Global net climate effects of anthropogenic reactive nitrogen

The study addresses how anthropogenic reactive nitrogen (Nr) affects global climate when accounting for all major pathways from pre-industrial (1850) to present (2019). Nr has increased rapidly since the industrial revolution due to agricultural activities, fertilizer application, and fossil fuel combustion. While elevated Nr causes air pollution, eutrophication, and biodiversity loss, it also influences climate via long-lived greenhouse gases (notably N₂O), short-lived aerosols (ammonium and nitrate) that scatter solar radiation, chemical interactions that alter the lifetimes of greenhouse gases (e.g., CH₄ via OH), and fertilization effects that enhance terrestrial carbon sequestration and lower atmospheric CO₂. Previous assessments have not comprehensively integrated these heterogeneous and timescale-dependent effects, often focusing on regional hotspots or present-day impacts without accounting for cumulative historical forcing. This study aims to quantify the net global direct radiative forcing attributable to anthropogenic Nr and to assess how future Nr trajectories alter climate forcing.

Earlier literature estimated a global net radiative forcing of anthropogenic Nr near −0.24 W m⁻² (with a wide uncertainty) based on synthesis of individual pathway sensitivities. Regional assessments (United States, Europe, China) evaluated components of Nr climate effects but were limited by focus on present-day levels, neglect of cumulative effects from long-lived gases since pre-industrial times, and incomplete treatment of spatial heterogeneity and nonlinear interactions among biogeochemical and atmospheric processes. Separate terrestrial biosphere and atmospheric chemistry model studies have also shown limitations when assessing specific Nr components in isolation, underscoring the need for an integrated framework capturing coupled transformations and feedbacks.

The authors developed an integrated modeling framework combining terrestrial biogeochemistry and atmospheric chemistry to estimate direct radiative forcing from anthropogenic Nr since 1850 and under future scenarios. Key components: (1) Anthropogenic emissions were compiled using the CEDS inventory. (2) Terrestrial responses were simulated using the NMIP2 (Global Nitrogen Model Intercomparison Project phase 2) model ensemble to quantify historical effects of anthropogenic Nr (fertilizer, manure, N deposition) on terrestrial carbon sequestration (NBP), soil nitrogen cycling, volatilization, and soil N₂O emissions. (3) Atmospheric composition and radiative effects were simulated with GEOS-Chem coupled with the RRTMG radiative transfer module to compute all-sky top-of-atmosphere direct radiative forcing for present-day (2019) relative to pre-industrial (1850), with and without anthropogenic Nr effects. A box model was used to quantify changes in CH₄ lifetime via OH perturbations driven by NOₓ. (4) Uncertainties were propagated from NMIP2 ensemble spread and atmospheric chemistry sensitivities (including ±30% ranges in OH and O₃), with additional consideration of emission uncertainties (e.g., NH₃). Anthropogenic Nr sources included agricultural practices (fertilizer, manure), N deposition, fossil fuel combustion, and related soil emissions. Indirect factors (irrigation, LUC, elevated CO₂, climate change) that affect the N cycle but cannot be robustly attributed to anthropogenic Nr were not assigned to Nr effects. The authors also attributed forcing to agricultural vs non-agricultural sources by separating soil emissions from fertilizer/manure (agricultural) and fossil fuel plus deposition-driven soil emissions (non-agricultural). Future Nr forcing was evaluated under SSP1-2.6, SSP3-7.0, and SSP5-8.5 scenarios using emissions trajectories and applying estimated historical percentage uncertainties to project error bars.

- Historical net direct radiative forcing from anthropogenic Nr in 2019 relative to 1850: −0.34 W m⁻², with an uncertainty range of [−0.20, −0.50]. This indicates a net cooling effect. - Decomposition of 2019 forcing (Fig. 3): - CO₂ (via enhanced terrestrial carbon sequestration): −0.12 W m⁻² [−0.07, −0.17] - N₂O (greenhouse gas warming): +0.16 W m⁻² [+0.14, +0.17] - CH₄ (via reduced lifetime from NOₓ–OH chemistry): −0.19 W m⁻² [−0.12, −0.29] - Aerosols (ammonium, nitrate, sulfate from Nr influences): −0.24 W m⁻² [−0.18, −0.28] - O₃ (tropospheric): −0.05 W m⁻² [−0.03, −0.07] - Spatial pattern: Aerosol cooling is uneven and concentrated in polluted regions (North America, Western Europe, Eastern and Southern Asia). Tropospheric O₃ burden increased from 280.1 to 325.0 Tg since pre-industrial, partially offsetting cooling from CH₄ lifetime reduction and aerosols. - Terrestrial and emission fluxes (2016–2020): - Anthropogenic Nr increased terrestrial carbon sinks by 0.55 ± 0.38 Pg C yr⁻¹. - Total N₂O emissions (soils + fossil fuels): 12.6 ± 1.5 Tg N yr⁻¹; anthropogenic contribution ~5.5 ± 0.97 Tg N yr⁻¹. - Soil N₂O emissions attributed to manure/fertilizer, deposition, and natural soils: 2.7 ± 0.95, 0.80 ± 0.22, and 2.6 ± 1.6 Tg N yr⁻¹, respectively. - Anthropogenic NOₓ emissions (2016–2020): 46.6 ± 2.7 Tg N yr⁻¹ (majority from fossil fuels). NMIP2 anthropogenic contribution to soil NOₓ: 12.2 ± 2.7 Tg N yr⁻¹. - NH₃ emissions are highly uncertain; a conservative total NH₃ of 50.7 Tg N yr⁻¹ in 2019 (CEDS) was used with NMIP2-derived anthropogenic shares. - Source attribution (2019): Agricultural and non-agricultural sources yield comparable net forcing: −0.19 [−0.03, −0.38] W m⁻² and −0.19 [−0.11, −0.31] W m⁻², respectively. Agricultural net cooling is dominated by direct aerosol effects from NH₃; CO₂ fertilization and N₂O tend to offset each other. - Future scenarios (relative to 2019): - SSP1-2.6: Net warming of +0.06 W m⁻² in the 2050s driven by increased CH₄ lifetime and decreased aerosol cooling due to reduced NOₓ; fertilizer/manure use remains relatively stable. - SSP3-7.0: Similar global aerosol forcing magnitude to 2019 but regionally variable; stronger N₂O warming (+0.06 W m⁻²) partly compensated by enhanced aerosol cooling (−0.03 W m⁻²) and increased terrestrial carbon sequestration (−0.04 W m⁻²). Potential N saturation could overestimate C-sequestration cooling by ~0.02 W m⁻². - SSP5-8.5: Anthropogenic Nr levels projected to be generally similar to 2019, implying limited changes in Nr-driven forcing components; broader climate forcing still depends on concurrent GHG trajectories.

The integrated assessment shows that anthropogenic Nr has exerted a net historical cooling primarily through increased aerosol burdens, reduced CH₄ lifetime, and enhanced terrestrial carbon uptake. These cooling influences outweigh the warming from increased N₂O (and to a lesser extent O₃). The analysis reconciles multiple pathways and their interactions, highlighting that the net effect results from competing processes with different lifetimes and spatial patterns. Aerosol effects are geographically concentrated, while chemically mediated changes in CH₄ and O₃ are more widespread. The comparison of agricultural and non-agricultural contributions underscores that both sectors significantly affect climate via distinct mechanisms (NH₃-driven aerosol cooling vs. fossil fuel NOₓ chemistry and deposition effects). Looking forward, reductions in NOₓ and NH₃ emissions consistent with air quality goals will diminish aerosol cooling and can lengthen CH₄ lifetime, thereby weakening or reversing the historical Nr-induced cooling, even as N₂O warming likely grows if agricultural emissions persist. Consequently, nitrogen management policies aimed at environmental protection should be coordinated with aggressive CO₂ and CH₄ mitigation to avoid unintended near-term warming from lost aerosol cooling and CH₄ lifetime changes.

Anthropogenic reactive nitrogen has produced a net direct radiative cooling effect of −0.34 W m⁻² in 2019 relative to 1850, arising from aerosol cooling, reduced methane lifetime, and increased terrestrial carbon sequestration, which overcome warming by N₂O and O₃. However, under plausible future scenarios, this cooling diminishes or can reverse due to cleaner air (reduced aerosols) and longer CH₄ lifetimes, while N₂O warming likely intensifies with continued agricultural emissions. The study provides a comprehensive, process-based synthesis across terrestrial and atmospheric pathways, emphasizing the need to incorporate nitrogen deposition and ecosystem responses in climate impact assessments. Future research should refine aerosol–cloud interactions, better constrain NH₃ and soil NOₓ emissions, quantify N saturation effects on carbon uptake, and improve attribution of indirect anthropogenic influences (e.g., LUC, elevated CO₂, and climate change) on Nr–climate linkages. Coordinated mitigation of CO₂ and CH₄ alongside nitrogen management is essential to achieve both air quality and climate goals.

Key limitations include uncertainties in each pathway (NH₃ emissions, aerosol radiative effects, OH and O₃ responses affecting CH₄ lifetime, and N₂O emission factors), challenges in representing aerosol–cloud interactions, and ambiguity in defining the scope of anthropogenic influences. Indirect anthropogenic factors (e.g., elevated CO₂, land-use change, climate change impacts such as wildfire) affect C, water, and N cycles but were not attributed to Nr-driven forcing due to attribution challenges. NMIP2 ensemble spread and conservative assumptions (e.g., NH₃ totals from CEDS, bounding N saturation) were used to bound uncertainties, yet residual structural and scenario uncertainties remain.