Tropical cloud feedback in near-Snowball Earth waterbelt states

Waterbelt states are an alternative scenario for Snowball Earth, where a narrow strip of ocean remains ice-free at the equator, providing a robust solution for the survival of life. Recent studies have shown that waterbelt states can be stabilised by subtropical low-level clouds, because they weaken the ice-albedo feedback created by the expanding sea ice beneath the clouds. Thick subtropical clouds are therefore needed to stabilise the waterbelt state.

However, clouds also have the opposite effect over the open ocean equatorward of the ice margin. Here they provide a destabilising cloud feedback that supports the ice-albedo feedback in favour of a Snowball Earth. When sea ice enters the subtropics, this effect becomes particularly strong, as the vertical structure and the phase partitioning of tropical clouds begin to change. As a result, tropical clouds can ultimately determine the stability of the waterbelt state.

Here we show a preliminary analysis of simulations with two versions of the atmospheric ICON model using the same setup, a slab-ocean aquaplanet with a thermodynamic sea-ice model and over a broad range of atmospheric CO₂ concentrations. While waterbelt states are easily found in ICON-A, they are absent in ICON-ESM due to a difference in tropical clouds. The tropical cloud feedback will be analysed by means of cloud controlling factors, and simulations with the cloud locking method will be employed to demonstrate the critical role of tropical cloud feedback for waterbelt states.