4 Jan 2018 - The Guardian - Areas starved of oxygen in open ocean and by coasts have soared in recent decades, risking dire consequences for marine life and humanity
Ocean dead zones with zero oxygen have quadrupled in size since 1950, scientists have warned, while the number of very low oxygen sites near coasts have multiplied tenfold. Most sea creatures cannot survive in these zones and current trends would lead to mass extinction in the long run, risking dire consequences for the hundreds of millions of people who depend on the sea.
Climate change caused by fossil fuel burning is the cause of the large-scale deoxygenation, as warmer waters hold less oxygen. The coastal dead zones result from fertiliser and sewage running off the land and into the seas.
The analysis, published in the journal Science, is the first comprehensive analysis of the areas and states: “Major extinction events in Earth’s history have been associated with warm climates and oxygen-deficient oceans.” Denise Breitburg, at the Smithsonian Environmental Research Center in the US and who led the analysis, said: “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.”
“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 recoveries in Chesapeake Bay in the US and the Thames river in the UK, where better farm and sewage practices led to dead zones disappearing.
However, Prof Robert Diaz at the Virginia Institute of Marine Science, who reviewed the new study, said: “Right now, the increasing expansion of coastal dead zones and decline in open ocean oxygen are not priority problems for governments around the world. Unfortunately, it will take severe and persistent mortality of fisheries for the seriousness of low oxygen to be realised.
The oceans feed more than 500 million people, especially in poorer nations, and provide jobs for 350 million people. But at least 500 dead zones have now been reported near coasts, up from fewer than 50 in 1950. Lack of monitoring in many regions means the true number may be much higher.
The open ocean has natural low oxygen areas, usually off the west coast of continents due to the way the rotation of the Earth affects ocean currents. But these dead zones have expanded dramatically, increasing by millions of square kilometres since 1950, roughly equivalent to the area of the European Union.
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