Methane mixing ratios of 100–300 ppm in the Proterozoic atmosphere (0.75–2.3 Ga) would have been sufficient to offset the climatic effects of the faint early sun and maintain the warm climate during those ∼1.5 b.y. The major argument against this type of the atmosphere is the short atmospheric oxidation time of methane after the first oxygenation event ca. 2.3 Ga. Here we argue that the net methane flux from the oxygen-poor Pro tero zo ic ocean could have been 10–20 times higher than the present total biological methane flux. We demonstrate that increased methane production would have been sufficient to maintain methane concentrations at 100–300 ppm, which would keep the surface warm throughout the Proterozoic without invoking high CO2 levels (although the CO2 abundance could have been higher as well). A second oxygenation event at the end of the Proterozoic would have resulted in a decrease of methane flux and could have caused the first Neoproterozoic “snowball” glaciation.

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