Ice-free tropical waterbelt for Snowball Earth events questioned by uncertain clouds

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.