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

Significance Atmospheric molecular hydrogen (H 2 ) levels influence the lifetime of greenhouse gases and the levels of tropospheric ozone. The extent to which human activities have influenced the global H 2 budget is not well established due to our limited knowledge of past variations in atmospheric H 2 levels. Here, we reconstruct atmospheric H 2 over the last 150 y using Antarctic firn air. We find a roughly 70% rise in atmospheric H 2 over the twentieth century that can be attributed to human activities. Surprisingly, there is no evidence that anthropogenic emissions of H 2 decreased during the late twentieth century due to reduced automotive emissions associated with air pollution controls. It is likely that nonautomotive anthropogenic H 2 emissions have been underestimated.