WASHINGTON (Reuters) -- Ancient volcanoes may have caused a dramatic warming of the Earth's atmosphere that raised sea temperatures and killed off many marine species, resulting in a "planetary emergency," U.S. and European scientists said Thursday.

The discovery shows the impact of the release of large amounts of greenhouse gases into the atmosphere and may shed light on planetary changes wrought by more recent causes of global warming, the scientists reported in the journal Science.

The giant prehistoric eruptions, they believe, may have been the catalyst that pushed Greenland and northwest Europe apart to create the North Atlantic Ocean.

"There has been evidence in the marine record of this period of global warming, and evidence in the geologic record of the eruptions at roughly the same time, but until now there has been no direct link between the two," Robert Duncan of Oregon State University, who worked on the study, said in a statement.

Duncan and a team of international scientists said the volcanoes erupted off the coast of Greenland and in the western British Isles about 55 million years ago, spewing carbon dioxide and methane into the atmosphere.

That triggered a 222,000-year period of warming that raised sea surface temperatures by 5 degrees Celsius (9 degrees Fahrenheit) in the tropics and more than 6 degrees Celsius (11 degrees Fahrenheit) in the Arctic and increased the acidity of the oceans.

The resulting "planetary emergency," as scientists have called it, wiped out 30 percent to 50 percent of the planet's deep sea creatures.

"We think the first volcanic eruptions began about 61 million years ago and then it took another 5 million years for the mantle to weaken, the continent to thin and the molten material to rise to the surface," Duncan said.

"It was like lifting a lid. The plate came apart and gave birth to the North Atlantic Ocean."

Scientists made the link between the volcanoes and the warming period, known as the Paleocene-Eocene thermal maximum, by matching layers of ash in east Greenland with those in marine sediments in the Atlantic Ocean.

They think greenhouse gases from lava flows and the heating of organic-rich sediments along the eastern edge of Greenland caused the global warming and changes in ocean chemistry.

While these volcanic eruptions caused warming of the atmosphere, they are extremely rare, scientists said.

Some more recent volcanoes, such as the eruption of Mount Tambora in Indonesia, actually cooled the Earth by adding more sulphate aerosols to the upper atmosphere, which reflected sunlight back into space, Dr. Ellie Highwood, senior lecturer in climate physics at the University of Reading in Britain, said in a statement.

"The type of volcano eruption described here has not been seen over the past 1,000 years, and therefore volcanoes are unlikely to have contributed significantly to recent climate change," Highwood said.

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