Warming Ocean Holding Less Oxygen. That Seems Like a Problem.

May 16, 2023

Look up some recipes for Jellyfish.


The mid-Pliocene, from 2.6 to 5.3 million years ago, may be the closest the Earth has ever been—climatologically speaking—to where we are now.

With an atmospheric carbon dioxide level about the same as today’s, “the Pliocene is the last time we had a stable, warm climate globally,” says Catherine Davis, an oceanographer at North Carolina State University. The average global temperature was about 2 °C to 3 °C warmer than it is now, she says. The Pliocene had something else climate experts are predicting for our future: massive oxygen minimum zones (OMZs)—largely lifeless stretches of ocean severely lacking in oxygen.

OMZs are common in the ocean. They occur in its deeper reaches, typically between 200 and 1,000 meters below sea level, where oxygen concentrations plummet from their typical level of around six to eight milligrams per liter nearer the surface to less than two milligrams per liter. At these depths, explains John Cigliano, a biologist at Pennsylvania’s Cedar Crest College, microbes deplete the oxygen in the water as they decompose sinking organic material. The lack of oxygen, Cigliano says, restricts many animals to the ocean’s surface areas.

During the Pliocene, however, OMZs were much more widespread. In recent research, Davis looked for traces of a specific foraminifera in records of seafloor sediments from around the world. This tiny shelled organism only lives in low-oxygen water, giving Davis a way to calculate the extent of Pliocene OMZs. “Most of the North Atlantic Ocean hosted an OMZ in the Pliocene, whereas there are virtually no Atlantic OMZs today,” she says. “I can’t speculate as to where OMZs could develop in the future, but it’s certainly suggestive.”

Yet with our modern climate changing to look more and more like the Pliocene, Davis says, this “could give us a glimpse of what Earth’s oceans might look like 100 years from now.”

Yale Environment 360:

Off the coast of southeastern China, one particular fish species is booming: the oddly named Bombay duck, a long, slim fish with a distinctive, gaping jaw and a texture like jelly. When research ships trawl the seafloor off that coast, they now catch upwards of 440 pounds of the gelatinous fish per hour — a more than tenfold increase over a decade ago. “It’s monstrous,” says University of British Columbia fisheries researcher Daniel Pauly of the explosion in numbers.

The reason for this mass invasion, says Pauly, is extremely low oxygen levels in these polluted waters. Fish species that can’t cope with less oxygen have fled, while the Bombay duck, part of a small subset of species that is physiologically better able to deal with less oxygen, has moved in.

The boom is making some people happy, since Bombay duck is perfectly edible. But the influx provides a peek at a bleak future for China and for the planet as a whole. As the atmosphere warms, oceans around the world are becoming ever more deprived of oxygen, forcing many species to migrate from their usual homes. Researchers expect many places to experience a decline in species diversity, ending up with just those few species that can cope with the harsher conditions. Lack of ecosystem diversity means lack of resilience. “Deoxygenation is a big problem,” Pauly summarizes.

Our future ocean — warmer and oxygen-deprived — will not only hold fewer kinds of fish, but also smaller, stunted fish and, to add insult to injury, more greenhouse-gas producing bacteria, scientists say. The tropics will empty as fish move to more oxygenated waters, says Pauly, and those specialist fish already living at the poles will face extinction.

Researchers complain that the oxygen problem doesn’t get the attention it deserves, with ocean acidification and warming grabbing the bulk of both news headlines and academic research. Just this April, for example, headlines screamed that global surface waters were hotter than they have ever been — a shockingly balmy average of 70 degrees F. That’s obviously not good for marine life. But when researchers take the time to compare the three effects — warming, acidification, and deoxygenation — the impacts of low oxygen are the worst.

“That’s not so surprising,” says Wilco Verberk, an eco-physiologist at Radboud University in the Netherlands. “If you run out of oxygen, the other problems are inconsequential.” Fish, like other animals, need to breathe.

Oxygen levels in the world’s oceans have already dropped more than 2 percent between 1960 and 2010, and they are expected to decline up to 7 percent below the 1960 level over the next century. Some patches are worse than others — the top of the northeast Pacific has lost more than 15 percent of its oxygen. According to the IPCC’s 2019 special report on the oceans, from 1970 to 2010, the volume of “oxygen minimum zones” in the global oceans — where big fish can’t thrive but jellyfish can — increased by between 3 and 8 percent.

The oxygen drop is driven by a few factors. First, the laws of physics dictate that warmer water can hold less dissolved gas than cooler water (this is why a warm soda is less fizzy than a cold one). As our world warms, the surface waters of our oceans lose oxygen, in addition to other dissolved gases. This simple solubility effect accounts for about half of the observed oxygen loss seen so far in the upper 1,000 meters of the ocean.

In 2018, hundreds of researchers concerned with oxygen loss signed the Kiel Declaration to urgently call for more awareness of the problem, alongside work to limit pollution and warming. Researchers are now in the midst of establishing a Global Ocean Oxygen Database and ATlas (GO2DAT) to consolidate and map all the data.

Andrew Babbin, a biogeochemist at MIT who is on the steering committee for GO2DAT, in 2021 mapped out huge areas of extremely low oxygen in the Pacific. “It’s concerning for sure,” says Babbin, who hopes to repeat the mapping exercise in a decade or so to see how things change. One issue, he notes, is that low-oxygen conditions tend to host a class of anoxic bacteria that produce methane or nitrous oxide — potent greenhouse gases.

Modelling the net impacts of the three factors — solubility, mixing, and microbiology — has proven tricky. “Any one of those is hard,” says Babbin. “And then you put them all together, and it’s dramatically difficult to make any predictions.” In the tropics, for example, one model suggests that a shifting balance of biological factors that deplete oxygen, versus ocean mixing that delivers oxygen, will drive oxygen levels down until about 2150 but then raise them — a spot of potentially good news for tropical fish. On the whole, though, climate models seem to have underestimated changes in oxygen levels, which have been dropping faster than expected.


5 Responses to “Warming Ocean Holding Less Oxygen. That Seems Like a Problem.”

  1. redskylite Says:

    “The findings do not surprise experts in the field, but they provide further confirmation that climate change is caused by humans, as well as detailed information that can be used to refine future models.”


  2. […] Warming Oceans Hold Less Oxygen. That Seems Like a Problem […]

  3. neilrieck Says:

    Cold water holds more infused gases (like Oxygen) than warm water. This drives many fish to migrate farther north (or south if on the other side of the equator). Meanwhile, jellyfish prefer warmer water with less oxygen so guess what fishermen are going to be netting?

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