High levels of pollution may be turning the planet's oceans acidic at a faster rate than at any time in the past 300 million years, with unknown consequences for future sea life, researchers said Thursday.

The acidification may be worse than during four major mass extinctions in history when natural pulses of carbon from asteroid impacts and volcanic eruptions caused global temperatures to soar, said the study in the journal Science.

An international team of researchers from the United States, Britain, Spain, Germany and the Netherlands examined hundreds of paleoceanographic studies, including fossils wedged in seafloor sediment from millions of years ago.

They found only one time in history that came close to what scientists are seeing today in terms of ocean life die-off -- a mysterious period known as the Paleocene-Eocene Thermal Maximum about 56 million years ago.