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Abstract Ice core measurements of the concentration and stable isotopic composition of atmospheric nitrous oxide (N₂O) 74,000–59,000 years ago constrain marine and terrestrial emissions. The data include two major Dansgaard‐Oeschger (D‐O) events and the N₂O decrease during global cooling at the Marine Isotope Stage (MIS) 5a‐4 transition. The N₂O increase associated with D‐O 19 (~73–71.5 ka) was driven by equal contributions from marine and terrestrial emissions. The N₂O decrease during the transition into MIS 4 (~71.5–67.5 ka) was caused by gradual reductions of similar magnitude in both marine and terrestrial sources. A 50 ppb increase in N₂O concentration at the end of MIS 4 was caused by gradual increases in marine and terrestrial emissions between ~64 and 61 ka, followed by an abrupt increase in marine emissions at the onset of D‐O 16/17 (59.5 ka). This suggests that the importance of marine versus terrestrial emissions in controlling millennial‐scale N₂O fluctuations varied in time. 

Abstract Molecular hydrogen (H₂) is an abundant and reactive constituent of Earth's atmosphere, with links to climate and air quality. Anthropogenic emissions of H₂are expected to rise as the use of H₂as an energy source increases. Documenting past variations in atmospheric H₂will help to validate current understanding of the global H₂budget. The modern instrumental record begins in the 1980s; there is little information about atmospheric H₂prior to that time. Here, we use firn air measurements from a 2001 South Pole campaign to reconstruct atmospheric H₂levels over the 20th century. Inversion of the measurements indicates that H₂over South Pole has increased from 350–540 ppb from 1910–2000. A biogeochemical box model indicates that the atmospheric burden of H₂increased by 37% over that time. The rise in H₂is consistent with increasing H₂emissions from fossil fuel combustion and increasing atmospheric production from methane oxidation.