The mechanisms of North Atlantic CO₂ uptake in a large Earth System Model ensemble

Abstract. The oceans currently take up around a quarter of the carbon dioxide (CO₂) emitted by human activity. While stored in the ocean, this CO₂ is not influencing Earth's radiation budget; the ocean CO₂ sink therefore plays an important role in mitigating global warming. CO₂ uptake by the oceans is heterogeneous, with the subpolar North Atlantic being the strongest CO₂ sink region. Observations over the last 2 decades have indicated that CO₂ uptake by the subpolar North Atlantic sink can vary rapidly. Given the importance of this sink and its apparent variability, it is critical that we understand the mechanisms behind its operation. Here we explore the combined natural and anthropogenic subpolar North Atlantic CO₂ uptake across a large ensemble of Earth System Model simulations, and find that models show a peak in sink strength around the middle of the century after which CO₂ uptake begins to decline. We identify different drivers of change on interannual and multidecadal timescales. Short-term variability appears to be driven by fluctuations in regional seawater temperature and alkalinity, whereas the longer-term evolution throughout the coming century is largely occurring through a counterintuitive response to rising atmospheric CO₂ concentrations. At high atmospheric CO₂ concentrations the contrasting Revelle factors between the low latitude water and the subpolar gyre, combined with the transport of surface waters from the low latitudes to the subpolar gyre, means that the subpolar CO₂ uptake capacity is largely satisfied from its southern boundary rather than through air–sea CO₂ flux. Our findings indicate that: (i) we can explain the mechanisms of subpolar North Atlantic CO₂ uptake variability across a broad range of Earth System Models; (ii) a focus on understanding the mechanisms behind contemporary variability may not directly tell us about how the sink will change in the future; (iii) to identify long-term change in the North Atlantic CO₂ sink we should focus observational resources on monitoring lower latitude as well as the subpolar seawater CO₂; (iv) recent observations of a weakening subpolar North Atlantic CO₂ sink may suggest that the sink strength has peaked and is in long-term decline.