The world's oceans appear to be acidifying faster than at any time in the last 300 million years, and sea life may well be unable to adapt.
The last time acidification took place anywhere near this fast was during the Paleocene-Eocene Thermal Maximum, or PETM.
In the last hundred years, rising carbon dioxide from human activities has lowered ocean pH by 0.1 unit, and the Intergovernmental Panel on Climate Change (IPCC) predicts that pH will fall another 0.2 units by 2100, raising the possibility that we may soon see the same sort of ocean changes as those observed during the PETM.
"What we're doing today really stands out in the geologic record," says lead author Bärbel Hönisch, a paleoceanographer at Columbia University's Lamont-Doherty Earth Observatory.
"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, oysters, salmon."
The oceans act like a sponge to draw down excess carbon dioxide from the air, which then reacts with seawater to form carbonic acid.
If too much carbon dioxide enters the ocean too quickly, it can deplete the carbonate ions that corals, mollusks and some plankton need for reef and shell-building.
And in a review of hundreds of paleoceanographic studies, the researchers say, only the PETM is comparable.
About 56 million years ago, a mysterious surge of carbon into the atmosphere warmed the planet and turned the oceans corrosive. In about 5,000 years, atmospheric carbon doubled to 1,800 parts per million (ppm), and average global temperatures rose by about 6 degrees Celsius.
The carbonate plankton shells littering the seafloor dissolved, leaving the brown clay layer that scientists see in sediment cores today.
As many as half of all species of benthic foraminifera, a group of one-celled organisms that live at the ocean bottom, went extinct, suggesting that deep-sea organisms higher on the food chain may have also disappeared, says Ellen Thomas, a paleoceanographer at Yale University.
"It's really unusual that you lose more than 5 to 10 percent of species," she said.
Scientists estimate that ocean acidity may have fallen as much as 0.45 units as the planet vented stores of carbon into the air.
"The ocean acidification we're seeing today is unprecedented, even when viewed through the lens of the past 300 million years - a result of the very fast rates at which we're changing the chemistry of the atmosphere and oceans," says Candace Major, program officer in the National Science Foundation's (NSF) Division of Ocean Sciences, which funded the research.