The thawing of organic carbon stored in arctic permafrost soils, and its oxidation to carbon dioxide (a greenhouse gas), is predicted to be a major, positive feedback on global warming. However, current estimates of the magnitude of this feedback do not include the oxidation of permafrost soil organic carbon flushed to sunlit lakes and rivers. We showed that ancient dissolved organic carbon (> 4,000 years old) draining permafrost soils is readily oxidized to carbon dioxide by sunlight. Our discovery dispels an assumption that ancient carbon released from permafrost soils is unreactive to sunlight, and was made possible by an advance in technology that measures the CO2 produced at specific wavelengths of sunlight rather than the traditional coarse assessment across all wavelengths. Our novel approach also revealed that iron catalyzes the sunlight oxidation of modern and ancient lignin and tannin from permafrost soils to CO2. As a consequence, current estimates of additional global warming from the permafrost carbon feedback may be too low by ~ 14%.