Divergent sensitivities of apparent oxygen utilization to ventilation changes in the deep ocean across Earth system models

Abstract. The biological carbon pump (BCP), involving photosynthesis at the surface and remineralisation at depth, maintains a significant vertical gradient in dissolved inorganic carbon (DIC), thereby promoting the ocean's ability to absorb atmospheric CO2. Remineralised DIC is a good indicator of the strength of the BCP. It can be estimated from apparent oxygen utilisation (AOU), which measures the deficit of oxygen relative to saturation. AOU is projected to increase under climate change due to changes in remineralisation rates and ventilation. However, the amplitude of the change remains uncertain. Here, we identify linear relationships between trends in AOU and ideal-age in the deep ocean, based on simulations of the contemporary (1982–2013) and future (2015–2099) periods from five Earth system models (ESMs). Our analysis underscores the substantial role of ventilation slowdown in increasing remineralised DIC. Furthermore, the study highlights considerable inter-model variability in their sensitivity of AOU to age changes, with this sensitivity remaining relatively stable over time. With more observational data, refined estimates of age changes from ocean tracers and a larger model ensemble, constraining this variability will become feasible. These insights emphasise both the challenges and opportunities for constraining future BCP projections arising from uncertainties in ventilation.