Coupled Carbon-Nitrogen Cycle in MAGICC v1.0.0: Model Description and Calibration

Abstract. The integration of a nitrogen cycle represents a recent advancement in Earth System Models (ESMs). However, diverse formulations and representations introduce uncertainties and inconsistencies in nitrogen effects on the carbon cycle, leaving the global carbon-nitrogen coupling effect unclear. In this study, we present the newly developed carbon-nitrogen coupling in MAGICC, a reduced complexity model (RCM). We have calibrated this coupled carbon-nitrogen cycle to two land surface models (CABLE and OCN) and (the land component of) a set of CMIP6 ESMs. The new carbon-nitrogen coupled model is able to capture the dynamics of the more complex models' carbon-nitrogen cycle at the global-mean, annual scale. The emulation results suggest a consistent nitrogen limitation on net primary production (NPP) in CMIP6 ESMs, persisting throughout the simulations (i.e. over the period 1850–2100) in most models. The emulation may provide a way to disentangle diverse nitrogen effects on carbon pool turnovers in CMIP6 ESMs, with our results suggesting that nitrogen deficiency generally inhibits litter production and decomposition while enhancing soil respiration (from a multi-model mean perspective). However, this disentanglement is limited due to a lack of simulations from CMIP6 ESMs which would allow us to cleanly separate the nitrogen and carbon responses. The results imply a potential reduction in land carbon sequestration in the future due to nitrogen deficiency. Future studies will use the newly developed model to further investigate the carbon-nitrogen coupling effect and its associated uncertainty.