Comment on bg-2022-134

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The ocean slows global warming by currently taking up around one quarter of all human-made CO2 emissions. However, estimates of the ocean anthropogenic carbon uptake vary across various observation-based and model-based approaches. Here, we show that the global ocean anthropogenic carbon sink simulated by Earth System Models can be constrained by two physical parameters, the present-day sea surface salinity in the subtropical-polar frontal zone in the Southern Ocean and the strength of the Atlantic Meridional Overturning Circulation, and one biogeochemical parameter, the Revelle factor of the global surface ocean. By exploiting this three-dimensional emergent constraint with observations, we provide a new model- and observation-based estimate of the past, present and future global ocean anthropogenic carbon sink and show that the ocean carbon sink is 9–11 % larger than previously estimated. Furthermore, the constraint reduces uncertainties of the past and present global ocean anthropogenic carbon sink by 42–59 % and the future sink by 32–62 % depending on the scenario, allowing for a better understanding of the global carbon cycle and better targeted climate and ocean policies. The here identified key parameters for the ocean carbon sink should be quantified when presenting simulated ocean anthropogenic carbon uptake as in the Global Carbon Budget and be used to adjust these simulated estimates if necessary. The larger ocean sink results in enhanced ocean acidification over the 21st century, which further threatens marine ecosystems by reducing the water volume that is projected to be undersaturated towards aragonite by around 3.7–7.4 million km3 more than originally projected.