Atmospheric H₂ variability over the past 1,100 years

Anthropogenic emissions of hydrogen (H₂) are expected to rise if H₂ energy technology is widely implemented as part of the green energy transition^1,2. Although atmospheric H₂ is not radiatively active, it warms the Earth's climate through chemical effects on methane, ozone and water vapour^1-6. Predicting the atmospheric response to anthropogenic perturbations is challenging, in part because of the limited duration of the modern instrumental record⁷. Ice core measurements of H₂ can extend the observational record, providing information about anthropogenic and natural perturbations and the biogeochemical controls on H₂ levels over long timescales. However, ice core measurements of H₂ are challenging because of the high permeability of H₂ in ice^8,9. Here we present an ice core record of atmospheric H₂ recovered from a Greenland ice core, spanning the past millennium. The record shows a 70-111% (2σ) rise in atmospheric H₂ from the pre-industrial to the modern era, consistent with increasing direct emissions from fossil fuel burning and increased atmospheric concentrations of H₂ precursors. The pre-industrial record also shows a 4-25% (2σ) decrease in H₂ levels during the Little Ice Age (LIA), indicating that H₂ biogeochemistry may be sensitive to climate change. The findings suggest that the sensitivity of H₂ sources and sinks to climate warming should be considered in estimates of the radiative consequences of rising anthropogenic H₂ emissions.