{"authors":[{"id":"orcid_______::23570bd375f490354db686d8d0684f37","fullName":"Gang Tang","name":"Gang","surname":"Tang","rank":1,"pid":{"id":{"scheme":"orcid","value":"0000-0003-1075-3826"},"provenance":null}},{"id":"orcid_______::40bdebfac76886acc97a58ec39951500","fullName":"Sönke Zaehle","name":"Sönke","surname":"Zaehle","rank":2,"pid":{"id":{"scheme":"orcid","value":"0000-0001-5602-7956"},"provenance":null}},{"id":"orcid_______::07a27a0c5631fc8f12b6c0f9f23ddc08","fullName":"Zebedee R.J. Nicholls","name":"Zebedee R.J.","surname":"Nicholls","rank":3,"pid":{"id":{"scheme":"orcid","value":"0000-0002-4767-2723"},"provenance":null}},{"id":"orcid_______::d7130f1c0bd9135f1e77efab551dd165","fullName":"Alexander Norton","name":"Alexander","surname":"Norton","rank":4,"pid":{"id":{"scheme":"orcid","value":"0000-0001-7708-3914"},"provenance":null}},{"id":"orcid_______::772d6f85d0e714e2d185aeb291f05172","fullName":"Tilo Ziehn","name":"Tilo","surname":"Ziehn","rank":5,"pid":{"id":{"scheme":"orcid","value":"0000-0001-9873-9775"},"provenance":null}},{"id":"orcid_______::8ef2a16c1d7385c0e04614aa2776271d","fullName":"Malte Alexander Meinshausen","name":"Malte Alexander","surname":"Meinshausen","rank":6,"pid":{"id":{"scheme":"orcid","value":"0000-0003-4048-3521"},"provenance":null}}],"openAccessColor":"gold","publiclyFunded":false,"eoscIfGuidelines":null,"type":"publication","language":{"code":"und","label":"Undetermined"},"countries":[{"code":"AT","label":"Austria","provenance":null}],"subjects":[{"subject":{"scheme":"FOS","value":"0401 agriculture, forestry, and fisheries"},"provenance":null},{"subject":{"scheme":"FOS","value":"04 agricultural and veterinary sciences"},"provenance":null}],"mainTitle":"Understanding the Drivers of Carbon-Nitrogen Cycle Variability in CMIP6 ESMs with MAGICC CNit v2.0: Model and Calibration Updates","subTitle":null,"descriptions":["<jats:p>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.</jats:p>"],"publicationDate":"2025-06-16","publisher":"Wiley","embargoEndDate":null,"sources":["Crossref","Journal of Advances in Modeling Earth Systems"],"formats":["application/pdf","text"],"contributors":null,"coverages":null,"bestAccessRight":{"code":"c_abf2","label":"OPEN","scheme":"http://vocabularies.coar-repositories.org/documentation/access_rights/"},"container":{"name":"Journal of Advances in Modeling Earth 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