The low O2 content of the Archean atmosphere implies that methane should have been present at levels ~102 to 103 parts per million volume (ppmv) (compared with 1.7 ppmv today) given a plausible biogenic source. CH4 is favored as the greenhouse gas that countered the lower luminosity of the early Sun. But abundant CH4 implies that hydrogen escapes to space (↑space) orders of magnitude faster than today. Such reductant loss oxidizes the Earth. Photosynthesis splits water into O2 and H, and methanogenesis transfers the H into CH4. Hydrogen escape after CH4 photolysis, therefore, causes a net gain of oxygen [CO2 + 2H2O --> CH4 + 2O2 --> CO2 + O2 + 4H(↑space)]. Expected irreversible oxidation (~1012 to 1013 moles oxygen per year) may help explain how Earth's surface environment became irreversibly oxidized.