ocean deoxygenation
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The oceans are becoming increasingly deprived of oxygen and are suffocating

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Scientists warn that the oceans are suffocating. Increasingly large areas are being deprived of oxygen. They are becoming "dead zones" and their number has, since the 1950s, quadrupled in the open sea and increased tenfold in coastal areas! The consequences of this situation are disastrous for marine life and for ours too.
 
Ahe number of oxygen-depleted ocean dead zones has quadrupled since 1950, scientists warn, while the number of very oxygen-poor sites near the coasts has increased tenfold. Most marine creatures cannot survive in these areas and current trends would lead to a long-term mass extinction, with potentially disastrous consequences for the hundreds of millions of people who depend on the sea.
 
Climate change caused by greenhouse gas emissions from fossil fuels is the cause of this large-scale deoxygenation. Warming of surface waters makes it more difficult for oxygen to reach deeper areas of the ocean. In addition, as the ocean warms, it contains less oxygen. In coastal waters, excessive nutrient pollution from land creates algal blooms that drain oxygen as they die and decompose.

Threat of extinction

The analysis, published in the journal Scienceis the first comprehensive study of its kind. In the preamble, the researchers state: " Major extinction events in Earth's history have been associated with warm climates and oxygen-deficient oceans. ». Denise Breitburg, marine ecologist with the Smithsonian Environmental Research Center in the U.S. and who conducted the analysis, says: " With the current trajectory, that's where we'd be heading. But the consequences for humans of staying on that trajectory are so terrible that it is hard to imagine that we could go that far in that direction. "
 
Not only do the oceans produce about half of the oxygen on Earth, but they feed more than 500 million people, especially in poor countries, and provide employment for 350 million people. But at least 500 dead zones have now been reported near the coasts, up from less than 50 in 1950. The lack of surveillance in many areas means that the actual number could be much higher.
 
Red areas mark coastal areas where oxygen has dropped to less than 2 milligrams per litre.
Blue areas indicate locations with oxygen depletion in the open ocean - Credit: GO2NE working group
 
The open ocean has natural areas of low oxygen content, generally off the west coast of continents because of the way the Earth's rotation affects ocean currents. But these dead zones have expanded considerably, increasing by millions of square kilometres since 1950, an area roughly the size of the European Union.
 
In addition, oxygen levels in all ocean waters are declining, with a loss of 2% - 77 billion tonnes - since 1950. This can reduce growth, harm reproduction and increase disease, scientists warn. The irony is that warmer waters not only contain less oxygen, but the marine organisms living in them have to breathe faster, consuming oxygen more quickly.
 
In areas traditionally known as dead zones such as the Chesapeake Bay and Gulf of Mexico, oxygen levels drop so low that many animals suffocate and die. As fish avoid these areas, their habitats shrink and become more vulnerable to predators or fishing.
But the problem goes far beyond the dead zones, the authors point out. Even lower oxygen declines can stunt animal growth, hinder reproduction and lead to disease and even death. Lower oxygen levels can also trigger the release of dangerous chemicals such as nitrous oxide, a greenhouse gas up to 300 times more potent than carbon dioxide and hydrogen sulphide. While some animals can thrive in dead zones, overall biodiversity is declining.
 
In coastal areas, pollution from fertilizers, manure and sewage cause algal blooms, and when the algae decompose, oxygen is sucked out of the water.

A problem to take on the body

It's a problem we can solve," warns Denise Breitburg in The Guardian. " Stopping climate change requires a global effort, but even local actions can contribute to nutrient-induced oxygen depletion.. "The scientist reported recovery in the Chesapeake Bay in the United States and the Thames River in the United Kingdom, where improved agricultural and sewage treatment practices have resulted in the disappearance of dead zones.
 
To preserve the level of oxygen in the ocean, scientists believe that the world needs to manage the issue from three angles:
- Address the two main causes: nutrient pollution and climate change.
- Protecting vulnerable marine life. According to the team GO2NE (Global Ocean Oxygen Network), this could mean the creation of marine protected areas or no-take zones in areas used by animals to escape the depletion of oxygen or to switch to fishing for fish that are not threatened by declining oxygen levels.
- Improving the monitoring of oxygen levels in the world's oceans. Scientists do not know the precise location of low-oxygen areas. Improved monitoring, especially in developing countries, and numerical models will help identify the most threatened locations and determine the most effective solutions.
 
However, Professor Robert Diaz of the Virginia Institute of Marine Science, who reviewed the new study, says bitterly: " Currently, the increasing expansion of coastal dead zones and the decline of oxygen in the open ocean are not priority issues for governments around the world. Unfortunately, it will take severe and persistent fisheries mortality to realize the seriousness of oxygen depletion.. »
 
The new analysis was produced by GO2NE, an international working group established in 2016 by the Intergovernmental Oceanographic Commission of UNESCO. Kirsten Isensee, a member of the Commission, sounds the alarm: "The new analysis has been produced by GO2NE, an international working group set up in 2016 by the Intergovernmental Oceanographic Commission of UNESCO. Deoxygenation of the oceans is taking place around the world due to the human footprint, so we must also combat it on a global scale. ".
 
 
Sources: Science, The Guardian
 
Header image: Lack of oxygen caused the death of these corals in Bocas del Toro, Panama. The dead crabs in the photo also succumbed to the loss of marine oxygen. (Credit: Arcadio Castillo/Smithsonian)

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