Environment First Look

Climate change may be sucking oxygen out of the sea. Here's why that matters.

Warm air and warm water bring more bad news for marine life. 

Researchers lower a CTD-rosette into the sea from the German RV METEOR. Scientists at the GEOMAR Hemholtz Centre for Ocean Research in Kiel, Germany analyzed hundreds of thousands of historical and current oxygen measurements.
Courtesy of Martin Visbeck/GEOMAR
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Take a deep breath and feel the oxygen rich air filling your lungs. But for fish and other marine life those "deep breaths" are becoming less plentiful in places, according to a new study.

Of course, gills take in water, not air, but it's still laden with oxygen critical to life on this planet. Assembling millions of ocean oxygen measurements, a team of oceanographers has found that underwater oxygen levels have already dropped an average of 2 percent worldwide since 1960.

“Since large fishes in particular avoid or do not survive in areas with low oxygen content, these changes can have far-reaching biological consequences,” said Sunke Schmidtko, the lead-author of the study, in a press release.

The culprit? It’s complicated, but results match what climate scientists have predicted for years.

Warm water can’t hold as many dissolved gas molecules as cold water can, because the more actively jostling and bumping molecules create more opportunities for, say, oxygen molecules to escape into the air. As warmer air warms the ocean surface, the seas become less able to absorb and keep oxygen.

"It's the same reason we keep our sparkling drinks pretty cold," Dr. Schmidtko told The Washington Post.

But the oceanographers of GEOMAR Helmholtz Centre for Ocean Research in Kiel, Germany, saw more than six times the oxygen drop they’d have expected if this surface absorption power were the only effect at work, according to their paper, published Wednesday in the journal Nature.  

By combining “millions” of regional oxygen measurements taken over the 50-year period between 1960 and 2010, the researchers constructed the most comprehensive map ever of the ocean’s oxygen levels and their evolution.

To fill in some blank spots between measurements, the team used interpolation techniques, connecting the dots between existing data points. “To quantify trends for the entire ocean, however, was more difficult since oxygen data from remote regions and the deep ocean is sparse,” Schmidtko explained in the statement. “We were able to document the oxygen distribution and its changes for the entire ocean for the first time."

Taking a rare look into the ocean’s depths, the scientists found that the oxygen decline didn’t stop at the surface. Hot air rises, and warm water does too. When the temperature difference between the upper layers and the lower layers grows, the oxygen mixing effect of sinking and rising seawater slows to a crawl.

"When the upper ocean warms, less water gets down deep, and so therefor, the oxygen supply to the deep ocean is shut down or significantly reduced," Schmidtko told The Washington Post.

And 2 percent is just the average oxygen drop. “A few limited areas” show a rise, and others a more precipitous fall, with the most significant losses being found in the North Pacific (by amount lost) and the Arctic (by percentage lost).

“The oxygen losses in the ocean can have far-reaching consequences because of the uneven distribution. For fisheries and coastal economies this process may have detrimental consequences,” emphasized co-author Dr. Lothar Stramma in the statement.

The oceanographers are careful not to imply that correlation with climate change proves causation of oxygen decline.

“With measurements alone, we cannot explain all the causes,” added co-author Professor Martin Visbeck, “natural processes occurring on timescales of a few decades may also have contributed to the observed decrease.”

However as oceanographer Matthew Long, who was not involved with the new study, told The Washington Post, this global-scale decline in oxygen “conforms to the patterns we expect from human-driven climate warming.” Models have predicted a loss of between 1 percent and 7 percent by 2100, according to the paper.

The study is the latest to reveal drastic changes in the planet’s oceans, including massive bleaching of coral reefs, and a nearly one full degree of warming in 50 years, which scientists say could bring sea level rises of 20 to 30 feet.

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