Volcanoes blamed for prehistoric warming

The culprit, the researchers reported Thursday in the journal Science, was a series of volcanic eruptions that set off a chain reaction releasing massive amounts of carbon into the atmosphere.

The eruptions occurred on the rift between two continental plates as Greenland and Europe separated.

In 10,000 years -- a blip in Earth's history -- the polar seas turned into tropical baths, deep-sea microorganisms went extinct, and mammals migrated poleward as their habitats warmed.

It took about 200,000 years for the atmospheric carbon to transfer to the deep ocean, allowing the planet to cool.

The event, known as the Paleocene-Eocene Thermal Maximum, was discovered in the early 1990s. Since then, scientists have studied it to better predict how Earth will respond to the current buildup of greenhouse gases.

The ancient warming was sparked by the release of 1,500 to 4,000 gigatons of carbon over several thousand years, scientists estimate. By comparison, emissions from human activities are about 7 gigatons a year -- a much faster rate.

During the thermal maximum, "carbon was released over thousands of years," said James Zachos, an earth sciences professor at UC Santa Cruz, who was not involved in the study. "We're going to do it in a few centuries."

Topic of dispute

The cause of the ancient warming has been a source of scientific debate.

In the latest study, researchers from the United States and Denmark analyzed volcanic ash found on basalt cliffs in Greenland and buried under the floor of the North Atlantic Ocean. The samples showed that the timing of the eruptions corresponded to the ancient warming.

Scientists knew that volcanic eruptions alone would not provide enough greenhouse gases to account for the warming -- a jump of more than 9 degrees Fahrenheit.

Previous research has suggested two possible sources of carbon: ocean floor sediments containing chemicals known as methyl hydrates, and land sediments rich in organic material.

The new study suggests that the eruptions triggered a chain reaction involving the land sediments.