World & Nation

Scientists believe they have solved the mystery of 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 Thursday 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 [HTML_REMOVED] a blip in Earth's history [HTML_REMOVED] 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. Since then, scientists have studied it to better 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 a year [HTML_REMOVED] a much faster rate.

"[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 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.

[HTML_REMOVED]Los Angeles Times/Washington Post wire service