Researchers propose adding aquatic deoxygenation as a tenth Planetary Boundary.
A Tenth Planetary Boundary for Earth’s Waters
Oxygen is disappearing from the world’s aquatic ecosystems, a rapidly escalating crisis that threatens to upend the biosphere according to a review published June 30 in the journal Limnology and Oceanography. Scientists from Scripps Institution of Oceanography at UC San Diego and other institutions argue that aquatic deoxygenation should be formally added to the Planetary Boundaries framework. First introduced in 2009, the framework identifies nine critical processes that maintain Earth’s stability and resilience, tracking how human activity pushes these systems past safe limits.
The nine established boundaries include climate change, ocean acidification, biodiversity loss, atmospheric aerosol loading, stratospheric ozone depletion, freshwater change, land-use change, chemical pollution, and biogeochemical flows. Researchers contend that dissolved oxygen levels belong alongside them as a core metric of planetary health.
Erica Ferrer, a postdoctoral scholar at the University of California, Santa Barbara’s National Center for Ecological Analysis and Synthesis, stated that the health and stability of the planet depends on the health and stability of aquatic ecosystems, which need oxygen to function normally. She added that the study aims to elevate the profile of aquatic deoxygenation as a global threat and show that it does not operate in isolation.
Warming and Nutrient Pollution Drive Oxygen Loss
The decline of dissolved oxygen in oceans, coastal waters, rivers, and lakes stems primarily from human-caused global warming and pollution. As greenhouse gas emissions drive up planetary temperatures, aquatic systems warm up. Warmer water holds less dissolved oxygen than cold water because higher temperatures reduce oxygen solubility. At the same time, warm surface layers prevent oxygen from mixing into deeper waters.
Compounding the temperature rise, industrial processes, agriculture, wastewater discharge, and stormwater runoff dump excess nitrogen and phosphorus into Earth’s waters. These nutrients fuel massive algal blooms that ultimately decompose, consuming large amounts of oxygen. Warming and excess nutrients together stimulate microbial oxygen consumption, driving down oxygen levels further across aquatic food webs according to research led by Scripps Institution of Oceanography.
Globally, the ocean has lost roughly 2% of its dissolved oxygen since the 1950s, according to Copernicus, the European Union’s Earth observation initiative. Scientists expect the ocean to lose another 1% to 7% by the end of the century. Marine organisms ranging from microscopic life to fish, sharks, and marine mammals face direct threats as oxygen depletion degrades habitats and shifts prey distributions.
Assessing Risks and Indicators for an Unsafe Space
The review’s authors synthesized interactions and feedbacks between deoxygenation and all nine established planetary boundaries, concluding that oxygen loss extensively modulates climate change, nutrient loading, biodiversity loss, and aerosol loading. The research effort originated after lead author Erica Ferrer and senior author Lisa Levin attended COP25, the 2019 United Nations Climate Change Conference in Madrid.
“Given these interactions and current rates of oxygen loss, we argue that aquatic deoxygenation is approaching an ‘unsafe space,’ with Earth-system impacts that are likely to be irreversible in our lifetimes.”
Erica Ferrer and research co-authors, via Limnology and Oceanography
The study was authored by Erica Ferrer alongside former Scripps PhD students Shailja Gangrade, Lillian McCormick, Ariel Pezner, and Yassir Eddebbar.
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