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Abstract Molecular hydrogen (H₂) is an abundant and reactive constituent of Earth's atmosphere, with links to climate and air quality. Anthropogenic emissions of H₂are expected to rise as the use of H₂as an energy source increases. Documenting past variations in atmospheric H₂will help to validate current understanding of the global H₂budget. The modern instrumental record begins in the 1980s; there is little information about atmospheric H₂prior to that time. Here, we use firn air measurements from a 2001 South Pole campaign to reconstruct atmospheric H₂levels over the 20th century. Inversion of the measurements indicates that H₂over South Pole has increased from 350–540 ppb from 1910–2000. A biogeochemical box model indicates that the atmospheric burden of H₂increased by 37% over that time. The rise in H₂is consistent with increasing H₂emissions from fossil fuel combustion and increasing atmospheric production from methane oxidation.