The world's oceans have lost more than two percent of their oxygen since 1960, with potentially devastating consequences for sea plants and animals, marine scientists said Wednesday.
In those five and a half decades, parts of the oceans devoid of oxygen, called anoxic waters, have quadrupled, said a study in the science journal Nature.
And the production and flow of nitrous oxide, a powerful greenhouse gas, "will probably have increased," it said.
Oceans cover nearly three-quarters of the Earth's surface, provide about half of the oxygen we breathe and feed billions of people every year.
In a comment on the study that Nature also published, research scientist Denis Gilbert of Fisheries and Oceans Canada wrote that a "two percent decrease of ocean oxygen content may not sound like much."
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But, he warned, "the implications of this for marine ecosystems could be severe in parts of the ocean where oxygen is already low."
The report found that the largest decrease happened near areas where oxygen was already low, in so-called "dead zones," where oxygen levels declined by four percent every decade.
Most oxygen was lost in the Equatorial and North Pacific Ocean, the Southern Ocean and the South Atlantic Ocean.
"Oxygen data in the Arctic, Equatorial and North Pacific... and Southern Ocean show a continuous decrease, and together are responsible for 60 percent of the global oceanic oxygen loss," the study reported.
The authors said they needed to conduct more research to determine how much of the oxygen loss was due to global warming and how much was related to natural climate cycles.
The study also reiterated an older warning that the loss of oxygen would accelerate - with predictions of a one to seven percent decline by 2100.
The findings "should ring yet more alarm bells about the consequences of global warming," Gilbert said.
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