Decades of data on world's oceans reveal a troubling oxygen decline

May 4, 2017, Georgia Institute of Technology
Global map of the linear trend of dissolved oxygen at the depth of 100 meters. Credit: Georgia Tech

A new analysis of decades of data on oceans across the globe has revealed that the amount of dissolved oxygen contained in the water - an important measure of ocean health - has been declining for more than 20 years.

Researchers at Georgia Institute of Technology looked at a historic dataset of ocean information stretching back more than 50 years and searched for long term trends and patterns. They found that levels started dropping in the 1980s as ocean temperatures began to climb.

"The oxygen in oceans has dynamic properties, and its concentration can change with natural climate variability," said Taka Ito, an associate professor in Georgia Tech's School of Earth and Atmospheric Sciences who led the research. "The important aspect of our result is that the rate of global oxygen loss appears to be exceeding the level of nature's random variability."

The study, which was published April in Geophysical Research Letters, was sponsored by the National Science Foundation and the National Oceanic and Atmospheric Administration. The team included researchers from the National Center for Atmospheric Research, the University of Washington-Seattle, and Hokkaido University in Japan.

Falling in have the potential to impact the habitat of marine organisms worldwide and in recent years led to more frequent "hypoxic events" that killed or displaced populations of fish, crabs and many other organisms.

Researchers have for years anticipated that rising water temperatures would affect the amount of oxygen in the oceans, since warmer water is capable of holding less dissolved gas than colder water. But the data showed that was falling more rapidly than the corresponding rise in water temperature.

"The trend of oxygen falling is about two to three times faster than what we predicted from the decrease of solubility associated with the ocean warming," Ito said. "This is most likely due to the changes in ocean circulation and mixing associated with the heating of the near-surface waters and melting of polar ice."

The majority of the oxygen in the ocean is absorbed from the atmosphere at the surface or created by photosynthesizing phytoplankton. Ocean currents then mix that more highly oxygenated water with subsurface water. But rising ocean water temperatures near the surface have made it more buoyant and harder for the warmer surface waters to mix downward with the cooler subsurface waters. Melting polar ice has added more freshwater to the ocean surface - another factor that hampers the natural mixing and leads to increased ocean stratification.

"After the mid-2000s, this trend became apparent, consistent and statistically significant—beyond the envelope of year-to-year fluctuations," Ito said. "The trends are particularly strong in the tropics, eastern margins of each basin and the subpolar North Pacific."

In an earlier study, Ito and other researchers explored why was more pronounced in tropical waters in the Pacific Ocean. They found that air pollution drifting from East Asia out over the world's largest contributed to oxygen levels falling in tropical waters thousands of miles away.

Once carried the iron and nitrogen pollution to the tropics, photosynthesizing phytoplankton went into overdrive consuming the excess nutrients. But rather than increasing oxygen, the net result of the chain reaction was the depletion oxygen in subsurface water.

That, too, is likely a contributing factor in waters across the globe, Ito said.

Explore further: Polluted dust can impact ocean life thousands of miles away, study says

More information: Takamitsu Ito et al, Upper Ocean Otrends: 1958-2015, Geophysical Research Letters (2017). DOI: 10.1002/2017GL073613

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17 comments

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gkam
2.8 / 5 (9) May 04, 2017
This will be challenged by those who get their opinions from political sources.

We are killing our Life-Support System, the Environment.
richdiggins
2 / 5 (4) May 04, 2017

The phytoplankton contribution to the carbon cycle is partially understood + documented.
"Oxygen can only enter water from two sources: it either dissolves into water from contact with the atmosphere or is produced by plants during photosynthesis. "
Personally, phytoplankton sound like a cure and not the cause.

The jury is still out on the cause of any oxygen level decreases. There are other contributing factors, other sources of pollution, causing this issue.

With the increased ship traffic of the 21st century, I would suspect that human waste pumped overboard is also a factor.

"How do human activities affect the dissolved oxygen concentrations in water?"

"Microorganisms such as bacteria decompose organic waste in water—a process which requires oxygen.

Organic waste is anything that was once part of a plant or animal, such as leaves
richdiggins
2.6 / 5 (5) May 04, 2017
.."and manure"

Source;
"Dissolved Oxygen - Utah State University Extension"
http://digitalcom...n_curall

Personally, I get my opinions from carefully considering all of the facts involved. Unlike the emotional members of this forum.
antigoracle
1.6 / 5 (7) May 05, 2017
Too many fishes using up the oxygen.

So, according to AGW Cult "science", warming oceans hold less Oxygen but absorbs more CO2
dogbert
2 / 5 (8) May 05, 2017
The article failed to quantify the decline in O2 dissolved in the oceans. Is the measured decline significant?

The article also failed to explain why O2 levels would decline with ocean warming but CO2 levels would increase with ocean warming.

AGW apparently has only bad outcomes and decreasing O2 in the ocean coupled with increasing CO2 would represent a bad outcome. Doesn't have to make sense if it promotes the agenda.
howhot3
4.5 / 5 (8) May 05, 2017
Here is a link just for you denier gooneys.

https://skepticalscience.com
zz5555
4.5 / 5 (8) May 05, 2017
The article also failed to explain why O2 levels would decline with ocean warming but CO2 levels would increase with ocean warming.

I think they didn't explain that because that's not what happens. As the ocean warms it's able to hold less and less CO2 and oxygen - all things being equal. But all things aren't equal. CO2 levels have risen a great deal and continue to increase. The increased partial pressures will cause atmospheric CO2 to be taken up by the oceans (https://www.pmel....n+Uptake ).

Perhaps they should have explained it, but the writers have to assume some basic level of understanding in their audience and this isn't all that complicated, so maybe they just assumed the audience had at least a high school education.
dogbert
2 / 5 (4) May 05, 2017
zz555,
Over 20 % of our atmosphere is oxygen. CO2 is measured in parts per million.

Yes, water at higher temperatures is less able to contain dissolved gasses, but there is hundreds of times as much oxygen as carbon dioxide to dissolve I water.

The question of why CO2 concentrations is rising while O2 concentrations are falling is a reasonable question.

Since it is do ready to answer, why don't you explain it?
Da Schneib
5 / 5 (4) May 05, 2017
@dogbert, the abstract of the paper gives the figures:
The global upper ocean O2 inventory (0-1,000m) changed at the rate of -243±124TmolO2 per decade.


As far as CO2 the paper doesn't discuss CO2. There are, however, a large number of papers on CO2 and a large body of evidence supporting the finding that its concentration in the world's oceans is increasing.

Explanation and mechanism are irrelevant here; these are observed facts.

You're obfuscating again.
dogbert
1 / 5 (4) May 05, 2017
Da Schneib,

So it is not a significant drop. Did not think it would be.
Da Schneib
5 / 5 (4) May 05, 2017
@dogbert apparently can't read:
The spatial patterns of the multi-decadal trend include regions of enhanced ocean deoxygenation including the subpolar North Pacific, eastern boundary upwelling systems and tropical oxygen minimum zones.
So we're going to get dead zones where fish can't live. How long? Nobody knows yet. That research remains to be done.

The drop is in teramoles. That's rather a lot of oxygen even for the entire ocean down to 1,000m, and considering it's predicted to exacerbate already oxygen poor areas, the impact could be more severe than indicated by a whole-ocean figure.

In this case, however, the effect doesn't appear to be heat-related.

It's pretty poor practice not to read the paper before commenting.
dogbert
1 / 5 (4) May 05, 2017
Da Schneib,

Your propensity to attack anyone who does not agree with you is not conducive to rational discourse.

I did read the article. I also wasted time reading your comments.

Those who have little to say attack instead of saying anything of import. You continue to define yourself as someone who retreats to insult because you cannot support your vacuous comments.
Da Schneib
5 / 5 (2) May 05, 2017
@dogbert, your comments are even more risible if you actually read the paper.

Your attempt to change the subject and refusal to address your technical errors is noted as more obfuscation.

If you don't like being attacked, don't say stupid stuff, and take the time to read the paper (not just the article) to make sure you don't. And don't obfuscate. It's irritating.
dogbert
1 / 5 (4) May 05, 2017
I read and commented on the article.

Your comments are mostly just insults.
Da Schneib
5 / 5 (2) May 05, 2017
But did you read the paper? That's the question. The article isn't science. The paper is science.

You must be new.
zz5555
5 / 5 (3) May 05, 2017
Over 20 % of our atmosphere is oxygen. CO2 is measured in parts per million.

I'm not sure why you would've thought that was relevant to your question.

but there is hundreds of times as much oxygen as carbon dioxide to dissolve I water.

Again, irrelevant.
The question of why CO2 concentrations is rising while O2 concentrations are falling is a reasonable question.

You might have bothered to read the link I provided:
Air-sea gas exchange is a physio-chemical process, primarily controlled by the air-sea difference in gas concentrations and the exchange coefficient, which determines how quickly a molecule of gas can move across the ocean-atmosphere boundary.

So as you increase the concentration of CO2 in the atmosphere, the concentration of CO2 in the ocean will equilibrate to that new concentration. Partial pressure, Henry's Law - look them up.
katesisco
5 / 5 (1) May 07, 2017
The article fails to answer the questions in my mind. Why is the North Sea a source of increased O2? Are the areas on the ocean map of increasing O2 actually upwellings?
Are the areas of decreasing O2 sited close to NA actually reflective of the ocean currents sweeping up and collecting CO from the land and having CO2 produced as it collects an oxygen to become carbon dioxide?

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