Global net cooling effects of anthropogenic reactive nitrogen: the unneglectable roles of short-lived nitrogen components

Anthropogenic activities have substantially enhanced the loadings 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 not only as a result of the increased terrestrial carbon sequestration, but also led by short-lived Nr components and the associated atmospheric chemical reactions, including increased aerosol loading and reduced methane lifetime induced by nitrogen oxide (NO_x). Such cooling effect is not offset by the warming effects of enhanced atmospheric nitrous oxide (N₂O) and ozone (O₃). However, despite the significant climate impacts of the short-lived nitrogen components, in particular, NO_x, the associated soil biogeochemical processes remain poorly constrained, thus leading to varied responses to N fertilizer application as well as the estimates of global soil emissions among different approaches. Our results highlight the urgent necessities to integrate knowledge between atmospheric chemistry and soil biogeochemistry to improve the understanding of the Nr climatic effects.