Three-dimensional climate model investigations of cirrus cloud warming on early Mars

The conclusive evidence for flowing liquid water on Mars in its early history requires a much warmer climate than today, at least intermittently. Clouds and aerosols are potentially promising contributors to climate warming because they have a relatively strong radiative forcing per unit mass compared to greenhouse gases, but constraining their effects requires careful analysis. Several previous studies in 1D and 3D have looked at the effects of one particular category of aerosol: high-altitude water ice clouds. While most previous work found limited to moderate warming effects, one recent study (Kite et al. 2021, PNAS) has reported strong cirrus cloud warming (up to ~265 K annual mean temperature) in a GCM simulation of a 0.6 bar CO2 atmosphere under 80% solar flux, in a specific climate scenario where obliquity is low and most water is trapped as ice in a south polar cap. In this presentation, we report results from independent 3D climate models in which we attempt to reproduce the results of this study. Some general remarks on how to assess the climate impact of clouds and aerosols on early Mars are also given.