These days the ocean is acidifying at a rate 10 times faster than it did during a similar upheaval 56 million years ago.

During those ancient days, researchers estimate that ocean acidity increased by about 100 percent in a few thousand years or more, and levels didn't bounce back to normal for another 70,000 years. Some species were able to adapt and evolve to such radical environmental changes, while others perished and died off. Also during this time, a wave of carbon dioxide (CO2) surged into the atmosphere, raising global temperatures, and scientists have long suspected that ocean acidification caused the crisis.

For the first time, researchers are using the chemical composition of fossils to reconstruct surface ocean acidity at the Paleocene-Eocene Thermal Maximum (PETM), a period of intense warming on land and throughout the oceans due to high CO2.

"This could be the closest geological analog to modern ocean acidification," study co-author Bärbel Hönisch, a paleoceanographer at Columbia University's Lamont-Doherty Earth Observatory, said in a statement. "As massive as it was, it still happened about 10 times more slowly than what we are doing today."

Since the Industrial Revolution, oceans have absorbed about a third of the carbon humans have pumped into the air, helping to cool the Earth. Consequentially, chemical reactions caused by that excess CO2 have made seawater grow more acidic, depleting it of the carbonate ions that corals, mollusks and calcifying plankton need to build their shells and skeletons.

"We are dumping carbon in the atmosphere and ocean at a much higher rate today - within centuries," said study co-author Richard Zeebe, a paleoceanographer at the University of Hawaii. "If we continue on the emissions path we are on right now, acidification of the surface ocean will be way more dramatic than during the PETM."

The studied fossils - ancient plankton taken from Japanese waters - reveal that the ocean pH has indeed dropped, and will continue to do so.

Researchers still aren't sure what caused the upheaval of CO2 into the atmosphere so long ago. They speculate that the Earth's warming may have sent methane from the seafloor into the air, triggering the aforementioned events.

Life was able to survive the last acidification process, and maybe most species can again, but there are already signs that marine life is in trouble.

"The real unknown is how individual organisms will respond and how that cascades through ecosystems," lead author Donald Penman commented.