The ocean's biological and preformed carbon pumps in future steady-state climate scenarios

Abstract. The future of the marine carbon cycle is vitally important for climate and the fertility of the oceans. However, predictions of future biogeochemistry are challenging because a myriad of processes needs parameterization and the future evolution of the physical ocean state is uncertain. Here, we embed a data-constrained model of the carbon cycle in steady circulations that correspond to perpetual 2090s conditions as simulated for the RCP4.5 and RCP8.5 scenarios. Focusing on steady-state changes from preindustrial conditions allows us to capture the response of the system on all timescales, not just on the sub-centennial timescales of typical transient simulations. We find that biological production experiences only modest declines because the reduced nutrient supply by a more sluggish future circulation is counteracted by warming-stimulated growth. Organic-matter export declines by 15–25 % due to reductions in both biological production and export ratios, the latter driven by warming-accelerated shallow respiration and reduced subduction of dissolved organic matter. The future biological pump cycles a 30–70 % larger regenerated inventory accumulated over longer sequestration times, while preformed DIC is shunted away from biological utilization to outgassing. We develop a conceptually new partitioning of preformed DIC to quantify the ocean's preformed carbon pump and its future changes. Near-surface paths of preformed DIC become more important in the future as weakened ventilation isolates the deep ocean. Thus, while regenerated DIC cycling becomes slower in the future, preformed DIC cycling speeds up for inventory changes of similar magnitude.