Marine carbon sink dominated by biological pump after temperature overshoot

In the event of insufficient mitigation efforts, net-negative CO2 emissions may be required to return climate warming to acceptable limits as defined by the Paris Agreement. The ocean acts as an important carbon sink under increasing atmospheric CO2 levels when the physico-chemical uptake of carbon dominates. However, the processes that govern the marine carbon sink under net-negative CO2 emission regimes are unclear. Here we assessed changes in marine CO2 uptake and storage mechanisms under a range of idealized temperature-overshoot scenarios using an Earth system model of intermediate complexity over centennial timescales. We show that while the fate of CO2 from physico-chemical uptake is very sensitive to future atmospheric boundary conditions and CO2 is partly lost from the ocean at times of net-negative CO2 emissions, storage associated with the biological carbon pump continues to increase and may even dominate marine excess CO2 storage on multi-centennial timescales. Our findings imply that excess carbon that is attributable to the biological carbon pump needs to be considered carefully when quantifying and projecting changes in the marine carbon sink.