Modeling the Archean Atmosphere and Climate

The Archean atmosphere was likely a weakly reduced mixture composed predominantly of N₂ and CO₂, with smaller concentrations of H₂, CO, and CH₄. Both CO₂ and N₂ may have been present in abundances exceeding today's values, by a factor of 2 or more for N₂ and by factors of 100 or more for CO₂. Published upper limits on CO₂ from paleosols and banded iron formations are probably invalid; hence, CO₂ could have been the dominant greenhouse gas that compensated for the fainter young Sun. The Archean greenhouse effect was likely supplemented by CH₄, which could have risen to levels of 1000ppmv or more once methanogens had evolved. Warming by CH₄ was limited to approximately 10-12°, however, by formation of organic haze. The key to analyzing Archean atmospheric composition is to understand the hydrogen budget of the atmosphere in which outgassing of H₂ and other reduced gases from volcanoes was balanced by loss of hydrogen to space and burial of organic carbon in sediments. The mixing ratio of O₂ in such a weakly reduced atmosphere would have been extremely low, roughly 10^-13 at the surface, increasing to ~10^-3 in the upper stratosphere. A rise in O₂ just after the end of the Archean may have eliminated the methane greenhouse and triggered the Paleoproterozoic glaciations.