A Greenland ice core record of H2 reveals enhanced sensitivity to climate

Anthropogenic emissions of hydrogen (H2) are expected to rise with increasing production and use during the green energy transition. Although atmospheric H2 is not radiatively active, it warms the Earth’s climate via chemical effects on methane, ozone, and water vapor1-6. Predicting the atmospheric response to anthropogenic perturbations is challenging due to the limitations of the modern instrumental record. Ice core measurements of H2 can greatly extend the observational record and provide enhanced dynamic range to test theories about the global biogeochemistry and climate sensitivity of H2. Ice core measurements of H2 are challenging because of the uniquely high permeability of H2 in ice. Here we present the first ice core record of atmospheric H2 recovered from a Greenland ice core, spanning the last millennium. The record shows a 60% rise in atmospheric H2 from the preindustrial to the modern, consistent with increasing direct emissions from fossil fuel burning and increased atmospheric concentrations of H2 precursors. The results limit the strength of the previously proposed geologic source of H2. The record also shows a 16% decrease in H2 levels during the Little Ice Age. No more than half of that decrease can be attributed to biomass burning changes. These results demonstrate that conventional estimates likely underestimate the climate sensitivity of H2 biogeochemistry. This sensitivity must be accounted for in estimates of the radiative consequences of anthropogenic emissions in a warming climate.