Wondering whether the sea can really run short of oxygen? It can. Ocean deoxygenation is the human-driven fall in dissolved oxygen, and studies suggest the ocean has lost 1-2% of it since the mid-20th century.
This explainer is part of the marine science section of the site.
| Item | Value |
|---|---|
| Ocean oxygen lost since the mid-20th century | 1-2% |
| Projected fall in global ocean oxygen over the next hundred years | Up to 7% |
| Model range for oxygen decline over the next century (2016 article) | 1.8% to 3.45% |
| Hypoxia threshold | Less than 2 mg of oxygen per liter |
| Ocean volume in a hypoxic state (2016 article) | 60.4 million cubic kilometers |
| Typical depth of oxygen minimum zones | 100-1000 m |
The figures come from Wikipedia’s overview of ocean deoxygenation and a 2016 article in The Conversation. Estimates change as new measurements and models arrive, so check the sources before you rely on them.
What is ocean deoxygenation?
Ocean deoxygenation is the reduction of oxygen content in different parts of the ocean because of human activities. International scientific bodies increasingly use the term because it captures the falling trend in the oxygen held by the world ocean.
It happens in two places, and the causes differ:
- Coastal zones: eutrophication, meaning an excess of nutrients, has driven quite rapid declines in oxygen to very low levels within a few decades. These are often called dead zones.
- Open and deep ocean: naturally low-oxygen areas, known as oxygen minimum zones, are slowly expanding as a consequence of human-caused climate change.
Scientists name the stages of oxygen shortage. Water holding less than 2 mg of oxygen per liter is called hypoxic, a level that is generally harmful for much aquatic life such as fish and shellfish, according to the 2016 Conversation piece. Water completely devoid of oxygen is called anoxic, and few higher organisms can live there.
Why is the ocean losing oxygen?
Three linked processes do most of the damage. Together they lower the supply of oxygen or raise the demand for it.
- Warmer water holds less oxygen. Warmer water is less dense and holds less oxygen than cold water, and salt lowers the carrying capacity too.
- Stronger stratification. Warm, fresh surface water floats on denser water below, so oxygen from the surface mixes down less.
- Nutrient runoff. Fertilizer and sewage feed algal blooms, and microbes that break down the dead algae use up oxygen.
Reduced oxygen solubility from warming can explain about 50% of oxygen loss in the upper ocean, according to the Wikipedia overview. Warmer water also raises the oxygen demand of living organisms, so less is left for marine life.
Stratification grows when the surface ocean warms or when more freshwater arrives from rivers and melting ice. Because the layers mix less, the deep ocean receives less oxygen from above while respiration keeps consuming what it has. That respiration is part of the wider ocean carbon cycle, in which sinking organic material is broken down at depth.
Oxygen enters the ocean at the surface through photosynthesis by phytoplankton and through mixing with the atmosphere. The role of phytoplankton in climate therefore includes supplying oxygen as well as taking up carbon.
One point stands out in the 2016 article: climate change worsens the problem, often independent of nutrient loads, and the decline is global in scale. In other words, cleaning up farm runoff helps coastal waters but does not remove the warming-driven part.
Where low-oxygen zones are spreading
The 2016 article lists dead zones that are on the increase from the waters off the Oregon coast to the upwelling zones off Peru. It also names the Baltic and Black seas, the Bay of Bengal, the South China Sea and the Gulf of Mexico. Low-oxygen areas in the deep ocean are expanding too, primarily because of warming.
Oxygen minimum zones generally sit at middle depths, from 100-1000 m deep. They are natural, produced by the respiration of sinking organic material from the surface, but they are growing both vertically and horizontally. Because circulation in these areas is slow, even small changes such as a 1-2% decline are easier to detect.
Long records from the Baltic and Adriatic
Long-term records make the trend concrete. In the Adriatic Sea, summertime oxygen was stable until the mid-20th century, then hypoxia began in 1960 and intensified.
In the Baltic Sea, one study using data from as far back as 1899 found increasing hypoxic and anoxic bottom waters. As of the 2016 article, that dead zone was about three times the size of the Gulf of Mexico dead zone.
How much water is affected
Worldwide, the ocean volume in a hypoxic state, at about 1.6 mg/liter of dissolved oxygen, was put at 60.4 million cubic kilometers in the 2016 article. That is the scale of the problem, though the figure is a snapshot from one source and not a live measurement.
How low oxygen affects marine life and fisheries
Even in the best case, water holds much less oxygen than air, so animals work harder to breathe in it. Experiments cited in the 2016 article show that reduced oxygen limits the metabolic scope of fish, and feeding and growth can fall sharply.
The article also describes a study that combined rising temperatures with lower oxygen. It predicted that, on average across all seas, maximum fish size would decline 10 percent, with large differences among species.
Baltic Sea cod show how this can play out:
- Cod condition, including size, has been in serious decline over the past two decades, per the 2016 article.
- Hypoxia was the most important explanatory variable, followed by lack of prey.
- Low oxygen also caused a 30 percent reduction in living space, which the article calls “habitat squeeze”.
The Wikipedia overview adds that a decrease in ocean oxygen affects how productive the ocean is, how nutrients and carbon move around and how marine habitats function. It poses a threat to marine life and to people who depend on it for food or livelihood.
What the projections say
Climate models run under very different assumptions, from business as usual to strong emission cuts, and still point the same way. The 2016 article reports a projected oxygen decline of 1.8 percent to 3.45 percent over the next century.
The Wikipedia overview gives a higher ceiling: model simulations predict a decline of up to 7% in global ocean oxygen over the next hundred years. It adds that the decline is projected to continue for a thousand years or more.
The effects will not be evenly spread. The 2016 article says the north Atlantic, north Pacific and Antarctic waters will be particularly affected.
The article was published on November 1, 2016, and the Wikipedia text is a rolling summary, so newer studies may give different ranges. Treat the numbers here as a guide to the size of the issue, not as forecasts for any one place.
The same article notes that a group of scientists formed the IOC-UNESCO working group called the Global Ocean Oxygen Network, or GO2NE, in 2015 to raise awareness and stimulate research and policy. That shows the topic has been on the international agenda for some years.
Frequently asked questions
Is ocean deoxygenation the same as a dead zone?
Show answer
Not quite. Dead zones are the coastal form, driven mainly by eutrophication, while the open-ocean form is the slow expansion of oxygen minimum zones linked to climate change.
What does hypoxic mean?
Show answer
Hypoxic water holds less than 2 mg of oxygen per liter, which is generally harmful to much aquatic life such as fish and shellfish. Water with no oxygen at all is called anoxic.
Will the ocean run out of oxygen?
Show answer
The sources do not suggest that. They report a 1-2% loss since the mid-20th century and projected declines of 1.8% to 3.45%, or up to 7%, over the next century, though local zones can become unlivable for many animals.
Does warming cause ocean deoxygenation on its own?
Show answer
Warming is a major driver, since warmer water holds less oxygen and mixes less with deeper layers. Nutrient pollution is a separate driver, mainly in coastal waters, and the two can add together.
