Warming Oceans Are Losing Oxygen at Breathtaking Rates, Threatening Mass Extinction of Marine Life, Scientists Warn

January 7, 2018 at 6:59 pm

Zero-oxygen “dead zones,” where marine life cannot survive, have quadrupled in size since 1950, new study finds

A fisherman Chile wading in dead sardines, a result of algae blooms that suck oxygen out of the water. Credit: Felix Marquez, Associated Press

Global warming and agricultural pollution have drastically reduced oxygen levels in the ocean over the last 70 years.

Coastal dead zones with zero oxygen have quadrupled in size since 1950, while the number of very low oxygen sites have multiplied tenfold, a new study published in the journal Science finds.

Most sea creatures cannot survive in these zones. If  current trends continue we are likely to see mass extinction of marine life within our lifetimes, along with the death of hundreds of millions of people who depend on the sea for food, an international group of scientists warn.

“Major extinction events in Earth’s history have been associated with warm climates and oxygen-deficient oceans,” Denise Breitburg, a marine ecologist for the Smithsonian Institute, told The Guardian.

“Under the current trajectory that is where we would be headed. But the consequences to humans of staying on that trajectory are so dire that it is hard to imagine we would go quite that far down that path.”

At least 500 coastal dead zones have now been reported. There were fewer than 50 in 1950.

Red dots indicate water where oxygen is less than 2 milligrams per liter. Source: Science

While researchers blame fertilizer and manure runoff from agricultural land for the coastal dead zones, they say large-scale deoxygenation is a result of global warming, as warmer waters hold less oxygen.

“Rising global temperatures decrease oxygen solubility in water, increase the rate of oxygen consumption via respiration, and are predicted to reduce the introduction of oxygen from the atmosphere and surface waters into the ocean interior by increasing stratification and weakening ocean overturning circulation,” the report says.

In total, the world’s oceans have lost a total of 2 percent — or 77 billion tons — of their oxygen since 1950.

This can slow reproduction, reduce growth size and increase disease, the scientists say. Additionally, the microbes that thrive in very low oxygen environments produce nitrous oxide, a greenhouse gas 300 times more potent than carbon dioxide, compounding the problem.

More than 500 million people are utterly dependent on the world’s oceans for food, especially in poorer nations. More than 350 million people are dependent on the world’s oceans for employment.

“This is a problem we can solve,” Breitburg said. “Halting climate change requires a global effort, but even local actions can help with nutrient-driven oxygen decline.” She pointed to former dead zones in Chesapeake Bay in the US and the Thames river in the UK, that cleared up after improved farming and sewage practices.

Robert Diaz, a marine biologist at the Virginia Institute of Marine Science who reviewed the new study, wasn’t as optimistic. Deoxygenating oceans are not yet a priority for governments around the world, he says.

“Unfortunately, it will take severe and persistent mortality of fisheries for the seriousness of low oxygen to be realized.”

Governments’ failure to take action doesn’t make the problem any less dire though, he says.

“No other variable of such ecological importance to coastal ecosystems has changed so drastically in such a short period of time from human activities as dissolved oxygen.”

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