of sea ice strongly influences snow cover on sea ice over the range of all climate states. Including heat capacity by using the 3-layer Winton model increases snow cover and enhances the ice-albedo feedback. The hysteresis of the stable waterbelt state found using the 0-layer model disappears when using the 3-layer model. This questions the relevance of a subtropical bare sea-ice edge for waterbelt states and might help explain drastically varying model results on waterbelt states in the literature.