Human activities have dramatically increased reactive nitrogen (Nr) in the Earth system since the pre-industrial era, primarily due to agricultural practices, fossil fuel combustion, and fertilizer application. This rise in Nr has led to widespread environmental problems like eutrophication and air pollution. However, Nr also plays a crucial role in global climate regulation. While long-lived greenhouse gases like nitrous oxide (N₂O) contribute to warming, short-lived gases like ammonia (NH₃) and nitrate (NO₃⁻) aerosols can have a cooling effect by altering solar radiation. Furthermore, Nr influences atmospheric chemical reactions, affecting the lifespan of other gases such as methane (CH₄) and CO₂. Nr deposition can also enhance terrestrial carbon sequestration, further impacting the climate. The net effect of Nr on global climate remains uncertain due to the complex interactions and varying timescales of these processes. Previous studies, often focusing on specific regions or individual Nr components, have lacked a comprehensive global perspective and the cumulative effects over a long timescale. This study aims to address this gap by providing a comprehensive assessment of the global net climate effect of anthropogenic Nr, incorporating both atmospheric chemistry and terrestrial biogeochemical processes, and projecting future impacts under different scenarios.

Existing literature highlights the detrimental environmental effects of increased reactive nitrogen, including air pollution, eutrophication, and biodiversity loss. While several studies have examined the regional climate impacts of specific Nr components (e.g., in the US, Europe, and China), these assessments were limited to present-day levels, neglecting the cumulative effects of long-lived greenhouse gases since the pre-industrial era and the spatial heterogeneity of impacts. These studies also faced challenges in extrapolating regional findings to the global scale due to significant uncertainties associated with individual processes. A previous study estimated a global net radiative forcing of -0.24 W m⁻², but this lacked a comprehensive modeling framework incorporating both atmospheric and terrestrial processes.

This study utilized a comprehensive model framework integrating terrestrial biogeochemistry and atmospheric chemistry modeling to quantify the global direct radiative forcing of anthropogenic Nr. Anthropogenic Nr emissions were assessed using the conventional emissions data system (CEDS). Terrestrial biosphere model outputs from the global nitrogen (N) model inter-comparison project phase 2 (NMIP2) were used to quantify the historical impact of Nr on terrestrial carbon sequestration, soil N, volatilization, and soil N₂O emissions. Two models were employed: a global greenhouse gas model and a global chemical transport model (GEOS-Chem) with a radiative transfer module (RRTMG) to estimate the global net radiative forcing associated with different emission sources. The net direct radiative forcing was calculated as the difference between present-day (2019) and pre-industrial (1850) conditions. Uncertainties were estimated based on the standard deviation across NMIP2 ensemble members and uncertainties in atmospheric chemistry. The study also separated agricultural and non-agricultural sources of Nr to better understand their individual contributions to the net climate effect. Future scenarios of anthropogenic Nr inputs were explored using shared socioeconomic pathways (SSPs) to project the potential changes in radiative forcing under different socioeconomic development trajectories.

The study found that anthropogenic Nr caused a net negative direct radiative forcing of -0.34 ± [-0.20, -0.50] W m⁻² in 2019 relative to 1850. This net cooling effect resulted from increased aerosol loading (-0.24 W m⁻²), reduced methane lifetime (-0.19 W m⁻²), and increased terrestrial carbon sequestration (-0.12 W m⁻²), which more than offset the warming effect of increased N₂O (+0.16 W m⁻²) and tropospheric O₃ (+0.05 W m⁻²). The analysis of agricultural versus non-agricultural sources showed comparable net cooling effects (-0.19 W m⁻² for each). Future scenarios (SSP1-2.6, SSP3-7.0, and SSP5-8.5) suggested a potential weakening of the net cooling effect primarily due to reduced aerosol loading and increased methane lifetime, whereas N₂O-induced warming was projected to increase. The magnitude of the radiative forcing estimates is associated with uncertainties in individual components and the inherent ambiguity in defining anthropogenic impacts. While indirect effects of human activities (e.g., elevated CO2, land-use change) on the N cycle could potentially amplify the overall climate effect, the study's direct assessment of anthropogenic Nr suggests these effects are less significant than the direct Nr inputs.

The findings demonstrate that anthropogenic reactive nitrogen has a net cooling effect on the global climate, primarily due to the combined effects of aerosol loading, reduced methane lifetime, and enhanced carbon sequestration. This result challenges the conventional understanding that nitrogen emissions primarily contribute to warming. The study highlights the importance of considering the complex interactions between atmospheric chemistry and terrestrial biogeochemical cycles in assessing the overall climate impact of Nr. The separation of agricultural and non-agricultural sources reveals comparable contributions to net cooling, suggesting that mitigation strategies need to address both sectors. The projections for future scenarios indicate that the net cooling effect could weaken, emphasizing the need for strategies to reduce greenhouse gas emissions while also managing nitrogen inputs.

This study provides a comprehensive assessment of the global net climate effects of anthropogenic reactive nitrogen, demonstrating a net cooling effect largely driven by aerosol loading, reduced methane lifetime, and enhanced carbon sequestration. Future scenarios indicate a potential weakening of this cooling effect, highlighting the need for integrated strategies to mitigate both nitrogen and greenhouse gas emissions. Further research should focus on refining the quantification of indirect effects of human activities on the nitrogen cycle and improving the representation of ecosystem responses to nitrogen deposition in climate models.

The study's conclusions rely on model simulations, and uncertainties exist in various parameters and processes, especially concerning indirect human impacts on the nitrogen cycle. The definition of 'anthropogenic' impact is simplified and may underestimate the full extent of human influence on the nitrogen cycle. The model's representation of complex interactions within the Earth system may not fully capture all relevant processes, leading to potential biases in the estimated radiative forcing.