Thermocline

Environmental chemistry

Oceanography

Geology

Biogeochemistry

Climate change

Catalysis

Organic chemistry

Chemistry

Ocean current

Deoxygenation

Environmental science

Climatology

Oxygen

Biochemistry

Biogeochemical cycle

Atmospheric sciences

Nature Geoscience

Direct observations indicate that the global ocean oxygen inventory is decreasing. Climate models consistently confirm this decline and predict continuing and accelerating ocean deoxygenation. However, current models (1) do not reproduce observed patterns for oxygen changes in the ocean’s thermocline; (2) underestimate the temporal variability of oxygen concentrations and air–sea fluxes inferred from time-series observations; and (3) generally simulate only about half the oceanic oxygen loss inferred from observations. We here review current knowledge about the mechanisms and drivers of oxygen changes and their variation with region and depth over the world’s oceans. Warming is considered a major driver: in part directly, via solubility effects, and in part indirectly, via changes in circulation, mixing and oxygen respiration. While solubility effects have been quantified and found to dominate deoxygenation near the surface, a quantitative understanding of contributions from other mechanisms is still lacking. Current models may underestimate deoxygenation because of unresolved transport processes, unaccounted for variations in respiratory oxygen demand, or missing biogeochemical feedbacks. Dedicated observational programmes are required to better constrain biological and physical processes and their representation in models to improve our understanding and predictions of patterns and intensity of future oxygen change. Ocean oxygen loss in a warming world is strongly affected by biogeochemical processes that are not fully accounted for in ocean models, suggests a literature synthesis.

Drivers and mechanisms of ocean deoxygenation