The last time massive amounts of greenhouse gases were released into the atmosphere, the North Pole was an ice-free expanse of open ocean that was teeming with tropical organisms, a study has found.
Scientists have discovered that the complete disappearance of the Arctic sea ice, 55 million years ago, coincided with a dramatic increase in concentrations of carbon dioxide or methane in the atmosphere - which must have caused global warming.
After analysiing the sediments on the floor of the Arctic Ocean, an international team of researchers working for the Arctic Core Expedition (Acex) came to the conclusion that the region was once extremely warm, unusually wet, and ice-free.
The scientists also found that the Arctic is currently experiencing one of the fastest temperature rises on record, with more sea ice melting each summer than at any time in hundreds and possibly thousands of years.
The Acex research team drilled frozen sedimentary cores from the ocean floor, which can be dated to 55 million years ago, a period known as the palaeocene-eocene thermal maximum (PETM). Surface temperatures in some parts of the world were then 8C (15F) higher than now.
"Building a picture of ancient climatic events is a lot like putting together a jigsaw puzzle, and what Acex allowed us to do was fill in a black section of the PETM picture," said Gerald Dickens, a geochemist at Rice University in Houston, Texas.
"It's difficult to overestimate the importance of this kind of experimental evidence. The Acex cores clearly show that the Arctic got very warm and wet during the PETM. Even tropical marine plants thrived in the balmy conditions [of the Arctic]," he said. The ice cores were drilled from the Lomonosov Ridge, an under-sea mountain range that stretches from northern Greenland, across the Polar Sea, to Siberia in northern Russia.
The task involved three icebreaker ships: a drilling vessel and two further ice-breakers, which protected the drilling operation from being crushed by drifting ice floes and metre-thick pack ice. This was the first time that scientists have had access to sedimentary cores drilled deep from under the Arctic.
Dr Dickens said the fossils of some microscopic plants, which can trigger algal blooms, suddenly become commonplace in the parts of the cores that are about 55 million years old. These organisms lived only in the tropics prior to the PETM warm period, suggesting the Arctic was also warm and ice-free during this period.
Scientists are not sure what caused the warming, which occurred over a relatively rapid geological period of 100,000 years. But they think it may have been due either to the release of vast deposits of carbon-containing methane stored on the sea floor, or the massive release of carbon dioxide from volcanic eruptions.
"The magnitude of the carbon input at the PETM outset is truly enormous. If it were all volcanic, you'd need something like a Vesuvius-sized eruption each day for centuries, which seems very unlikely," Dr Dickens said.
Another possibility is that there was a sudden release of massive amounts of methane and carbon dioxide that had been locked away in the deep ocean in a frozen form known as ocean gas hydrates. Even today, it is estimated that there is more carbon locked away as ocean gas hydrates than all of the oil and gas reserves of the world combined.
The latest study suggests this huge reservoir of carbon has been released in the past with devastating effects on the climate.