The Influence of Air‐Sea CO2 Disequilibrium on Carbon Sequestration by the Ocean's Biological Pump.

The ocean's biological carbon pump (BCP) affects the Earth's climate by sequestering CO2 away from the atmosphere for decades to millennia. One primary control on the amount of carbon sequestered by the biological pump is air‐sea CO2 disequilibrium, which is controlled by the rate of air‐sea CO2 exchange and the residence time of CO2 in surface waters. Here, we use a data‐assimilated model of the soft tissue BCP to quantify carbon sequestration inventories and time scales from remineralization in the water column to equilibration with the atmosphere. We find that air‐sea CO2 disequilibrium enhances the global biogenic carbon inventory by ∼35% and its sequestration time by ∼70 years compared to identical calculations made assuming instantaneous air‐sea CO2 exchange. Locally, the greatest enhancement occurs in the subpolar Southern Ocean, where air‐sea disequilibrium increases sequestration times by up to 600 years and the biogenic dissolved inorganic carbon inventory by >100% in the upper ocean. Contrastingly, in deep‐water formation regions of the North Atlantic and Antarctic margins, where biological production creates undersaturated surface waters which are subducted before fully equilibrating with the atmosphere, air‐sea CO2 disequilibrium decreases the depth‐integrated sequestration inventory by up to ∼150%. The global enhancement of carbon sequestration by air‐sea disequilibrium is particularly important for carbon respired in deep waters that upwell in the Southern Ocean. These results highlight the importance of accounting for air‐sea CO2 disequilibrium when evaluating carbon sequestration by the biological pump and for assessing the efficacy of ocean‐based CO2 removal methods. Plain Language Summary: In the surface ocean, tiny organisms called phytoplankton convert CO2 to organic matter, a portion of which is transferred to the deep ocean where it then releases CO2. This regenerated CO2 can be stored for hundreds to thousands of years before it is brought back to the surface ocean and reenters the atmosphere. Through this transfer of carbon from the surface to the depth, the "biological carbon pump" helps the ocean take up more CO2 from the atmosphere. In this study, we used computer models to determine how the rate of CO2 exchange between the ocean and the atmosphere impacts the amount of time CO2 can be stored in the ocean. Since the air‐sea exchange of CO2 is slow, the amount of carbon stored by the biological pump is about 35% greater than it would be if the exchange happened instantly. The slow air‐sea exchange rate also increases the length of time this carbon will be stored in the ocean. It is important to take this effect into account when assessing methods for deliberately storing CO2 in the ocean. Key Points: Impact of air‐sea CO2 disequilibrium on biogenic carbon inventories and sequestration times is quantified using an ocean biogeochemical modelAir‐sea CO2 disequilibrium enhances global sequestration time by ∼70 years and biogenic dissolved inorganic carbon inventory by ∼35%Disequilibrium effect is the strongest in the Southern Ocean and North Atlantic and weakest in the Pacific Ocean [ABSTRACT FROM AUTHOR]