On the Potential of the Southern Ocean Biological Pump to Maintain the Ocean Carbon Sink under Negative Emissions

Climate change is progressing fast and net negative emissions will most likely be needed to achieve ambitious climate targets. Simultaneously, the strength of the ocean carbon sink is likely to decline with ongoing climate change. Subsequent negative emissions might lead to the ocean reverting into a carbon source and will impact the amount of negative emissions needed.

We analyze an idealized scenario with a strong atmospheric CO₂ peak and decline with the Norwegian Earth System Model to gain a better understanding of the ocean sink under high emissions and subsequent negative emissions. We find that the strength of the ocean carbon sink declines under high CO₂ emission. When subsequently applying negative emissions, the ocean carbon uptake shows strong seasonal differences. In late boreal summer, the global ocean becomes a carbon source already within a few years, whereas it remains a sink for several decades when regarding the austral summer.

We find that both entrainment of DIC-rich waters and sluggish cooling in high latitudes dominate the global carbon outgassing in July to October. Contrarily, the high latitude biological production allows for a regionally continued carbon uptake. Here, the temporal evolution of the summer oceanic partial pressure of CO₂ mimics the peak and decline behavior of atmospheric CO₂. For March to June, the biological carbon uptake of the northern high latitudes is counteracted by the simultaneous DIC-entrainment of the Southern Ocean leading to a mixed behaviour of the global carbon uptake. In austral summer, however, the Southern Ocean biological production is the dominant driver for maintaining the global ocean carbon sink.

Our study shows that seasonal mechanisms are of high importance when considering the strength of the ocean carbon sink under negative emissions. Regional monthly trajectories visualize different aspects of biological and physical mechanisms, which can potentially be observed early on.