Scientists believe they have figured out 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 higher than today's.
The culprit, the researchers reported yesterday in the journal Science, was a series of volcanic eruptions that set off a chain reaction releasing huge 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 blink in Earth's history - the polar seas turned into tropical baths, deep-sea-dwelling microorganisms became extinct, and mammals migrated poleward as their habitats warmed. It took 200,000 years for the atmospheric carbon to be transferred to the deep ocean, enabling the planet to cool.
The event, known as the Paleocene-Eocene Thermal Maximum, was discovered in the 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 - a much faster rate.
During the thermal maximum, "carbon was released over thousands of years," said James Zachos, a professor of earth sciences at 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.
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 the eruptions triggered a chain reaction involving the land sediments.
Hot lava flows "cooked" organic material as the continents divided, releasing greenhouse gases, said co-author 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.
Alan Zarembo writes for the Los Angeles Times.