Atmospheric levels of molecular hydrogen (H₂) can impact climate and air quality. Constraining past changes to atmospheric H₂ is useful for understanding how H₂ cycles through the Earth system and predicting the impacts of increasing anthropogenic emissions under the hydrogen economy. Here, we use the aging air found in the polar snowpack to reconstruct H₂ levels over the past 100 years. We find that H₂ levels increased by 30 % over Greenland and 60 % over Antarctica during the 20th century.