The International Union for Conservation of Nature (IUCN)’s 2019 report Ocean deoxygenation: Everyone’s problem is the largest peer-reviewed study of ocean deoxygenation. The report was created with contributions from 67 scientific experts representing 51 institutes in 17 countries. Ocean deoxygenation may be the most important threat facing the ocean, and it is  the fastest-changing variable of marine health. The purpose of the IUCN report is to better inform policy and decision-makers with the existing science and to encourage further research where it is needed.

Almost all life in the ocean relies in some way on oxygen in the water. Further, the ocean supplies about half of all oxygen in the atmosphere. However, evidence is building that oxygen levels in the ocean are falling. Water oxygen levels have fallen by approximately 2% in the past 50 years, and it appears that human activity is likely to blame. Climate change is a major contributor to this effect - the ocean absorbs over a fourth of carbon dioxide emitted to the atmosphere as well as excess heat in the atmosphere from global warming. Carbon dioxide leads to ocean acidification (see more here) while heat promotes the release of oxygen from sea water. In fact, the majority of the oxygen lost has been through changes in ocean circulation and the associated gas exchange between the ocean and the atmosphere that are a consequence of warmer waters. On the other hand, agricultural runoff can contribute to the process of eutrophication, another cause of deoxynation. This occurs when high levels of fertilizers and nutrients in the water allow the growth of invasive algae blooms, which can cover large areas and eventually use up all the nutrients and oxygen in the water forming 'dead zones'. Management strategies for recovering areas impacted by eutrophication have been created and are in use. Increasingly, it appears that ocean deoxygenation is becoming an imminent problem. Indeed, despite having just 0.6% of atmospheric oxygen levels, the ocean is losing oxygen to the atmosphere faster than it is absorbing it.

Reduced oxygen levels create new challenges for marine species and their ecosystems. Marine animal physiology and behavior is heavily dependent on their ability to extract oxygen from water, and decreased oxygen in the water makes it difficult for marine animals to breathe. Low oxygen and increased temperatures reduce species habitat which can lead to changes in species migration, vulnerability, and abundance. Ecosystems are being further altered as species that are more tolerant to lower oxygen levels are becoming favored at the expense of others. Thus, the amount of life in the ocean is expected to significantly decrease. This will have future economic impacts by affecting fisheries and tourism.

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So what can we do? Ocean deoxygenation is expected to worsen over the next century as climate change continues and agricultural space expands, leading to warmer waters and increased eutrophication. Policies and projects for nutrient reduction strategies have often been effective at preventing algae blooms. Further, emissions must be decreased to decrease the rate of carbon dioxide and heat absorption in the ocean. The report acknowledges that this is a global challenge and will require contributions from all governments - ocean deoxygenation is, indeed, everyone's problem.

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Next year will be the beginning of the United Nations Decade of Ocean Science for Sustainable Development (2021-2030). The 2019 IUCN deoxygenation report contains timely findings that highlight dissolved oxygen levels as an incredibly important variable in the overall health of the ocean. The report also provides new knowledge and urgency to the need to reduce greenhouse gas emissions in order to curb the loss of oxygen in the ocean that is predicted to increase under a business-as-usual scenario.

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Find the full report here.

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References

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1. Laffoley, D. & Baxter, J.M. (eds.) Ocean deoxygenation: Everyone’s problem - Causes, impacts, consequences and solutions. Full report, 2019. Gland, Switzerland: IUCN. 580pp. Available from https://portals.iucn.org/library/sites/library/files/documents/2019-048-En.pdf