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. Ocean deoxygenation is recognized as key ecosystem stressor of the futureocean and associated climate-related ocean risks are relevant for current policydecisions. In particular, benefits of reaching the ambitious1.5  ∘ C warming target mentioned by the Paris Agreement compared tohigher temperature targets are of high interest. Here, we model oceanicoxygen, warming and their compound hazard in terms of metabolic conditionson multi-millennial timescales for a range of equilibrium temperaturetargets. Scenarios where radiative forcing is stabilized by 2300 are usedin ensemble simulations with the Bern3D Earth System Model of IntermediateComplexity. Transiently, the global mean ocean oxygen concentration decreasesby a few percent under low forcing and by 40 % under high forcing. Deoxygenationpeaks about a thousand years after stabilization of radiative forcing and newsteady-state conditions are established after AD 8000 in our model. Hypoxic watersexpand over the next millennium and recovery is slow and remains incompleteunder high forcing. Largest transient decreases in oxygen are projected forthe deep sea. Distinct and near-linear relationships between theequilibrium temperature response and marine O2 loss emerge. These point tothe effectiveness of the Paris climate target in reducing marine hazards andrisks. Mitigation measures are projected to reduce peak decreases in oceanicoxygen inventory by 4.4 %  ∘ C −1 of avoided equilibrium warming.In the upper ocean, the decline of a metabolic index, quantified by the ratioof O2 supply to an organism's O2 demand, is reduced by6.2 %  ∘ C −1 of avoided equilibrium warming. Definitions of peakhypoxia demonstrate strong sensitivity to additional warming. Volumes of waterwith less than 50 mmol  O2  m −3 , for instance, increase between36 % and 76 %  ∘ C −1 of equilibrium temperature response. Our resultsshow that millennial-scale responses should be considered in assessments ofocean deoxygenation and associated climate-related ocean risks. Peak hazardsoccur long after stabilization of radiative forcing and new steady-stateconditions establish after AD 8000.

Hazards of decreasing marine oxygen: the near-term and millennial-scale benefits of meeting the Paris climate targets