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 CO 2 and CH 4 were likely responsible for keeping the early climate warm. CH 4 concentrations of 1000 ppm or higher are predicted for the Late Archean/Paleoproterozoic atmosphere prior to the rise of O 2. Photolysis of this CH 4 may have created an optically thin organic haze during much of this time. The rise of O 2 at 2.3 Ga eliminated most of the methane and probably triggered the Paleoproterozoic glaciations. CH 4 concentrations could have remained elevated throughout much of the Proterozoic, however, as a consequence of low concentrations of dissolved O 2 and sulfate in the deep oceans and a corresponding increase in organic matter recycling by fermentation and methanogenesis.