The Ocean Is Running Out of Oxygen
The ocean has lost 2% of its oxygen in 50 years, but up to 40% in key regions.
I’m currently on my summer holiday, and as like most ocean lovers, I am obsessed with snorkelling. Yesterday, I was lucky enough to be swimming in the Ionian Sea, off a quiet stretch of the Albanian coast, where the water is so clear you can count the pebbles on the seabed. Beneath that serenity, something else was happening.
The slow disappearance of oxygen.
They came to talk about protecting the ocean, but no one is talking about the fact it’s losing its breath.
Right now, leaders from around the world are gathered in Nice for UNOC 3, the third United Nations Ocean Conference. They’re discussing the likes of marine protected areas, seabed mining, plastics, fisheries and high seas governance.
Still, one of the most serious and accelerating crises in the ocean, deoxygenation, is nowhere to be found on the agenda.
A slow, large-scale loss of oxygen is unfolding and no one is explaining it. That is why I’m writing this now. We are not just heating the sea, we are quietly draining the oxygen from it.
The consequences could accelerate everything else we’re already struggling to contain.
A Breathless Sea
The ocean is suffocating.
Global ocean oxygen levels have dropped by about 2% over the last fifty years. In some regions, including the eastern tropical Pacific and the California Current, losses have reached 40% or more in deeper layers. This is a slow, planetary decline.
The ocean is losing its breath, and barely anyone is paying attention.
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Where Oxygen Comes From and Why It's Disappearing
Oxygen enters the sea in two main ways. First, through surface mixing, where wind and waves draw air into the upper layers. Second, through photosynthesis by microscopic plankton and algae. It is then transported into deeper layers by ocean mixing, a vital process that is now under threat.
Oxygen is being lost from the ocean due to three major forces, all driven by human activity:
1. Warming Water Holds Less Oxygen
As global temperatures rise, so do sea surface temperatures. Warmer water can’t hold as much dissolved oxygen as cooler water can. Just like a warm can of Coke goes flat faster, warmer oceans are gradually releasing the oxygen they once stored.
2. Stratification Blocks Oxygen Flow
Warmer surface water is more buoyant and floats on top of cooler, heavier layers, creating stratification, a kind of layering that prevents oxygen from mixing downward and nutrients from coming up. This blocks the ocean’s internal conveyor belt, cutting off oxygen from the depths.
3. Nutrient Pollution Supercharges Algae
Fertilisers rich in nitrogen and phosphorus wash off fields into rivers and coasts, fuelling explosive algal blooms. When these algae die, bacteria break them down, a process that consumes enormous amounts of oxygen and creates “dead zones.” This is known as eutrophication.
Together, these three drivers are acting in concert, not isolation. One feeds the next. The result is an ocean that is warmer, stiller, more polluted and increasingly unbreathable.
Welcome to the Dead Zones
There are now more than 500 hypoxic (low-oxygen) zones recorded globally.
The Baltic Sea holds seven of them, stretching across nearly 80,000 square kilometres.
In the Black Sea, the oxygen-rich surface layer has thinned by over a third since the 1950s.
In the Gulf of Mexico, summer eutrophication creates seasonal dead zones that suffocate coastal life.
In the North Pacific, entire oxygen minimum zones, naturally occurring low-oxygen areas between 100–1000m deep, are now expanding faster than predicted.
Fish are retreating to the shallows. Shellfish stocks are collapsing. Jellyfish are thriving. Entire habitats are being driven into thinner slices of the ocean column, a phenomenon known as habitat compression. This forces predators and prey into crowded zones, increasing the risk of overfishing, disease and ecosystem collapse.
A Climate Feedback No One Is Talking About
Low-oxygen areas don’t just impact marine life. They start to reshape the planet’s climate. In these zones, microbes begin releasing nitrous oxide (a greenhouse gas nearly 300 times more potent than carbon dioxide). This creates a self-reinforcing loop:
Less oxygen → More nitrous oxide → More warming → Even less oxygen
This feedback cycle could become one of the most dangerous tipping points in the entire ocean–climate system, but because it’s unfolding quietly, invisibly, and without drama, it barely registers in the news.
If you regularly read Voice for the Blue, you’ll know the pattern by now, some of the most serious threats to life on Earth are happening underwater and barely anyone’s talking about them.
Fish Are Evolving and That Should Worry Us
Imagine walking into a room and realising there’s no air. That’s what all marine species are facing in parts of the ocean right now. We are witnessing real-time evolutionary pressure in species that can no longer rely on the ocean they evolved in. I want to pull out just a few examples of the many out there:
Ray-finned fish, including species like cod and herring, are showing changes in how they sense oxygen at the genetic level. Adaptations that typically take thousands of years, now unfolding over decades.
Mummichogs, a hardy estuarine fish, have developed both behavioural shifts and gill restructuring to survive polluted, low-oxygen water.
Epaulette sharks can survive complete loss of oxygen for up to two hours by rerouting blood to their brains and shutting down non-essential functions.
Crucian carp survive ice-covered, oxygen-depleted lakes by converting lactate into ethanol, essentially fermenting themselves through winter.
These aren’t quirks. They are survival tactics.
Every adaptation tells the same story, the ocean’s oxygen baseline is changing so fast, species are evolving just to keep up. Many won’t.
Scientists Are Responding but Policy Isn’t
In May 2025, several major research institutions launched a global initiative to study ocean deoxygenation. New tools, from optical sensors and Argo floats (these observe temperature, salinity and currents), to animal-borne tags and AI-powered models like OXYGENERATOR, are helping scientists build a clearer picture of how oxygen levels are shifting.
However, even with better data, the policies aren’t following.
Most international climate agreements still don’t mention oxygen loss at all.
This Should Be a Red Line
A growing number of researchers are calling for ocean deoxygenation to be recognised as a planetary boundary (one of the limits humanity must not cross if we want to avoid catastrophic Earth system change). It is a credible proposal. This trend is long-term, human-driven and threatens food systems, ecosystems and climate stability.
Why This Matters Right Now
Deoxygenation is not a theory. It is already reshaping the ocean.
In the Arabian Sea, the oxygen minimum zone is expanding and intensifying. Fish stocks have plummeted, particularly those tied to deeper waters.
Across multiple regions, species are migrating into new areas in search of breathable water, leaving traditional fishing zones empty and coastal food webs out of balance.
Predators and prey are being compressed into shallower waters, restructuring entire ecosystems. In some areas, jellyfish are taking over.
The research is out there!
These are measurable shifts in the structure and survival of marine life.
We notice the surface changes but no one is asking what is happening to the sea’s ability to breathe.
The Bottom Line
I’ve worked in ocean conservation for around twenty years and I’ve seen policy ignore these warning signs before. I’ve watched critical changes beneath the surface go unnoticed until it was too late.
If this continues, the ocean will not simply become warmer or more acidic. It will become uninhabitable. Not for all life, but for the systems we depend on and the balance we still barely understand.
The ocean does not need to boil to change how we live forever, it just needs to stop breathing.
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