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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 CO 2 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 CO 2 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.
Tang et al. (Thu,) studied this question.