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Significance Atmospheric molecular hydrogen (H2 ) levels influence the lifetime of greenhouse gases and the levels of tropospheric ozone. The extent to which human activities have influenced the global H2 budget is not well established due to our limited knowledge of past variations in atmospheric H2 levels. Here, we reconstruct atmospheric H2 over the last 150 y using Antarctic firn air. We find a roughly 70% rise in atmospheric H2 over the twentieth century that can be attributed to human activities. Surprisingly, there is no evidence that anthropogenic emissions of H2 decreased during the late twentieth century due to reduced automotive emissions associated with air pollution controls. It is likely that nonautomotive anthropogenic H2 emissions have been underestimated.

H 2 in Antarctic firn air: Atmospheric reconstructions and implications for anthropogenic emissions

H ₂ in Antarctic firn air: Atmospheric reconstructions and implications for anthropogenic emissions