The Arctic may have crossed key threshold, emitting billions of tons of carbon into the air, in a lo

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13. On average, world's oceans have lost 2% of oxygen content since the 1950s; some areas far worse

The world's oceans are struggling to breathe, rapidly running out of oxygen at an unprecedented rate. Climate change is dangerously exacerbating the issue, scientists warned in a new study.

The International Union for Conservation of Nature (IUCN) released the largest report of its kind — combining the efforts of 67 scientists from 17 countries — at the global climate summit in Madrid on Saturday. It found that the oxygen level of the ocean has declined by about 2% since the 1950s, and the volume of water completely depleted of oxygen has quadrupled since the 1960s.

Sixty years ago, only 45 ocean sites suffered from low oxygen levels. That number skyrocketed to 700 in 2011. According to the study, about 50% of oxygen loss in the upper part of the ocean is a result of temperature increase.
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"With this report, the scale of damage climate change is wreaking upon the ocean comes into stark focus," Dr. Grethel Aguilar, IUCN acting director general, said in a statement. "As the warming ocean loses oxygen, the delicate balance of marine life is thrown into disarray."

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https://www.cbsnews.com/news/worlds-oceans-are-losing-oxygen-at-a-dangerous-unprecedented-rate-as-temperatures-rise/

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To tease out what was primarily responsible for all this death in the oceans—in this, “the Great Dying”—Penn and his colleagues developed a climate model, not dissimilar to those used to project future warming on our own modern world. Only in Penn’s model, the continents were reunited. This was Pangaea, the mythic supercontinent that reached its apotheosis in the Permian period, joining Morocco with New Jersey and India with Antarctica. Then the team lit this ancient world on fire.

By jacking up the CO2 in their model high enough, Penn and his colleagues were able to re-create the scorching temperatures of the end-Permian mass extinction, searing their ancient climate model maps in worrying shades of red. The flip side of a hot ocean is one with less oxygen, and it has long been known that the oceans of the end-Permian were gasping for the stuff. This paleoceanographic fact has been uncovered by geologists who have found the sickly presence of laminated, pyrite-rich ocean rocks in end-Permian rock outcrops around the word—from the Salt Range in Pakistan to the old whaling redoubt of Spitsbergen in the Arctic Ocean. Even subtler, uranium isotopes in rocks from the catastrophe whisper dark rumors about the asphyxiation of the entire ancient ocean.

By driving up the temperature in his model, Penn re-created this end-Permian oxygen loss as well. This is because oxygen is less soluble in hot water, so heat alone can cause oxygen to plummet in the ocean. But a hotter ocean is also a more sluggish and stratified one, so the depths become starved of oxygen’s delivery as well. Even worse, the hotter it gets, the more oxygen animals need to power their metabolism, so hot water quickly creates a crisis of supply and demand. By populating his model ancient ocean with modern creatures, such as sharks and crabs and corals—creatures with a variety of tolerances to things like deoxygenation, temperature, and pH—and then letting all climate hell break loose, Penn found that it was the heat-driven loss of oxygen in the ocean, more than any other factor, that could explain the end of the world.
While essentially nothing was spared in this mass extinction, it appears from the fossil record that life at the poles was especially marked for destruction. And this makes sense in a world losing its oxygen. There’s more oxygen in colder waters at the poles, so the creatures there have adapted to it—and are especially sensitive to its diminishment. Meanwhile, tropical creatures are adapted to warmer, more oxygen-poor waters. So if anyone is to survive on a world that’s about to become outrageously hot and lose three-quarters of its oxygen, it will be those hardy inhabitants of the low latitudes. And that’s what’s seen in the fossil record. The tropics still faced an unthinkable cataclysm, but they fared mildly better. The predicament for polar creatures was not unlike those luckless creatures that today inhabit the tops of mountains, and that will have no higher altitude to which to escape in the coming decades. “They’re shit out of luck,” Deutsch says.

When the ocean loses its oxygen, it’s something of a great leveler compared with other hypothesized mass-extinction mechanisms. Ocean acidification, for instance (what happens when too much CO2 reacts with seawater), has previously been proposed as the great killer of the end-Permian. But while acidification can have a surprisingly variable effect on the survival of different kinds of sea life, there is hardly any selectivity at all when oxygen disappears from an ecosystem. Everyone dies, matching the near-universal signal of slaughter in the ancient ocean. “This study suggests we should be worrying much more about hypoxia than about ocean acidification,” Deutsch says. “There’s vastly more resources being put into [studying] organisms’ responses to pH in seawater than there is into understanding temperature-dependent hypoxia. I think that the field has basically allocated those resources in exactly the wrong way.”

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https://www.theatlantic.com/science/archive/2018/12/oxygen-loss-during-mass-extinction/577537/