| Summary: | The ocean is the only cumulative sink of atmospheric CO2. It has absorbed approximately 40% of the CO2 from fossil fuel burning and cement production, lowering atmospheric CO2 and limiting climate change. Here we will examine the regional and global mechanisms controlling the evolution of ocean uptake of this additional carbon from human activities (anthropogenic carbon, Cant) using ocean models and observations. Cant is rapidly injected into the deep ocean, sequestering it from the atmosphere for centuries. It is currently uncertain whether any of this sequestered Cant was absorbed from the atmosphere in the subpolar North Atlantic. Here we present evidence that the upper limb of the ocean’s overturning circulation supplies the subpolar North Atlantic with capacity to absorb Cant from the atmosphere. Using a coupled ocean model, we find that surface freshening of the subpolar North Atlantic reduces the volume available for Cant storage. We also investigate whether global ocean Cant uptake is reduced due to changing ocean circulation, this time across multiple emission scenarios, including scenarios with aggressive emission mitigation. Though it is clear that emission mitigation will reduce the magnitude of the ocean carbon sink, the mechanisms governing the decline in uptake have not been studied in detail. We find that the ocean sink becomes less efficient due to kinematic effects wherein Cant escapes from the surface ocean as atmospheric CO2 plateaus and then declines. In emission scenarios ranging from high to low emissions, projected changes in global Cant uptake due to ocean circulation are small. This is in contrast with the subpolar North Atlantic, where future circulation change plays a important role in the declining Cant uptake.
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