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.

Though the reason for the carbon upsurge back then remains a source of debate, scientists believe that the doubling of harmful emissions drove up global temperatures by about six degrees Celsius and caused big losses of ocean life.

Oceans are particularly vulnerable because they soak up excess carbon dioxide from the air which turns the waters more acidic, a state that can kill corals, mollusks and other forms of reef and shell organisms.

"We know that life during past ocean acidification events was not wiped out -- new species evolved to replace those that died off," said lead author Barbel Honisch, a paleoceanographer at Columbia University's Lamont-Doherty Earth Observatory.

"But if industrial carbon emissions continue at the current pace, we may lose organisms we care about -- coral reefs, oysters, salmon."

Honish and colleagues said the current rate of ocean acidification is at least 10 times faster than it was 56 million years ago.

"The geological record suggests that the current acidification is potentially unparalleled in at least the last 300 million years of Earth history, and raises the possibility that we are entering an unknown territory of marine ecosystem change," said co-author Andy Ridgwell of Bristol University.

The UN Environment Program released a report in 2010 that warned carbon emissions from fossil fuels may bear a greater risk for the marine environment than previously thought.

Rising acidity levels have an impact on calcium-based lifeforms, ranging from tiny organisms called ptetropods that are the primary food source, to crabs, fish, lobsters and coral, it said.

The UN report called for cuts in human-made CO2 emissions to reduce acidification and support for further work to quantify the risk and identify species that could be most in peril.