Deoxygenation and Its Drivers Analyzed in Steady State for Perpetually Slower and Warmer Oceans.

Ocean deoxygenation is an important consequence of climate change that poses an imminent threat to marine life and global food security. However, our understanding of the complex interactions between changes in circulation, solubility, and respiration that drive global‐scale deoxygenation is incomplete. Here, we consider idealized biogeochemical steady states in equilibrium with perpetually slower and warmer oceans constructed from climate‐model simulations of the 2090s that we hold constant in time. In contrast to simulations of the end‐of‐century transient state, our idealized states are intensely deoxygenated in the abyss, consistent with perpetually reduced ventilation and throttled Antarctic Bottom Water formation. We disentangle the effects of the deoxygenation drivers on preformed oxygen and true oxygen utilization (TOU) using the novel concept of upstream exposure time, which precisely connects TOU to oxygen utilization rates and preformed oxygen to ventilation. For our idealized steady states, deoxygenation below 2,000 m depth is due to increased TOU, driven dominantly by slower circulations that allow respiration to act roughly 2–3 times longer thereby overwhelming the effects of reduced respiration rates. Above 500 m depth, decreased respiration and slower circulation closely compensate, resulting in little expansion of upper‐ocean hypoxia. The bulk of preformed oxygen loss is driven by ventilation shifting equatorward to where warmer surface waters hold less oxygen. Warming‐driven declines in solubility account for less than 10% of the total oxygen loss. Although idealized, our analysis suggests that long‐term changes in the marine oxygen cycle could be driven dominantly by changes in circulation rather than by thermodynamics or biology. Plain Language Summary: Climate change is driving oxygen out of the ocean, threatening marine life and global food security. However, the precise contributions of the chemical, physical, and biological processes that control oxygen levels are not well known because of their complex interactions. To better understand these interactions, we consider idealized simulations of a global oxygen cycle that is fully equilibrated with a perpetually warmer and slower ocean constructed from climate‐model simulations of the 2090s but held constant in time for our analyses. Compared to typical predictions, these idealized states exhibit intense deep‐ocean deoxygenation, for which we precisely quantify the contributions from changes in solubility, respiration, and ocean circulation. We find that deep‐ocean deoxygenation is driven by the slower circulation allowing respiration to act for 2–3 times longer thereby overcoming lower respiration rates. The surface origin of oxygen shifts away from cold high‐latitude waters toward warmer waters, in which atmospheric oxygen is less soluble, further reducing oxygen levels. Warming‐driven decreases in solubility alone only account for a mere 10% of the total oxygen loss. The upper ocean remains well oxygenated because changes in respiration and circulation compensate almost perfectly. Our results highlight the central importance of circulation in controlling oxygen in the ocean. Key Points: Key drivers of deoxygenation are quantified for oxygen cycles idealized by being in equilibrium with perpetually slower oceansWidespread intense abyssal ocean deoxygenation is driven predominantly by slower circulations allowing respiration to act over longer timesMost of the reduction in preformed oxygen is driven by changes in ventilation patterns and not by warming‐driven reduced solubility [ABSTRACT FROM AUTHOR]