Global Net Climate Effects of Anthropogenic Reactive Nitrogen
Anthropogenic activities have significantly increased reactive nitrogen (Nr) in the Earth system since pre-industrial times, contributing to eutrophication, air pollution, and climate change. Nr affects the climate through various pathways, including the emission of nitrous oxide (N2O), a potent greenhouse gas, and the formation of aerosols like ammonium and nitrate, which have cooling effects by scattering solar radiation. This work quantifies the net climate effects of anthropogenic Nr, considering both warming and cooling influences, by integrating terrestrial biogeochemistry and atmospheric chemistry models.
The study finds that in 2019, anthropogenic Nr caused a net negative direct radiative forcing of -0.34 W m-² compared to 1850, indicating a cooling effect. This net cooling is a result of increased aerosol loading, reduced methane (CH4) lifetime, and enhanced terrestrial carbon sequestration, which offset the warming effects of increased atmospheric N2O and ozone. Future scenarios predict that this cooling effect will weaken primarily due to reduced aerosol loading and increased CH4 lifetime, while N2O-induced warming is expected to continue rising under all scenarios.
This work provides a comprehensive assessment of the global climate effects of anthropogenic Nr, highlighting a current net cooling effect due to various atmospheric and land processes. However, this cooling effect is expected to diminish in the future, necessitating stronger reductions in greenhouse gas emissions to meet climate change mitigation goals. The study emphasizes the need for enhanced efforts to reduce both Nr emissions and other greenhouse gases to achieve the environmental targets set by the Paris Agreement.