Constraining Projection-Based Estimates of the Future North Atlantic Carbon Uptake

The North Atlantic is one of the major sinks for anthropogenic carbon in the global ocean. Improved understanding of the underlying mechanisms is vital for constraining future projections, which presently have high uncertainties. To identify some of the causes behind this uncertainty, this study investigates the North Atlantic’s anthropogenically altered carbon uptake and inventory, that is, changes in carbon uptake and inventory due to rising atmospheric CO2and climate change (abbreviated as [Formula: see text]-uptake and [Formula: see text]-inventory). Focus is set on an ensemble of 11 Earth system models and their simulations of a future with high atmospheric CO2. Results show that the model spread in the [Formula: see text]-uptake originates in middle and high latitudes. Here, the annual cycle of oceanic pCO2reveals inherent model mechanisms that are responsible for different model behavior: while it is SST-dominated for models with a low future [Formula: see text]-uptake, it is dominated by deep winter mixing and biological production for models with a high future [Formula: see text]-uptake. Models with a high future [Formula: see text]-uptake show an efficient carbon sequestration and hence store a large fraction of their contemporary North Atlantic [Formula: see text]-inventory below 1000-m depth, while the opposite is true for models with a low future [Formula: see text]-uptake. Constraining the model ensemble with observation-based estimates of carbon sequestration and summer oceanic pCO2anomalies yields later flattening of the [Formula: see text]-uptake than previously estimated. This result highlights the need to depart from the concept of unconstrained model ensembles in order to reduce uncertainties associated with future projections.