The open uncertainty window in ocean biogeochemical responses to fossil carbon emissions

Abstract. Automated parameter optimisation can effectively reduce model-data mismatch and provide information about ocean biogeochemistry parameter sensitivities, but to what extent it reduces uncertainty in responses to climate change is not well understood. To explore the robustness of simulated ocean biogeochemical responses to a “business-as-usual” RCP 8.5-equivalent fossil CO₂ emission scenario until year 2300, six biogeochemical parameters are optimised against contemporary oxygen, phosphate, and nitrate fields in three different physical configurations of the UVic ESCM. The phytoplankton maximum growth rate parameter value and the nitrogen-to-oxygen molar ratio of organic matter remineralisation were found to be robust across the calibration ensemble, with the former varying less than 1.6 % across the calibrated ensemble and the latter varying less than 4 %. While each calibrated model performs nearly equivalently in terms of optimal total misfit (a result of the model calibration objective), significant differences in biogeochemical fluxes and rates (e.g., global net primary and particulate organic carbon; POC) remain. Application of CO₂ emissions to the calibrated models to the year 2300 produces model responses that start diverging in global average trends in the early 2000s. Models with more sluggish pre-industrial circulations respond less to climate warming in terms of physical changes, but more in their primary production (as these models have been tuned to have weaker limitation on production). Models that have larger changes in circulation metrics due to anthropogenic CO₂ emissions, in particular the Southern Ocean overturning, demonstrate larger changes in shallow (130 m) and deep (2 km) POC fluxes (due to stronger parametric control on particle flux). Suboxic volume trends are smaller in models with larger changes to the physical circulation metrics owing to compensation between model tuning favouring strong biological oxygen consumption in their vigorous pre-industrial state and these models’ having a greater physical sensitivity to warming. By the year 2300, the resulting model responses vary by a factor 2.8 in net primary production, 5 in shallow POC fluxes, 1.7 in deep POC fluxes, 1.5 in nitrogen fixation and 1.2 in suboxic volume, but the model spread in absolute values relative to the pre-industrial remains largely unchanged, demonstrating the utility of constraining the pre-industrial ocean state in diverse model ensembles for robust projections of ocean change. These results demonstrate the potential to reduce, but not to close the open uncertainty window in ocean biogeochemical responses to CO₂ emissions.