Maximum Drawdown of Atmospheric CO2 due to Biological Uptake in the Ocean and the Ocean Temperature Effect

During glacials, atmospheric CO₂ is significantly lowered; the decrease is about 1/3 or 90 ppm during the last four glacial cycles. Since the ocean reservoir of carbon, and hence the ocean capacity for storing carbon, is substantially larger than the atmospheric and terrestrial counterparts, it is likely that this lowering was caused by ocean processes, drawing the CO₂ into the deep ocean. The Southern Ocean circulation and biological efficiency are widely accepted as having played an important part in this CO₂ drawdown. However, the relative effects of different processes contributing to this oceanic uptake have not yet been well constrained. In this work, we focus on better constraining two of these processes; 1) the effect of increased efficiency of the biological carbon uptake, and 2) the effect of changes in global mean ocean temperature on the abiotic ocean-atmosphere CO₂ equilibrium. By performing ensemble runs using an Earth System Model of Intermediate Complexity (EMIC) we examine the changes in atmospheric pCO₂ achieved by 100% nutrient utilization efficiency of biology. The simulations display different ocean circulation patterns and hence different global ocean mean temperatures. By restoring the atmospheric pCO₂ to a target value during the spin-up phase, the total carbon content differs between each of the ensemble members. The difference is due to circulation having direct effects on biology, but also on global ocean mean temperature, changing the solubility of CO₂. This study reveals the relative importance of of the processes 1 and 2 (mentioned above) for atmospheric pCO₂ in a changed climate. The results of this study also show that a difference in carbon content after spin-up can have a significant effect on the drawdown potential of a maximised biological efficiency. Thus, the choice of spin-up characteristics in a model study of climate change CO₂ dynamics may significantly affect the outcome of the study.