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Are we causing a mass extinction in our oceans?

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The rate of carbon dioxide release matters, Kump said, because life has to have time to adapt.

"It's: Would you rather be squeezed or punched?" Kump said. "The Permian extinction was a squeeze that gradually got tighter and tighter … It may ultimately have been more fatal than the punch we are going to get, but the punch is going to hurt more."

Crumbling at the base

The parallel in ocean chemistry between the past and present isn't limited to oxygen depletion. The Permian ocean became more acidic as the climate changed, just as the modern ocean is doing.

The ocean has absorbed about 30 percent of human-produced carbon dioxide to date, and as a result, its waters have experienced a 30 percent increase in acidity, according to Richard Feely, a senior scientist with NOAA's Pacific Marine Environmental Laboratory. If trends continue, ocean acidity will increase by up to 150 percent by the end of this century, he said.

Increased carbon dioxide and ocean acidity played a role in all of the Big Five mass extinctions, but in those cases the change in acidity was tens to hundreds of times slower than what's happening now. When changes happen quickly, "the ocean system itself doesn't have time to adapt," Feely said.

Increasingly acidic waters affect a number of species that are key parts of the ocean's ecosystems.

Acidification interferes with the ability of oysters, marine snails and other creatures to build shells or skeletons from calcium carbonate. In oyster hatcheries on the West Coast of the United States, more-acidic waters prevent oyster larvae from forming shells, and have been shown to dissolve the shells of pteropods — small marine snails that feed salmon and other commercially caught fish — from around the living creatures, Feely said.

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