Marine N2O Emissions From Nitrification and Denitrification Constrained by Modern Observations and Projected in Multimillennial Global Warming Simulations

Nitrous oxide (N₂O) is a potent greenhouse gas (GHG) and ozone destructing agent; yet global estimates of N₂O emissions are uncertain. Marine N₂O stems from nitrification and denitrification processes which depend on organic matter cycling and dissolved oxygen (O₂). We introduce N₂O as an obligate intermediate product of denitrification and as an O₂-dependent by-product from nitrification in the Bern3D ocean model. A large model ensemble is used to probabilistically constrain modern and to project marine N₂O production for a low (Representative Concentration Pathway (RCP)2.6) and high GHG (RCP8.5) scenario extended to A.D. 10,000. Water column N₂O and surface ocean partial pressure N₂O data serve as constraints in this Bayesian framework. The constrained median for modern N₂O production is 4.5 (±1σ range: 3.0 to 6.1) Tg N yr⁻¹, where 4.5% stems from denitrification. Modeled denitrification is 65.1 (40.9 to 91.6) Tg N yr⁻¹, well within current estimates. For high GHG forcing, N2O production decreases by 7.7% over this century due to decreasing organic matter export and remineralization. Thereafter, production increases slowly by 21% due to widespread deoxygenation and high remineralization. Deoxygenation peaks in two millennia, and the global O₂ inventory is reduced by a factor of 2 compared to today. Net denitrification is responsible for 7.8% of the long-term increase in N2O production. On millennial timescales, marine N₂O emissions constitute a small, positive feedback to climate change. Our simulations reveal tight coupling between the marine carbon cycle, O₂, N₂O, and climate.