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The last time concentrations of the greenhouse gas carbon dioxide were as high as they are today, big chunks of the seemingly stable East Antarctic ice sheet melted and helped raise global sea levels more than 65 feet higher than they are now, a new study suggests.
Scientists have long known that seas were higher during the Pliocene, a geological epoch that ran from 5.3 million to 2.6 million years ago. At the time, atmospheric carbon dioxide levels were similar to today's 400 parts per million (ppm).
"Overall, it was a warmer climate than today, but similar to what we expect to reach by the end of this century," Carys Cook, a graduate student at the Grantham Institute for Climate Change at Imperial College London and the study's lead author, told NBC News in an email.
The West Antarctic and Greenland ice sheets were likely completely melted at the time, she added. The fate of the East Antarctic ice sheet has been less clear, though at least some of it must have melted to fully account for the highest global sea levels predicted by some reconstructions of the ancient Earth.
Cook and her colleagues studied the chemical composition of sediments drilled from the ocean floor near East Antarctica. They identified the signature of a specific type of rock "only found in large quantities hundreds of kilometers inland from the current ice sheet edge," Cook said.
The signature was strongest during the warmest phases of the Pliocene. "The only way it could have been eroded is by retreating the East Antarctic ice sheet inland, which means it must have melted significantly," she explained.
Cook and colleagues suggest that much of the ice that melted was in basins that were below sea level, putting it in direct contact with the seawater. As the ocean warmed, the ice was more vulnerable to melting.
That interpretation fits with recent airborne surveys that revealed large under-ice fjords in this part of Antarctica that appeared geologically young and carved by ice, and not as a result of plate tectonics, according to Duncan Young, a geophysicist at the University of Texas at Austin, who flew some of the surveys.
"This work reinforces that result," he told NBC News in an email. The new study is also "a shot in favor" of the argument that the East Antarctic ice sheet is less stable than previously believed, "which may be significant for future sea level change estimates," said Duncan, who was not involved in the new research.
Given the similarity between the Pliocene's estimated atmospheric carbon dioxide levels and those of today, scientists consider the epoch an analog for understanding how the present-day climate will evolve.
"What the study shows is that there is a clear record of rapid(-ish) sea level response to past climate shifts," Ted Scambos, an Antarctic ice expert at the National Snow and Ice Data Center in Boulder, Colo., said in an email to NBC News. He was not involved in the new research.
While the East Antarctic basins are covered in ice today, they might begin to melt as the oceans continue to warm, Scambos said. He noted that a mile-thick, Colorado-sized chunk of ice sloughing into the ocean would have a "big impact" on sea levels.
"And what we're seeing in other parts of Antarctica and Greenland today tells us that the transitions can be very abrupt by geologic standards," Scambos said. "They are mercifully more manageable by human standards, at least if we decide to start managing."
John Roach is a contributing writer for NBC News. To learn more about him, visit his website.