CO₂ condensation and the climate of early Mars

A one-dimensional, radiative-convective climate model was used to reexamine the question of whether early Mars could have been kept warm by the greenhouse effect of a dense, CO₂ atmosphere. The new model differs from previous models by considering the influence of CO₂ clouds on the convective lapse rate and on the planetary radiation budget. Condensation of CO₂ decreases the lapse rate and, hence, reduces the magnitude of the greenhouse effect. This phenomenon becomes increasingly important at low solar luminosities and may preclude warm (0°C), globally averaged surface temperatures prior to ∼2 billion years ago unless other greenhouse gases were present in addition to CO₂ and H₂O. Alternative mechanisms for warming early Mars and explaining channel formation are discussed.