The looming crisis of aquatic deoxygenation

And the much bigger hidden problem

Recently — or should I say, finally?— scientists are recognizing that aquatic deoxygenation—the rapid loss of dissolved oxygen in Earth’s water bodies—should be officially recognized as a new planetary boundary. A planetary boundary describes processes critical to sustaining Earth’s ecological and societal integrity, and defines a safe space where societies and environments can function at a sustainable level. The authors propose that aquatic oxygen loss is a critical tenth boundary because it regulates other Earth systems and is approaching dangerous thresholds.

Concerted global monitoring, research and policy efforts are needed to address the challenges brought on by rapid deoxygenation.

The current planetary boundaries

Crossing any planetary boundary is predicted to set off the rest. As confirmation of this, the authors of this paper say, “…most planetary boundary processes have already exceeded safe levels.”

For the record, here are the current agreed-upon planetary boundaries:

1. Climate Change: The concentration of greenhouse gases in the atmosphere, particularly carbon dioxide and methane.
2. Change in Biosphere Integrity (Biodiversity Loss): The erosion of genetic and functional diversity in ecosystems due to species extinction and other factors.
3. Biogeochemical Flows: The disruption of the natural cycles of essential elements, specifically nitrogen and phosphorus.
4. Land-System Change: The conversion of natural landscapes like forests and wetlands into other uses, such as agriculture.
5. Freshwater Use: The consumption of both “blue” water in rivers and lakes and “green” water from land use changes.
6. Ocean Acidification: The decrease in the pH of the world’s oceans due to the absorption of atmospheric of carbon dioxide. (I think deoxygenation would lead to increased acidity, also.)
7. Stratospheric Ozone Depletion: The thinning of the ozone layer, which protects the Earth from harmful ultraviolet radiation.
8. Atmospheric Aerosol Loading: The concentration of microscopic particles (aerosols) in the atmosphere, which can affect both climate and human health.
9. Novel Entities (Chemical Pollution): The introduction of new chemicals, substances, and materials, like plastics and other pollutants, into the environment. 

Key findings & arguments for aquatic deoxygenation

The article in Nature is a formal call to the scientific and policy communities to treat aquatic deoxygenation as a global planetary emergency. By defining it as a planetary boundary, they hope to prioritize global monitoring, funding, and policy interventions similar to those established for climate change. Here is a summary of their reasons:

• Widespread Oxygen Loss: The study compiles data showing that oxygen is declining rapidly in all types of aquatic ecosystems:
  • Lakes & Reservoirs: Have experienced significant declines (referenced as ~5.5% and ~18.6% losses respectively since 1980 in related reports).
  • Oceans: Have lost approximately 2% of their dissolved oxygen since 1960. While this percentage sounds small, it affects vast areas of the ocean, some of which have almost no oxygen at all.
• Drivers of Deoxygenation:
  1. Climate Change: Warmer water holds less gas, so the oxygen is literally boiling off.
  2. Reduced Circulation: Warming increases stratification—the density difference between water layers—which reduces vertical mixing and circulation, thereby hindering the delivery of oxygen to deep waters in both freshwater and marine environments. (There is concern that the Gulf Stream may stop flowing.)
  3. Nutrient Runoff: Pollution from agriculture and sewage feeds algae blooms; when these algae die and decompose, the process consumes vast amounts of oxygen (eutrophication).
• Why It Matters (the “Tipping Point”):
  • Feedback Loops: Low oxygen conditions can trigger the release of potent greenhouse gases like methane and nitrous oxide from aquatic sediments, which further accelerates climate change (a positive feedback loop). Likewise, the ocean absorbs most of the carbon dioxide emissions, and as it warms, it will absorb less CO2.
  • Biodiversity Collapse: Oxygen is fundamental for aquatic life. Dead zones are expanding, leading to mass die-offs, shifts in food webs, and the collapse of fisheries.
  • Planetary Stability: The authors argue that oxygen availability is a “key regulator” of Earth’s system stability, linking the carbon, nitrogen, and phosphorus cycles.
  • Point of no return: “Even if climate warming stopped immediately, deep oceans
    would probably continue to lose DO [dissolved oxygen] for centuries, with cumulative losses >10% of pre-industrial levels.”

The hidden existential threat

The article in Nature was a great win. And I agree with the findings. They make the ominous prediction: “The observed, substantial and pervasive ecological consequences of DO losses demonstrate that the global deoxygenation of aquatic systems is approaching critical thresholds… In many freshwater and marine systems, regional thresholds have already been crossed, and severe ecological impairment is pervasive.” However, I believe there is a much bigger problem that no one is talking about — the deoxygenation of the atmosphere. Allow me to explain.

As I mentioned, the oceans have already lost 2% of their oxygen. But where does it go? Most of that outgasses into the atmosphere. Global warming increases the temperature of the ocean, which decreases the solubility of oxygen in water. So, quite literally, the oxygen boils off. ²

Imagine opening a can of soda on a hot day and leaving it in the sun. What happens? The dissolved carbon dioxide bubbles out, and the soda goes flat. In a similar way, this is what is happening to our oceans. As a liquid warms up, it can’t hold as many dissolved gases. So, as the oceans warm due to global warming, they release their dissolved oxygen and other gases.

However, measurements of atmospheric oxygen levels don’t show an increase; they actually show a decrease. In other words, atmospheric deoxygenation is not yet recognized because, ironically, the oxygen lost from the ocean goes back into the atmosphere, which offsets the loss of oxygen due to human (anthropogenic) activity, like combustion engines and chemical manufacturing.

So, the oxygen levels in the atmosphere may appear stable for now, but what happens when the oceans reach a new equilibrium and stop venting the stored oxygen reserves?

How much oceanic oxygen has been lost?

For fun, let’s calculate a rough number for the loss of oxygen. As I discussed in my other articles, oxygen is surprisingly difficult to measure. For example, the density can go down, but the percentage can remain constant. Fortunately, scientists have already done a lot of work to measure these things. Let’s put it into perspective.

The estimated amount of dissolved oxygen in the ocean is ~7,000–8,000 gigatonnes (Gt). And the ocean loses 1.5–3.1 Gt of oxygen per year. ³

If I average all those numbers out, 150 gigatonnes of oxygen have been lost, which confirms a 2% loss.

The total amount of oxygen in the atmosphere is 1,080,000-1,200,000 gigatonnes. This is a gigantic number, so 150 Gt loss is only about 0.01–0.02% of the atmospheric oxygen reservoir (by mass).

In other words, the oceans have added back into the atmosphere 0.01–0.02% of its oxygen.

Why this matters

Okay, I admit I expected a much bigger number. The oceans hold a small amount of oxygen compared to the atmosphere. However, it is still significant for a lot of reasons. And it shortens the timeline I laid out in this article: How long before the Earth runs out of oxygen?

First off, a similar thing is happening as the glaciers are melting. They are outgassing, too.

Next, visualize this: If the oceans are losing 3.1 Gt of oxygen per year, that amount of oxygen, when mixed with air at the standard 20.946%, would fill the Grand Canyon about 3 times.

Now, add this 3.1 gigatonnes to the 55.418 gigatonnes of oxygen that humans burn/oxidize every year. These are big numbers, and unfortunately, they are always getting bigger.

Other bigger factors also come into play. These dead zones in the ocean aren’t going away. And as I’ve discussed, almost all the free breathable oxygen comes from the phytoplankton in the ocean. So as the ocean warms and the biosphere erodes, the ocean will produce less and less oxygen.

Conclusion

The oxygen levels in the atmosphere may appear stable for now, but what happens when the oceans reach a new equilibrium and stop venting the stored oxygen reserves? And what happens when the ocean’s ecosystem collapses and phytoplankton no longer produce oxygen? I know that sounds like a lot of what-ifs, but years of research have shown humanity approaching these planetary boundaries/tipping points very fast, and crossing some already. (If you feel skeptical, I recommend researching similar events in the past, like the Permian Extinction as related to ocean warming and oxygen production.)

My research leads me to believe that aquatic deoxygenation is a precursor to atmospheric deoxygenation. And there is another tipping point where the oxygen drops so far that the lifeforms that produce it begin to die.

So, let me be the first to declare an 11th planetary boundary: atmospheric oxygen depletion.