A reduced atmosphere for early Mars?

One-dimensional, radiative-convective climate calculations indicate that the old model of a warm, dense, CO2 atmosphere on early Mars is no longer viable. The magnitude of the greenhouse effect in a CO2/H2O atmosphere is limited by condensation of CO2 clouds; this phenomenon is not important for Mars today, but has a pronounced cooling effect at the low solar luminosities thought to apply during early solar system history. The failure of this model indicates one of four things: (1) the new climate calculations are incorrect; (2) current solar evolution models are incorrect; (3) the idea that early Mars was warm and wet is incorrect; or (4) the atmosphere of early Mars contained other greenhouse gases (or particles) in addition to CO2 and H2O. Of these explanations, the most plausible is number (4) and the rest of the investigation is a further explanation of number (4).