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

Abstract

Abstract. Ocean deoxygenation is recognized as key ecosystem stressor of the future ocean and associated climate-related ocean risks are relevant for current policy decisions. In particular, benefits of reaching the ambitious 1.5 ∘C warming target mentioned by the Paris Agreement compared to higher temperature targets are of high interest. Here, we model oceanic oxygen, warming and their compound hazard in terms of metabolic conditions on multi-millennial timescales for a range of equilibrium temperature targets. Scenarios where radiative forcing is stabilized by 2300 are used in ensemble simulations with the Bern3D Earth System Model of Intermediate Complexity. Transiently, the global mean ocean oxygen concentration decreases by a few percent under low forcing and by 40 % under high forcing. Deoxygenation peaks about a thousand years after stabilization of radiative forcing and new steady-state conditions are established after AD 8000 in our model. Hypoxic waters expand over the next millennium and recovery is slow and remains incomplete under high forcing. Largest transient decreases in oxygen are projected for the deep sea. Distinct and near-linear relationships between the equilibrium temperature response and marine O2 loss emerge. These point to the effectiveness of the Paris climate target in reducing marine hazards and risks. Mitigation measures are projected to reduce peak decreases in oceanic oxygen inventory by 4.4 % ∘C−1 of avoided equilibrium warming. In the upper ocean, the decline of a metabolic index, quantified by the ratio of O2 supply to an organism's O2 demand, is reduced by 6.2 % ∘C−1 of avoided equilibrium warming. Definitions of peak hypoxia demonstrate strong sensitivity to additional warming. Volumes of water with less than 50 mmol O2 m−3, for instance, increase between 36 % and 76 % ∘C−1 of equilibrium temperature response. Our results show that millennial-scale responses should be considered in assessments of ocean deoxygenation and associated climate-related ocean risks. Peak hazards occur long after stabilization of radiative forcing and new steady-state conditions establish after AD 8000.