Degrees of reversibility of ocean deoxygenation in an atmospheric carbon dioxide removal scenario

Abstract Over the last century, increasing atmospheric carbon dioxide (CO 2 ) concentrations, among other greenhouse gases, and resulting climate change have greatly impacted the ocean. Observed impacts include lower oxygen solubility and changes in ocean stratification, circulation and biological activity. To reduce the carbon burden in the atmosphere in the future and thereby mitigate anthropogenic climate change, carbon dioxide removal (CDR) techniques have been increasingly studied and tested. However, information on the impact of CDR on oceanic oxygen is still scarce. In the current study we explore dissolved oxygen responses from an idealized CDR implementation, with atmospheric CO 2 ramp-up and ramp-down simulations following the CDR model intercomparison project protocol. We find that over the timescale of a few centuries, the degree of recovery of marine oxygen, after atmospheric CO 2 has returned to pre-industrial levels, differs for different water depths. Oxygen concentrations strongly recover in the upper ocean, achieving a near reversibility within 97%–99% across models, and even overshoot pre-industrial levels at depths of 100–600 m. Conversely, oxygen responses show a long-lasting deoxygenation signal in the deep ocean, with a much smaller initial recovery signal by the end of the experiment. The main factor driving oxygen changes in the deep ocean is indicated by the apparent oxygen utilization, related to changes in circulation and ventilation, as inferred by the simulated age of deep water masses. According to our models and despite the effective recovery of oxygen in the upper ocean, the effects of time lags and hysteresis on deep ocean responses could lead to longstanding and deleterious impacts on redox-sensitive biogeochemical processes and on marine biota throughout the ocean.