Emerging Constraints on the H2 Budget From Polar Firn Air Reconstructions

Abstract

Comparison of modeled atmospheric hydrogen (H₂) levels with observations is an important way of testing the scientific understanding of H₂ biogeochemistry. But the instrumental record is short and provides a limited dynamic range with which to test assumptions about H₂ cycling. Here, we compare twentieth century H₂ levels reconstructed from polar firn air to the output from a historical run of the Geophysical Fluid Dynamics Laboratory Atmospheric Model 4.1 (GFDL-AM4.1). Simulated H₂ exhibits reasonable agreement with the reconstruction. The twentieth century increase in H₂ levels is consistent with rising atmospheric methane levels and changing emissions from fossil fuel combustion. However, the model fails to capture the reconstructed reversal in the interpolar H₂ gradient between 1960 and 1990. We invert an 8-box model of the atmosphere to show that the reversal in the interpolar difference requires a large increase (decrease) in the simulated NH (SH) net source of H₂ peaking circa 1980. Revisions to our estimates of historical biomass burning emissions, photochemical production, and/or anthropogenic emissions could help resolve the discrepancy. Our optimization also implies a 15% increase in the strength of the soil sink in the NH mid to high-latitudes from 1970 to 2000, which is qualitatively consistent with expectations based on changing soil moisture and temperature. We also show that smaller adjustments to the H₂ budget would be needed to explain the reversal in the interpolar gradient if the H₂ deposition lifetime were shorter than commonly accepted. A shorter lifetime may be supported by recently identified geological sources of H₂.