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Coupled Fertilizer Nitrogen and Aerosol Radiative Feedbacks in CESM2

Abstract: Synthetic nitrogen fertilization is ubiquitous in modern agriculture, yet global Earth System Models (\(\text{ESMs}\)) traditionally treat agricultural ammonia (\(\text{NH}_3\)) emissions as prescribed, uncoupled offline boundary conditions. This structural separation severs the bidirectional feedbacks connecting soil nitrogen dynamics, atmospheric aerosol formation, and radiative climate forcing. Here, we construct and implement a fully coupled, online land-atmosphere reactive nitrogen module linking the Community Land Model (\(\text{CLM5}\)) and the Community Atmosphere Model (\(\text{CAM6}\)) within \(\text{CESM2}\). Our coupled simulations reveal that fertilizer-induced \(\text{NH}_3\) emissions (\(42.3\text{ Tg N yr}^{-1}\) globally) accelerate atmospheric ammonium nitrate (\(\text{NH}_4\text{NO}_3\)) aerosol formation, generating a global net effective radiative forcing of \(-0.24\text{ W m}^{-2}\) (\(-1.8\text{ W m}^{-2}\) regionally over East Asia and India). This aerosol-mediated solar dimming lowers regional surface air temperatures by \(0.15–0.35^\circ\text{C}\), partially masking regional greenhouse warming while redirecting \(18.4\text{ Tg N yr}^{-1}\) of reactive nitrogen deposition back to natural vegetation. These results establish that agricultural nitrogen management is an active driver of the Earth's radiative energy balance.

"Synthetic fertilizer does not stay in the soil: it evaporates into aerosol mirrors that cool the climate while fertilizing distant ecosystems through atmospheric transport."

1. The Disconnected Nitrogen Cycle in Earth System Models

In the real Earth system, agricultural nitrogen operates in a continuous, closed loop across spheres:

flowchart TD
    Soil["Agricultural Soil (CLM5)<br/>Synthetic N Fertilizer + Manure"] -->|Dynamic Volatilization (pH, Moisture, Temp)| AtmGas["Atmospheric NH3 Gas"]
    AtmGas -->|Thermodynamic Gas-Aerosol Partitioning (MOSAIC/ISORROPIA)| NitrateAerosol["Ammonium Nitrate Aerosols (NH4NO3)"]

    NitrateAerosol -->|Direct Aerosol Scattering| DirectCooling["Direct Radiative Cooling (-0.08 W/m²)"]
    NitrateAerosol -->|Cloud Condensation Nuclei (CCN)| IndirectCooling["Cloud Albedo Indirect Cooling (-0.16 W/m²)"]

    DirectCooling & IndirectCooling --> TempDrop["Regional Surface Temperature Dimming (-0.15 to -0.35°C)"]

    NitrateAerosol -->|Wet & Dry Atmospheric Deposition| NaturalEcosystems["Remote Natural Forests & Grasslands (N Deposition)"]
    NaturalEcosystems -->|Carbon Sequestration (N-Stimulated Growth)| CarbonSink["Enhanced Terrestrial Carbon Sink"]

    style Soil fill:#059669,stroke:#10b981,stroke-width:2px,color:#fff
    style AtmGas fill:#f59e0b,stroke:#d97706,stroke-width:2px,color:#fff
    style NitrateAerosol fill:#0284c7,stroke:#0369a1,stroke-width:2px,color:#fff
    style TempDrop fill:#1e3a8a,stroke:#1e40af,stroke-width:2px,color:#fff
    style CarbonSink fill:#065f46,stroke:#047857,stroke-width:2px,color:#fff

Historically, global models broke this loop: land models calculated soil nitrogen without atmospheric chemistry, while atmospheric models prescribed static, fixed monthly ammonia emission inventories[^1].

2. Developing the Bidirectional Nitrogen Coupler in CESM2

In our research published in Biogeosciences[^2], we built a bidirectional land-atmosphere nitrogen coupling inside CESM2:

  1. Dynamic Soil Volatilization in CLM5: Rather than static emissions, soil \(\text{NH}_3\) volatilization is calculated at every 30-minute model time step as a function of soil ammonium pool size, instantaneous topsoil \(\text{pH}\), soil temperature, and moisture content: $\(E_{\text{NH}_3} = k_{\text{vol}}(T_{\text{soil}}, \theta_{\text{soil}}) \cdot \left[ \text{NH}_4^+ \right]_{\text{soil}} \cdot f(\text{pH})\)$
  2. Thermodynamic Gas-Aerosol Equilibrium in CAM6: Gas-phase \(\text{NH}_3\) is dynamically passed through the CESM coupler into CAM6-chem, where it interacts with sulfate and nitric acid (\(\text{HNO}_3\)) to form fine ammonium nitrate (\(\text{NH}_4\text{NO}_3\)) and ammonium sulfate (\((\text{NH}_4)_2\text{SO}_4\)) aerosols via the Modal Aerosol Module (MAM4)[^3].
  3. Closed Nitrogen Deposition Return: Wet and dry deposition fluxes are continuously fed back to natural ecosystems in CLM5, stimulating vegetation carbon uptake.

3. Radiative Climate Forcing & Temperature Impacts

Global Budget & Radiative Shifts:

Component Uncoupled Baseline Bidirectional Coupled CESM2 Climate Impact
Global Agricultural \(\text{NH}_3\) Flux \(34.2\text{ Tg N yr}^{-1}\) (fixed) \(42.3\text{ Tg N yr}^{-1}\) (dynamic) Captures hot-season spikes
Global \(\text{NH}_4\text{NO}_3\) Aerosol Burden \(0.18\text{ Tg}\) \(0.31\text{ Tg}\) (\(+72\%\)) Substantial aerosol expansion
Direct Aerosol Radiative Forcing \(-0.03\text{ W m}^{-2}\) \(-0.08\text{ W m}^{-2}\) Direct solar reflection
Indirect Cloud Albedo Forcing \(-0.07\text{ W m}^{-2}\) \(-0.16\text{ W m}^{-2}\) Brighter agricultural clouds
Total Effective Radiative Forcing (\(\Delta F_{\text{net}}\)) \(-0.10\text{ W m}^{-2}\) \(-0.24\text{ W m}^{-2}\) Net cooling amplification
Regional Cooling (East Asia & Indo-Gangetic Plain) \(-0.05^\circ\text{C}\) \(-0.15\text{ to }-0.35^\circ\text{C}\) Masks greenhouse warming 
       Regional Radiative Forcing Breakdown (East Asia / Indo-Gangetic Plain):

       Direct Solar Reflection (ΔF_dir):   [████████] -0.58 W/m²
       Cloud Albedo Modification (ΔF_ind): [████████████████] -1.22 W/m²
       ────────────────────────────────────────────────────────────────────────
       Net Regional Radiative Forcing:     -1.80 W/m² (Local Climate Masking)

4. Interactive Simulator: Fertilizer Nitrogen-Climate Coupling

Explore how synthetic fertilizer input levels dynamically propagate into atmospheric aerosol optical depth (\(\text{AOD}\)), surface solar dimming, and net radiative cooling:

🌐 Coupled Land-Atmosphere Nitrogen-Climate Gauge

NH₃ Volatilization
42.3 Tg N/yr
NH₄NO₃ Aerosol Burden
0.31 Tg
Global Radiative Forcing
-0.24 W/m²
Regional Cooling Mask
-0.25 °C
At baseline fertilization, agricultural aerosol formation exerts a noticeable regional cooling masking effect over East Asia.

5. The Policy Dilemma: Air Quality vs. Unmasking Warming

This research reveals a profound climate-air quality policy paradox:

  1. The Air Quality Mandate: Reducing agricultural ammonia and industrial \(\text{NO}_x\) is urgently needed to protect millions of lives from toxic \(\text{PM}_{2.5}\) smog.
  2. The Climate Unmasking Penalty: As countries successfully clean up sulfate and nitrate aerosols, the loss of this \(-0.24\text{ W m}^{-2}\) global cooling mask will accelerate near-term regional surface warming.
  3. Integrated Climate-Chemistry Strategy: To prevent sudden warming spikes as clean air policies take effect, aerosol abatement must be accompanied by aggressive simultaneous reductions in short-lived climate warmers (\(\text{CH}_4\), black carbon, and hydrofluorocarbons).

Data and Code Availability

  • Earth System Model: The bidirectional nitrogen coupler code is implemented in the Community Earth System Model (CESM2.1 / CAM6-chem / CLM5).
  • Model Archives: Simulation output files and namelists are archived on the NCAR Climate Data Gateway.
  • Source Scripts: Analysis and plotting code are hosted on GitHub: kamingfung/CESM2-Coupled-Nitrogen.

References

[^1]: Lamarque, J.-F. et al. The Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP): overview and description of models, simulations and evaluation. Atmos. Chem. Phys. 13, 1793–1832 (2013). https://doi.org/10.5194/acp-13-1793-2013 [^2]: Fung, K. M., Val Martin, M. & Tai, A. P. K. Modeling the interinfluence of fertilizer-induced \(\text{NH}_3\) emission, nitrogen deposition, and aerosol radiative effects using modified CESM2. Biogeosciences 19, 1635–1655 (2022). https://doi.org/10.5194/bg-19-1635-2022 [^3]: Liu, X. et al. Description and evaluation of a new four-mode version of the Modal Aerosol Module (MAM4) in the Community Atmosphere Model (CAM5). Geosci. Model Dev. 9, 505–526 (2016). https://doi.org/10.5194/gmd-9-505-2016