The importance of the biological pump for reducing uncertainties in future carbon uptake projections

At present, the largest oceanic CO₂ sinks are situated in the North Atlantic and the Southern Ocean. Knowing how these carbon sinks will be affected by climate change and increasing carbon emissions is vital for humankind, yet there is no scientific consensus.

In order to help to identify the mechanisms responsible for this uncertainty, we analyze the CO₂ uptake of 11 Earth System Models for a high-CO₂ future. As the carbon uptake of North Atlantic and Southern Ocean is enabled by biological drawdown and deep convection / subduction, the focus of our investigation lies on the seasonal cycle of the oceanic pCO₂. The latter is an indicator for both biological and physical processes and has the advantage that it can be validated by observational estimates.

For the North Atlantic, the observed annual cycle of oceanic pCO₂ is biogeochemically dominated (i.e. by dissolved inorganic carbon and alkalinity influenced by deep winter mixing and biological drawdown). Models with a high future CO₂ uptake show the same behavior, while models with a low future CO₂ uptake have a temperature-dominated annual cycle. Imposing these constraints on the model ensemble, we find a stronger future carbon uptake of the North Atlantic than previous estimates.

For the Southern Ocean, the annual cycle of oceanic pCO₂ divides the model-ensemble into two groups featuring either dominance of temperature or biogeochemistry. Observational constraints point towards a biogeochemical dominance but with a much smaller amplitude. None of the considered models perform within observational uncertainty.

We find that there is a direct link between the biogeochemical dominance of the annual cycle of pCO₂ and the strength of the future oceanic CO₂ uptake. Unfortunately, most of the models reproduce the biogeochemical dominance only in either North Atlantic or Southern Ocean. In order to reduce uncertainty, we need to increase our ability to model the strength of the biological pump in both oceans.