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 2x10(-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.