The world's oceans may be turning acidic faster today from human carbon emissions¹ than they did during four major extinctions in the last 300 million years, when natural pulses of carbon sent global temperatures soaring, says a new study in Science. The study is the first of its kind to survey the geologic² record for evidence of ocean acidification（酸化） over this vast time period. "What we're doing today really stands out," said lead author Bärbel Hönisch, a paleoceanographer at Columbia University's Lamont-Doherty Earth Observatory³. "We know that life during past ocean acidification events was not wiped out -- new species evolved to replace those that died off. But if industrial carbon emissions continue at the current pace, we may lose organisms we care about -- coral reefs, oysters⁴, salmon⁵."

The oceans act like a sponge to draw down excess carbon dioxide from the air; the gas reacts with seawater to form carbonic acid, which over time is neutralized⁶ by fossil carbonate shells on the seafloor. But if CO2 goes into the oceans too quickly, it can deplete⁷（耗尽） the carbonate ions that corals, mollusks（软体动物） and some plankton⁸ need for reef and shell-building.

That is what is happening now. In a review of hundreds of paleoceanographic studies, a team of researchers from five countries found evidence for only one period in the last 300 million years when the oceans changed even remotely as fast as today: the Paleocene-Eocene Thermal⁹ Maximum, or PETM, some 56 million years ago. In the early 1990s, scientists extracting sediments¹⁰ from the seafloor off Antarctica found a layer of mud from this period wedged between thick deposits of white plankton fossils. In a span of about 5,000 years, they estimated, a mysterious surge of carbon doubled atmospheric¹¹ concentrations, pushed average global temperatures up by about6 degrees C, and dramatically changed the ecological¹² landscape.

The result: carbonate plankton shells littering the seafloor dissolved, leaving the brown layer of mud. As many as half of all species of benthic foraminifers, a group of single-celled organisms that live at the ocean bottom, went extinct, suggesting that organisms higher in the food chain may have also disappeared, said study co-author Ellen Thomas, a paleoceanographer at Yale University who was on that pivotal Antarctic cruise. "It's really unusual that you lose more than 5 to 10 percent of species over less than 20,000 years," she said. "It's usually on the order of a few percent over a million years." During this time, scientists estimate, ocean pH -- a measure of acidity¹³--may have fallen as much as 0.45 units. (As pH falls, acidity rises.)