The effects of CO2 clouds on the thermal structure of the early Martian atmosphere

Characterizing the influence of clouds in the early Martian atmosphere is critical to understanding the nature of the climate. In this paper, we present simple simulations of massive CO₂ atmospheres with varying CO₂ cloud treatments in order to isolate their effects on the thermal structure of the atmosphere. We use the 3-D NASA Ames Legacy early Mars Global Climate Model with a self-consistent CO₂ cloud scheme to simulate 500 mbar, 1 bar, and 2 bar CO₂ atmospheres with either no CO₂ clouds, radiatively inert CO₂ clouds, or radiatively active CO₂ clouds. We find that clouds affect the atmosphere in two primary ways, consistent with previous work. First, CO₂ cloud condensation releases latent heat and warms temperatures aloft, causing cooling in the lower atmosphere in order to maintain energy balance. Second, CO₂ cloud scattering in the visible and infrared leads to competing cooling and warming effects respectively, though ultimately the infrared scattering leads to lower atmosphere warming in our simulations. Overall, these effects lead to significant changes in the thermal structure of these atmospheres. At ∼30 km in altitude, CO₂ clouds warm the atmosphere by up to 20 K, 30 K, and 45 K in 500 mbar, 1 bar, and 2 bar atmospheres, respectively. CO₂ clouds are ubiquitous in these atmospheres, spanning a large range of latitudes, longitudes, and altitudes. We conclude that CO₂ clouds are important to account for in global climate modeling studies of early Mars. Future work should explore potential interactions between CO₂ clouds, water ice clouds, and the dust cycle as well as the behavior of CO₂ clouds in warmer early Mars scenarios, as they may have the ability to limit greenhouse warming.