Effects of high CO₂ levels on surface temperature and atmospheric oxidation state of the early Earth

One-dimensional radiative-convective and photochemical models are used to examine the effects of enhanced CO₂ concentrations on the surface temperature of the early Earth and the composition of the prebiotic atmosphere. Carbon dioxide concentrations of the order of 100-1000 times the present level are required to compensate for an expected solar luminosity decrease of 25-30%, if CO₂ and H₂O were the only greenhouse gases present. The primitive stratosphere was cold and dry, with a maximum H₂O volume mixing ratio of 10^-6. The atmospheric oxidation state was controlled by the balance between volcanic emission of reduced gases, photo-stimulated oxidation of dissolved Fe⁺² in the oceans, escape of hydrogen to space, and rainout of H₂O₂ and H₂CO. At high CO₂ levels, production of hydrogen owing to rainout of H₂O₂ would have kept the H₂ mixing ratio above 2×10^-4 and the ground-level O₂ mixing ratio below 10^-11, even if no other sources of hydrogen were present. Increased solar UV fluxes could have led to small changes in the ground-level mixing ratios of both O₂ and H₂.