Vast swathes of the ocean are now completely devoid of oxygen, according to a warning from scientists based on new analysis. 

Very few marine creatures can survive in these areas, known as “dead zones”, and it could spell mass extinction if we continue on the same trajectory. 

“If we were creating vast areas on land that were uninhabitable by most animals, we’d notice,” said Denise Breitburg, from the Smithsonian Environmental Research Center in the US. “But we don’t always see things like this when they are happening in the water.” 

“Major extinction events in Earth’s history have been associated with warm climates and oxygen-deficient oceans,” she added.

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The “dead zones” are now four times larger than they were in 1950 — increasing by more than 4.5 million square kilometres since 1950, which is an area roughly the size of the European Union.

What’s more, the number of areas near coasts that have very low oxygen are now 10 times larger than in 1950 — up from fewer than 50 to at least 500. 

“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,” continued Breitburg. 

But, she added, this is a problem we can solve, with “a global effort” and “even local actions can help with nutrient-driven oxygen decline.” 

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The analysis, which examined all the major research on ocean oxygen loss, was published in the journal “Science”, and was produced by an international working group launched in 2016 by Unesco’s Intergovernmental Oceanographic Commission.

The “large-scale deoxygenation” is mainly caused by climate change, because warmer waters carry less oxygen. 

Warmer waters also increase the metabolism of microbes and marine creatures, meaning they’re using up more of the oxygen that is there. 

Meanwhile, dead zones near to coasts are the result of fertiliser and sewage running off the land and polluting the water.

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These low-oxygen areas — including major areas in the eastern Pacific and Baltic Sea, and an area of southern California that has seen a drop in oxygen levels of 30% in just 25 years — are expanding towards to surface by as much as a meter every year. 

And it’s changing the habits of marine animals, as well as threatening the livelihoods of the 500 million people around the world — predominantly in developing countries — who rely on the ocean to survive. 

But Robert Diaz, from the Virginia Institute of Marine Science, is worried that governments around the world are not yet wise to the severity of the problem. 

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“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,” he said.

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

But there is still hope. 

Both the River Thames in London, and Chesapeake Bay in the US, are examples of oxygen dead zones that have recovered, after the problem was addressed and better farm and sewage practices were established. 

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