The progressive deoxygenation of the oceans has become a serious problem for Earth’s ecosystems, serious enough that scientists are now proposing it be added to the list of “planetary boundaries”
Oxygen has always been considered one of the fundamental building blocks of life on Earth, so the fact that the oceans are steadily losing it is no small concern. That’s the conclusion of a study published in June in the journal Limnology and Oceanography, which argues that ocean deoxygenation should be added to the roster of “planetary boundaries,” a set of nine parameters (since their introduction in 2009) used to track the limits of Earth’s overall health, covering areas like climate change, ocean acidification, biodiversity loss, ozone depletion and others.
Erica Ferrer, a postdoctoral researcher at the University of California, Santa Barbara and the study’s lead author, put it this way: the health and stability of the planet depend on the health and stability of aquatic ecosystems, which need oxygen to sustain their natural life processes. Much of the blame falls on carbon emissions, which raise the average temperature of oceans, lakes, rivers and other water bodies, triggering a chain reaction: warmer water holds less dissolved oxygen, since oxygen’s solubility itself declines as temperatures rise.
Nutrient pollution is the other major culprit, encompassing activities like agriculture, wastewater discharge and various industrial processes that introduce nitrogen and phosphorus into water bodies worldwide. Higher levels of these nutrients drive uncontrolled algae growth, and the decomposition of that algae consumes large amounts of oxygen in turn.
Since the 1950s, the world’s oceans have lost 2% of their dissolved oxygen. According to Copernicus, the EU’s Earth observation program, scientists project a further loss of between 1% and 7% by the end of the century. That may sound like a small figure, but this steady, ongoing loss could deprive marine organisms of an element essential to their survival, threatening the fragile balance of marine ecosystems.
In the study, Ferrer and her colleagues focused in particular on the link between ocean deoxygenation and the existing planetary boundaries, showing how it affects processes, such as climate change and atmospheric aerosol loading, that are meant to stay within safe limits to protect ecosystems.
The outlook the researchers describe is far from reassuring: taking these interactions and current rates of oxygen loss into account, they believe ocean deoxygenation is approaching an “unsafe” threshold, with consequences for the planet that could prove irreversible.
Given this, the researchers propose four indicators scientists could use to track the “health status” of ocean deoxygenation and potentially set a global limit not to be crossed. These include, first, the concentration of dissolved oxygen itself, followed by a measure of how close water is to its maximum oxygen-holding capacity, changes observed in oxygen-sensitive plants and animals, and finally what’s called the metabolic index, the ratio between the amount of oxygen organisms need to survive and the amount actually available to them.
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