Climate Change and Suffocating Oceans

Principal Investigator

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At a Glance

The Resplandy group’s CMI research in the last year focused on the ocean response to climate change, in particular the loss of oxygen associated with climate change and how it influences ecosystems and ecosystem services, such as fisheries. The Resplandy group uses the latest generation of climate and ocean models to evaluate how ocean oxygen has evolved in the past and will evolve in the future. This is a key step in understanding and anticipating how greenhouse gas emissions will impact ecosystems. This work has led to two publications in the past year focusing on constraining drivers of global de-oxygenation and the fate of oxygen minimum zones (OMZs). Understanding the causes and mechanisms of reduced ocean oxygen in a warming world is important for energy industry policymakers, especially in helping them to make informed decisions about energy transition and mitigation.

 

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Research Highlight

The ocean has lost dissolved oxygen (O2) in response to global warming in the past 50 years. A serious threat of this systematic ocean deoxygenation is the expansion of tropical oxygen minimum zones located in the subsurface ocean. Equally threatening is the more frequent occurrence of coastal dead zones where human pollution originating from the land, such as fertilizers, urbanization, wastewater, etc., reinforces the depletion of oxygen (e.g., Resplandy, 2018).

The first project focused on explaining the unexpected and counter-intuitive weak change in oxygen (and even increase in some places) observed in parts of the Atlantic Ocean in the past 50 years. The researchers showed that this pattern can be explained by the amplification of the hydrological cycle, a response to climate change. Using a series of Earth system model experiments, they found that the hydrological effect increases the supply of oxygen in “salty-get-saltier” subtropical waters, countering the oxygen loss tied to warming. At the same time, this reduced the supply of oxygen in “fresh-get-fresher” deep waters, reinforcing the oxygen loss tied to warming. This work reveals that long overlooked indirect effects of warming, through hydrological changes, can substantially influence ocean de-oxygenation patterns.

The second project expanded on work published in 2022 on the Pacific Ocean oxygen minimum zone (Busecke et al., 2022). This work showed that three regimes control the future of the oxygen minimum zone under climate change in all basins (Figure 11.1). The three regimes include: the expansion regime of “low oxygenated waters;” the contraction regime of the “core waters” (lowest oxygen levels within the oxygen minimum zone); and in-between, a transition regime with spatial redistributions but little change in volume. This three-regime view of oxygen minimum zones is crucial to assess impacts. For example, the expansion of “low oxygenated waters,” which delimits the optimum habitat of numerous marine species, could impact marine ecosystems and ecosystem services. Likewise, the contracting “core waters” could limit the production of oceanic nitrous oxide.

 

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References

Busecke, J.J.M., L. Resplandy, S.J. Ditkovsky, and J.G. John, 2022. Diverging fates of the Pacific Ocean oxygen minimum zone and its core in a warming world. AGU Advances 3:e2021AV000470. (https://doi.org/10.1029/2021AV000470).

Ditkovsky, S., L. Resplandy, and J. Busecke, 2023. Unique ocean circulation pathways reshape the Indian Ocean oxygen minimum zone with warming. Biogeosciences 20:4711–4736. (https://doi.org/10.5194/bg-20-4711-2023).

Hogikyan, A., L. Resplandy, M. Liu, and G. Vecchi, 2024. Hydrological cycle amplification reshapes warming-driven oxygen loss in the Atlantic Ocean. Nature Climate Change 14:82–90. (https://doi.org/10.1038/s41558-023-01897-w).

Resplandy, L., 2018. Will ocean zones with low oxygen levels expand or shrink? Nature 557:314–315. (https://doi.org/10.1038/d41586-018-05034-y).