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

[摘要] The ocean is a substantial source of nitrous oxide (N₂O) to theatmosphere, but little is known about how this flux might change in the future.Here, we investigate the potential evolution of marine N₂O emissions inthe 21st century in response to anthropogenic climate change using theglobal ocean biogeochemical model NEMO-PISCES. Assuming nitrification as thedominant N₂O formation pathway, we implemented two differentparameterizations of N₂O production which differ primarily under low-oxygen (O₂) conditions. When forced with output from a climate modelsimulation run under the business-as-usual high-CO₂ concentrationscenario (RCP8.5), our simulations suggest a decrease of 4 to 12 % inN₂O emissions from 2005 to 2100, i.e., a reduction from 4.03/3.71 to3.54/3.56 TgN yr⁻¹ depending on the parameterization. The emissionsdecrease strongly in the western basins of the Pacific and Atlantic oceans,while they tend to increase above the oxygen minimum zones (OMZs), i.e., inthe eastern tropical Pacific and in the northern Indian Ocean. The reductionin N₂O emissions is caused on the one hand by weakened nitrification asa consequence of reduced primary and export production, and on the otherhand by stronger vertical stratification, which reduces the transport ofN₂O from the ocean interior to the ocean surface. The higher emissionsover the OMZ are linked to an expansion of these zones under global warming,which leads to increased N₂O production, associated primarily withdenitrification. While there are many uncertainties in the relativecontribution and changes in the N₂O production pathways, the increasingstorage seems unequivocal and determines largely the decrease in N₂Oemissions in the future. From the perspective of a global climate system,the averaged feedback strength associated with the projected decrease inoceanic N₂O emissions amounts to around −0.009 W m⁻² K⁻¹,which is comparable to the potential increase from terrestrial N₂Osources. However, the assessment for a potential balance between the terrestrialand marine feedbacks calls for an improved representation of N₂Oproduction terms in fully coupled next-generation Earth system models.

[学科分类] 地球化学与岩石