Projections of oceanic N₂O emissions in the 21st century using the IPSL Earth system model

Abstract. The ocean is a substantial source of nitrous oxide (N₂O) to the atmosphere, but little is known about how this flux might change in the future. Here, we investigate the potential evolution of marine N₂O emissions in the 21st century in response to anthropogenic climate change using the global ocean biogeochemical model NEMO-PISCES. Assuming nitrification as the dominant N₂O formation pathway, we implemented two different parameterizations of N₂O production which differ primarily under low-oxygen (O₂) conditions. When forced with output from a climate model simulation run under the business-as-usual high-CO₂ concentration scenario (RCP8.5), our simulations suggest a decrease of 4 to 12 % in N₂O emissions from 2005 to 2100, i.e., a reduction from 4.03/3.71 to 3.54/3.56 TgN yr⁻¹ depending on the parameterization. The emissions decrease strongly in the western basins of the Pacific and Atlantic oceans, while they tend to increase above the oxygen minimum zones (OMZs), i.e., in the eastern tropical Pacific and in the northern Indian Ocean. The reduction in N₂O emissions is caused on the one hand by weakened nitrification as a consequence of reduced primary and export production, and on the other hand by stronger vertical stratification, which reduces the transport of N₂O from the ocean interior to the ocean surface. The higher emissions over the OMZ are linked to an expansion of these zones under global warming, which leads to increased N₂O production, associated primarily with denitrification. While there are many uncertainties in the relative contribution and changes in the N₂O production pathways, the increasing storage seems unequivocal and determines largely the decrease in N₂O emissions in the future. From the perspective of a global climate system, the averaged feedback strength associated with the projected decrease in oceanic N₂O emissions amounts to around −0.009 W m⁻² K⁻¹, which is comparable to the potential increase from terrestrial N₂O sources. However, the assessment for a potential balance between the terrestrial and marine feedbacks calls for an improved representation of N₂O production terms in fully coupled next-generation Earth system models.