Strength and reversibility of 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. To determine the amount of negative emissions needed, it is key to identify the reversibility of our carbon sinks, i.e. to establish how much their of their strength is lost during declining emissions. Specifically, the strength of the ocean carbon sink is likely to decline with ongoing rising emissions and subsequent negative emissions might lead to the ocean reverting into a carbon source.

In light of these challenges, we analyze strength and reversibility of the ocean carbon sink with the Norwegian Earth System Model under an idealized scenario, the 'Climate and carbon reversibility experiment' of CDRMIP. Here, a strong atmospheric CO₂ increase of 1% per year is assumed until CO₂-concentrations have quadrupled, followed by a decrease of 1% per year until pre-industrial concentrations are restored. Our model results indicate that the oceanic CO₂-uptake is not able to keep pace with the atmospheric rise and descent, but shows only a slow increase of oceanic CO₂-uptake and a sudden decrease with the onset of negative emissions. However, the seasonal envelope illustrates that this is not true for all months but that the oceanic CO₂-uptake during austral winter months shows both a strong uptake and high reversibility.

A regional analysis of seasonal characteristics shows that a strong and reversible CO₂-uptake throughout the experiment is only maintained by the biological pump in high latitudes during spring and summer (austral and boreal, respectively). For other months and latitudes, the oceanic CO₂-uptake is weak or even turns into outgassing due to ongoing warming and subsequent sluggish cooling of sea surface temperature. In our model simulation, the inertia of sea surface temperature is the main cause for the irreversibility of the oceanic CO₂-uptake. This result, however, is highly dependent on the amount of CO₂ taken up during rising emissions. In a corresponding simulation without warming, our model’s ocean takes up more CO₂ during rising emissions, leading to dissolved inorganic carbon being the main cause for the irreversability of the oceanic CO₂ 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 be observed early on and help to verify strength and reversibility of the ocean carbon sink.