A committed fourfold increase in ocean oxygen loss.

Less than a quarter of ocean deoxygenation that will ultimately be caused by historical CO emissions is already realized, according to millennial-scale model simulations that assume zero CO emissions from year 2021 onwards. About 80% of the committed oxygen loss occurs below 2000 m depth, where a more sluggish overturning circulation will increase water residence times and accumulation of respiratory oxygen demand. According to the model results, the deep ocean will thereby lose more than 10% of its pre-industrial oxygen content even if CO emissions and thus global warming were stopped today. In the surface layer, however, the ongoing deoxygenation will largely stop once CO emissions are stopped. Accounting for the joint effects of committed oxygen loss and ocean warming, metabolic viability representative for marine animals declines by up to 25% over large regions of the deep ocean, posing an unavoidable escalation of anthropogenic pressure on deep-ocean ecosystems.