05 Aug 2010: Analysis

A Looming Oxygen Crisis and
Its Impact on World’s Oceans

As warming intensifies, scientists warn, the oxygen content of oceans across the planet could be more and more diminished, with serious consequences for the future of fish and other sea life.

by carl zimmer

The Deepwater Horizon oil spill is overshadowing another catastrophe that’s also unfolding in the Gulf of Mexico this summer: The oxygen dissolved in the Gulf waters is disappearing. In some places, the oxygen is getting so scarce that fish and other animals cannot survive. They can either leave the oxygen-free waters or die. The Louisiana Universities Marine Consortium reported this week that this year’s so-called “dead zone” covers 7,722 square miles.

Unlike the Deepwater Horizon disaster, this summer’s dead zone is not a new phenomenon in the Gulf. It first appeared in the 1970s, and each summer it has returned, growing bigger as the years have passed. Its expansion reflects the rising level of fertilizers that farmers in the U.S. Midwest have spread across their fields. Rain carries much of that fertilizer into the Mississippi River, which then delivers it to the sea. Once the fertilizer reaches the Gulf, it spurs algae to grow, providing a feast for bacteria, which grow so fast they use up all the oxygen in their neighborhood. The same phenomenon is repeating itself along many coastlines around the world. This summer, a 377,000-square-kilometer (145,000-square-mile) dead zone appeared in the Baltic Sea. In 2008, scientists reported that new dead zones have been popping up at an alarming rate for the past 50 years. There are now more than 400 coastal dead zones around the world.

As serious as these dead zones are, however, they may be just a foreshadowing of a much more severe crisis to come. Agricultural runoff
Warming is expected to reduce the mixing of the ocean by making surface seawater lighter.
can only strip oxygen from the ocean around the mouths of fertilizer-rich rivers. But global warming has the potential to reduce the ocean’s oxygen content across the entire planet. Combined with acidification — another global impact of our carbon emissions — the loss of oxygen could have a major impact on marine life.

Scientists point to two reasons to expect a worldwide drop in ocean oxygen. One is the simple fact that as water gets warmer, it can hold less dissolved oxygen. The other reason is subtler. The entire ocean gets its oxygen from the surface — either from the atmosphere, or from photosynthesizing algae floating at the top of the sea. The oxygen then spreads to the deep ocean as the surface waters slowly sink.

Global warming is expected to reduce the mixing of the ocean by making surface seawater lighter. That’s because in a warmer world we can expect more rainfall and more melting of glaciers, icebergs, and ice sheets. Since freshwater is less dense than salt water, the water at the ocean’s surface will become lighter. The extra heat from the warming atmosphere will also make surface waters expand — and thus make them lighter still. The light surface water will be less likely to sink — and thus the deep ocean will get less oxygen. Instead, more of the oxygen will linger near the surface, where it will be used up by oxygen-breathing organisms.

The prospect that global warming could reduce the ocean’s oxygen has led some scientists to wonder if the predicted decline has already begun. It’s a
A Looming Oxygen Crisis and Its Impact on World’s Oceans
Scientists predict declining oxygen levels could have a major effect on marine life.
maddeningly hard thing to determine, however. We can be very confident that humans have driven up the concentration of carbon dioxide in the atmosphere because scientists have recorded a steady increase over the course of decades. The signal of human-produced carbon dioxide is stronger than the noise of nature’s ups and downs.

Fluctuations in oxygen levels, on the other hand, are a lot noisier. As ocean currents oscillate naturally, upwellings of deep-ocean water can deliver nutrients to coastal waters, triggering an explosion of growth and driving down oxygen levels. Volcanoes can alter oxygen levels, too, by creating a haze that blocks sunlight, thus temporarily cooling the ocean’s surface and allowing more oxygen to dissolve into the water.

In recent years some worrying signals have started to emerge from the noise. In 2006, for example, oxygen levels off the coast of Oregon dropped to record lows. Reefs that had been packed with rockfish and other animals suddenly became ecological ghost towns. Instead of agricultural run-off, studies on the Oregon dead zone suggest that global warming was partly responsible. Higher temperatures have reduced the oxygen in the ocean currents that deliver water to the Oregon coast.

It’s much harder for scientists to figure out what’s happening in the open ocean than along the coastlines, because the records are far spottier. But some recent studies have also offered cause for worry. In April, for
Because records of oxygen levels are incomplete, scientists are calling for a new push for more research.
example, Lothar Stramma of the University of Kiel and his colleagues published a study in Deep Sea Research in which they compared records of oxygen levels in the tropical ocean from two periods: from 1960 to 1974 and from 1990 to 2008. In some regions, the oxygen levels have gone up, the scientists found, but in most places they’ve gone down. In fact, the area of the global ocean without enough oxygen for animals to survive (less than 70 micromoles per kilogram to be exact) expanded by 4.5 million square kilometers (1.7 million square miles). That’s an area about half the size of the United States.

Because the records of oxygen levels in the past are so incomplete, many scientists are calling for a push for more research. An international collaboration started in 1995, the Climate Variability and Predictability Repeat Hydrography Program — CLIVAR for short — is beginning to gather better data. But in the latest issue of Annual Review of Marine Science, Ralph Keeling of Scripps Institution of Oceanography and his colleagues warn that the CLIVAR program may need 20 to 30 years to establish long-term trends of oxygen levels. To speed up the process, they call for a global network of floating sensors known as Argo to be brought into the effort. If scientists put oxygen sensors on a few hundred of the 3,000 Argo floats, Keeling and his colleagues predict that a clear pattern would emerge in as little as five years. Keeling and his colleagues believe that it’s urgent to speed up this research, because the deoxygenation of the oceans could have a major impact on marine life.

In order to project how global warming will alter oxygen in the oceans, climate scientists are developing a new generation of computer models. The models are still too crude to capture some important features of the real world, such as the way winds can change how deep water rises in upwellings. But the models are good enough to replicate some of the changes in oxygen levels that have already been recorded. And they all predict that the oxygen in the world’s oceans will drop; depending on the model, the next century will see a drop of between 1 and 7 percent.

That could be enough to have a profound effect on life in the ocean, according to Daniel Pauly, a fisheries biologist at the University of British
Jellyfish, which can tolerate lower oxygen levels than fish, may thrive in the new conditions.
Columbia. In his new book, Gasping Fish and Panting Squids: Oxygen, Temperature and the Growth of Water-Breathing Animals, Pauly argues that getting oxygen is the most important constraint on the growth of fishes and many other marine animals. That’s because it takes a lot of energy to extract oxygen from water, and the bigger animals get, the more energy they have to invest.

Pauly and his colleagues are working on computer models to project how global warming will affect the world’s fisheries. Many species of fishes will shift their ranges away from water that’s too warm for them. But this flight from heat may force them into regions of the ocean with low levels of oxygen, where their growth will be limited. Pauly and his colleagues predict that the drop in the ocean’s oxygen and pH levels will together reduce the world’s fish catch by 20 to 30 percent by 2050.

While fishes and other animals with high oxygen demands suffer, jellyfish may thrive. Jellyfish can tolerate lower oxygen levels than fish, in part because they can store reserves of the gas in their jelly. Free from competition and predators, jellyfish will be able to feast on the microscopic animals and protozoans that feed on algae. They may thus leave more food for bacteria, spurring a further drop in oxygen levels.


An Ominous Warning on the
Effects of Ocean Acidification

An Ominous Warning on the Effects of Ocean Acidification
A new study says the seas are acidifying ten times faster today than 55 million years ago when a mass extinction of marine species occurred. And current changes in ocean chemistry due to the burning of fossil fuels may portend a new wave of die-offs, reports science writer Carl Zimmer.
A drop in oxygen may also cause the ocean's bacteria to change. Bacteria that need oxygen will no longer be able to thrive in oxygen-free zones of the ocean. But these dead zones will foster the growth of many species of bacteria for whom oxygen is toxic. Some of these oxygen-hating microbes produce nitrogen compounds that are among the most potent greenhouse gases ever measured. In other words, a drop in oxygen levels could further intensify global warming.

Unless we find a way to rein in our carbon emissions very soon, a low-oxygen ocean may become an inescapable feature of our planet. A team of Danish researchers published a particularly sobering study last year. They wondered how long oxygen levels would drop if we could somehow reduce our carbon dioxide emissions to zero by 2100. They determined that over the next few thousand years oxygen levels would continue to fall, until they declined by 30 percent. The oxygen would slowly return to the oceans, but even 100,000 years from now they will not have fully recovered. If they’re right, fish will be gasping and squid will be panting for a long time to come.

POSTED ON 05 Aug 2010 IN Biodiversity Biodiversity Climate Climate Oceans Science & Technology Antarctica and the Arctic Central & South America North America 


The toughest thing to separate out from all of this is exactly how much oxygen levels in the oceans and seas changed previously as we have only been measuring them in the last few decades of the industrial era. How do natural cycles of warming and cooling of the earth in general affect this? Does this generally happen during the end of ice ages, as we are apparently still coming out of the last one? Assuming that you would expect the Arctic and Antarctic to melt and affect these environments even more. Is this change in makeup of ecosystems a spur for evolutionary changes and adaptation of species or new species? the other thing is that in the history of the earth oxygen levels have been much higher than they are now, up at 30%, not the 21% we have all been taught about early in school. So maybe oxygen fluctuations are also a natural part of the earth's cycle?

Posted by Ed on 05 Aug 2010

An excellent article on ocean deoxygenation, which is arguably of much greater concern than acidification and much less well known. The 1-7% predicted decline is averaged over the *entire* volume of the ocean - i.e., this is a massive decrease in oxygen that will be more intense in certain areas. How marine ecosystems respond to warming, acidification, deoxygenation, overfishing, and different forms of pollution - all occurring simultaneously - is the trillion dollar question.

Posted by Michael Beman on 05 Aug 2010

I've also read that the oceans are responsible for a significant portion of out atmospheric oxygen. If that's true, fish won't be the only ones gasping.

Can anyone with better knowledge than I speak to that?

Posted by David Veale on 05 Aug 2010

Green plants that photosynthesis live in the oceans too. These plants produce oxygen. The vast majority of these plants are single cell phytoplankton that are not visable to the naked eye. They are said to produce something like 20 percent (I don't have the exact number) of the oxygen produced on earth. The atmosphere is something like 21 percent O2 so there are no worries here as this is plenty for us for a long time coming.

Posted by Vale Contros on 05 Aug 2010

With the recent gulf oil spill, I was wondering if it were possible to increase the dissolved oxygen in the water by pumping air down to the bottom. I've heard that the bacteria that eat the oil are oxygen limited, so increasing the level would help clean the water. Is there a way to do this cheaply? On a global scale?

Posted by Gary Godfrey on 06 Aug 2010

Re; air pumping. How about a liquid oxygen "bomb" that would burst at great depth. An "artesian diver" type buoy would release on the bottom and float back to the surface for a refill.

Posted by JackJack on 10 Aug 2010

According to Nature the biomass of phytoplankton is now 40 percent less than in 1950 and is decreasing by 1 percent a year.

This is an extinction event at the base of the oceanic food chain happening right before our eyes.

World War IV is being fought right now. It's a War against CO2 and it's not going well.

Posted by co2hound on 10 Aug 2010

I was reading that excellent biology textbook by Capmbell and Reece, and I found this part about the Permian mass extinction:

"[...] temperature differences between the equator and the poles would have slowed the mixing of ocean water, which in turn would have reduced the amount of oxygen available to marine organisms. This oxygen deficit may have been a major cause of the Permian extinction of marine life"

We had better take very seriously this looming oxygen crisis...

Posted by Walter S. Andriuzzi on 12 Aug 2010

This is nonsense. Sea life has been surviving the El Niño -- La Niña alternation for millions of years; this alternation changes surface temperature distribution and relative O2 levels over half the planet on a three to five-year cycle.

The Oregon coastal data would be meaningful only if we had accurate records over several centuries; in fact, discovery of the Pacific Decadal Oscillation is quite recent -- and this has a major effect on the northern Pacific coast of America.

Give it a rest.

Posted by Craig Goodrich on 16 Aug 2010


Can you please provide the reference for this paper?

"A team of Danish researchers published a particularly sobering study last year. ... The oxygen would slowly return to the oceans, but even 100,000 years from now they will not have fully recovered." Thank you

Posted by Tony Noerpel on 17 Aug 2010


A link to the 2009 Nature study has been added to the story. Here's the URL: http://www.nature.com/ngeo/journal/v2/n2/

Posted by e360 on 17 Aug 2010

Phytoplankton produce most of the oxygen you and I breathe. They must be feeling the heat too.

So it goes. Destruction of these single cell creatures means less O2 for all life on planet earth.

Do we care?

Posted by Justine Ashton on 18 Oct 2010

Denial, too, has been extensively studied (by for example Elisabeth Kubler-Ross, from memory) and is to be expected from a large part of the population as part of our human response to imminent catastrophe. One hopes the action required won't be stymied by those who can't yet accept the scale of the disaster we face.

Posted by Richard Middleton on 20 Dec 2010

A great post on the causes of oxygen depletion in the water bodies and the effects it will have on sea life.

Posted by Sam on 06 Apr 2011

A small ray of hope re: the Deepwater Horizon spill. For a year or so after WWII, significant amounts of oil were found in the Sargasso Sea are in the Atlantic Ocean. This was the apparent result of ruptured and corroded oil bunkers and tanker hulls from the Battle of the Atlantic.
Within a couple of more years, the oil had rotted or evaporated away, with no apparent permanent damage.

I'm not saying this will happen in the Gulf of Mexico, but at least there is a natural process that will mitigate some of the spillage.

Posted by Roy Mendez on 22 Aug 2011

With the recent gulf oil spill, I was wondering if it were possible to increase the dissolved oxygen in the water by pumping air down to the bottom.

I've heard that the bacteria that eat the oil are oxygen limited, so increasing the level would help clean the water. Is there a way to do this cheaply? On a global scale?

Posted by Cioara on 09 Jul 2012

Comments have been closed on this feature.
Carl Zimmer writes about science for The New York Times and a number of magazines. A 2007 winner of the National Academies of Science Communication Award, Zimmer is the author of six books, including Microcosm: E. coli and the New Science of Life. In previous articles for Yale Environment 360, he has written about the prospects of passing planetary tipping points and the consequences of increased acidification in the world’s oceans.



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