Global net climate effects of anthropogenic reactive nitrogen

Anthropogenic activities have substantially enhanced the loading of reactive nitrogen (Nr) in the Earth system since pre-industrial times, contributing to widespread eutrophication and air pollution. Increased Nr can also influence global climate through a variety of effects on atmospheric and land processes but the cumulative net climate effect is yet to be unravelled. Here, we show that anthropogenic Nr causes a net negative direct radiative forcing of −0.34 ± [−0.20, −0.50] W m⁻² in the year 2019 relative to the year 1850. This net cooling effect is the result of increased aerosol loading, reduced methane lifetime and increased terrestrial carbon sequestration associated with increases in anthropogenic Nr, which are not offset by the warming effects of enhanced atmospheric nitrous oxide and ozone. Future predictions using representative scenarios show that this cooling effect may be weakened primarily as a result of reduced aerosol loading and increased lifetime of methane, whereas N₂O-induced warming will probably continue to increase. Our results indicate that future studies of the climate impact of anthropogenic Nr should incorporate the effects of enhanced nitrogen deposition and the associated changes in ecosystem structure and function, as well as their interaction with climate mitigation efforts.