Scientists think they now know what caused the most rapid global warming in known geologic history, a cataclysmic temperature spike 55 million years ago driven by concentrations of greenhouse gases hundreds of times greater than today.

The culprit, the researchers reported yesterday in the journal Science, was a series of volcanic eruptions that set off a chain reaction releasing massive quantities 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 dwelling microorganisms went extinct, and mammals migrated poleward as their habitats warmed. It took about 200,000 years for the atmospheric carbon to be transferred to the deep ocean, allowing the planet to cool.

The event, known as the Paleocene-Eocene Thermal Maximum, was discovered in the early 1990s. Scientists have studied it to understand 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 each year - a much faster rate.

'A few centuries'

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

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 the timing of the eruptions corresponded to the ancient warming.

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

'Cooked' material

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. Hot lava "cooked" organic material as the continents divided, releasing greenhouse gases, said coauthor Robert Duncan, a professor of oceanography and atmospheric sciences at Oregon State University.

He described the organic material as the "turbocharger" that accelerated the warming.

Some scientists say the mystery is not quite solved.

James Kennett, an oceanographer at UC Santa Barbara, said he was not convinced that the localized eruptions were enough to set off a global warming.

Jerry Dickens, an oceanographer at Rice University in Houston, said there was not sufficient evidence showing that a volcano chain reaction would provide enough carbon emissions for such a sharp warming.

Still, the eruptions' timing is difficult to dismiss as coincidence, other researchers said.

"This looks like the perfect fit," Zachos said.