Methane and climate during the Precambrian era

The Sun was substantially less bright in the distant past, yet Earth's surface temperature remained above freezing. Higher concentrations of the greenhouse gases CO₂ and CH₄ were likely responsible for keeping the early climate warm. CH₄ concentrations of 1000 ppm or higher are predicted for the Late Archean/Paleoproterozoic atmosphere prior to the rise of O₂. Photolysis of this CH₄ may have created an optically thin organic haze during much of this time. The rise of O₂ at 2.3 Ga eliminated most of the methane and probably triggered the Paleoproterozoic glaciations. CH₄ concentrations could have remained elevated throughout much of the Proterozoic, however, as a consequence of low concentrations of dissolved O₂ and sulfate in the deep oceans and a corresponding increase in organic matter recycling by fermentation and methanogenesis.