Atmospheric nitrogen deposition is an important external nutrient source to the global ocean, with significant impacts on marine ecosystems and biogeochemical cycles. However, its future evolution under changing anthropogenic emissions and the consequences for ocean productivity and climate-relevant gases such as N2O remain unclear. Here, the global atmospheric chemistry model GEOS-Chem is used to evaluate changes in global oceanic nitrogen deposition between 2015 and 2050 under three representative future emission scenarios and to assess their responses to multiple levels of NH3 and NOx emission reductions. Global oceanic nitrogen deposition is projected to change by −24% to +6% between 2015 and 2050, with a substantially increasing share contributed by reduced nitrogen across all scenarios. There is strong chemical compensation between reduced and oxidized nitrogen deposition: reducing emissions of one species (e.g., NH3 or NOₓ) can enhance dry deposition of the other to the oceans, particularly in coastal regions, suggesting that joint control of NH3 and NOₓ is required to reduce total nitrogen deposition effectively. The potential ocean productivity induced by nitrogen deposition may decrease from 290 Tg C in 2015 to 222 Tg C under SSP1-RCP2.6, or increase to 306 Tg C under SSP3-RCP7.0 by 2050, while concurrent changes in oceanic N2O emissions largely offset the associated climatic effects. These results highlight overlooked impacts of future climate mitigation and nitrogen abatement on marine ecosystems via altered atmospheric nitrogen deposition, and call for more holistic assessments of nitrogen impacts across the air, terrestrial, and ocean systems.
Deng et al. (Thu,) studied this question.