Constraining cloud uncertainty in near-snowball waterbelt scenarios at the outer limit of the habitable zone

Abstract:

Waterbelt climate states with close to global ice cover may extend the outer limit of the habitable zone (OHZ) and help to explain the survival of advanced marine species during the Neoproterozoic glaciations on Earth. However, the stability of waterbelt climate states is expected to be sensitive to variations in external forcings and a planet’s internal climate response. Close to the OHZ the internal climate response is impacted by the interaction of cloud-radiative effects (CRE) and albedo feedbacks. In present-day climate on Earth, CRE are known to cause a substantial part of the uncertainty of the internal climate response to a given external forcing, contributing to a considerable spread of climate sensitivity across global climate models. We here study the impact of uncertainty associated with CRE on the existence of geologically relevant waterbelt climate regimes using two global climate models and an idealized energy balance model. We find that the stable range of the waterbelt climate regime is very sensitive to the abundance, or lack, of low-level mixed-phase clouds at low latitudes. If cloud cover is low in these regions climate sensitivity becomes too high to allow for stable waterbelt states. Because the treatment of mixed-phase clouds is highly uncertain in global climate models, we aim to constrain the uncertainty associated with their CRE by means of a hierarchy of global and regional simulations at different grid resolutions that include large-eddy simulations of subtropical mixed-phase clouds. By conducting simulations with two extreme scenarios for atmospheric aerosols, which serve as ice-nucleating particles and therefore can control mixed-phase cloud physics, we aim to estimate the spread of CRE associated with subtropical mixed-phase clouds. We expect our results to highlight the benefits of applying a hierarchy of models - from global energy balance models down to fine-scale regional large eddy-models - in the assessment of planetary climate and to indicate to which extent our ability to evaluate the existence of waterbelt climate regimes close to the OHZ is currently limited.