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Tang, G., Zaehle, S., Nicholls, Z. 
ORCID: https://orcid.org/0000-0002-4767-2723, Norton, A., Ziehn, T., & Meinshausen, M.
(2026).
Understanding the Drivers of Carbon–Nitrogen Cycle Variability in CMIP6 ESMs With MAGICC CNit v2.0: Model and Calibration Updates.
Journal of Advances in Modeling Earth Systems 18 (1) e2025MS005270. 10.1029/2025MS005270.
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J Adv Model Earth Syst - 2026 - Tang - Understanding the Drivers of Carbon Nitrogen Cycle Variability in CMIP6 ESMs With.pdf - Published Version Available under License Creative Commons Attribution. Download (4MB) | Preview |
Abstract
Carbon–nitrogen coupling is a critical constraint for improving carbon cycle and climate simulations in Earth system models (ESMs), yet large uncertainties hinder inter-model comparisons. Here, we present CNit v2.0, an updated representation of the carbon–nitrogen cycle in MAGICC—a widely used reduced-complexity model (RCM). CNit v2.0 is calibrated to emulate carbon–nitrogen cycle dynamics in various ESMs across historical, idealized (1pctCO2, 1pctCO2-bgc), and multiple Shared Socioeconomic Pathway (SSP) experiments, demonstrating strong emulation performance. The global annual-mean emulation from historical to SSP5-8.5 (1850–2100) reveals increasing nitrogen limitation on net primary production (NPP), with a multi-model mean inhibition of 10.2 ± 5.6% by 2100 due to nitrogen deficits limiting plant uptake. The stronger CO2 fertilization effect in carbon-only (C-only) ESMs exceeds the mitigating influence of nitrogen limitation in CN-coupled ESMs, implying a risk of continued NPP overestimation in C-only ESMs—even if a nitrogen cycle is later added—due to insufficient constraints on CO2 sensitivity. The climate response of litter production is sign-changing between C-only (inhibition) and CN-coupled (enhancement) ESMs, suggesting nitrogen effects may be misattributed as climate effects in C-only ESMs. Divergent climate responses and nitrogen effects on litter decomposition—particularly litter respiration and labile soil organic matter decomposition—are the primary drivers of total heterotrophic respiration differences between C-only and CN-coupled ESMs. Alongside NPP, these factors shape distinct carbon cycle dynamics. While nitrogen pools and fluxes generally follow carbon trends, they exhibit greater inter-model spread. In light of the calibration updates, we propose practical strategies to improve carbon cycle calibration in future RCMs.
| Item Type: | Article |
|---|---|
| Research Programs: | Energy, Climate, and Environment (ECE) Energy, Climate, and Environment (ECE) > Integrated Assessment and Climate Change (IACC) |
| Depositing User: | Luke Kirwan |
| Date Deposited: | 28 Jan 2026 08:48 |
| Last Modified: | 28 Jan 2026 08:48 |
| URI: | https://pure.iiasa.ac.at/21264 |
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