More than 400,000 years ago, the Greenland ice sheet collapsed, causing worldwide sea levels to rise between 13 and 20 feet, reports a new study.

Unlike current global warming due to the buildup of atmospheric CO2, the ice sheet melted during this long warm period between ice ages because of changes to the Earth's orbit around the sun.

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“The climate 400,000 years ago was not that much different than what we see today, or at least what is predicted for the end of the century,” Anders Carlson, an associate professor at Oregon State University and co-author on the study, said in a press release. “The forcing was different, but what is important is that the region crossed the threshold allowing the southern portion of the ice sheet to all but disappear."

Other studies show the sea level rise during that time could have been greater than 40 feet, the researchers reported. The ice sheet melted beyond Greenland's southern edges, but the melting didn't reach the center, which hasn't been without ice for 1 million years, Carlson said.

The researchers studied glacial stream sediment cores taken off of the coast of Greenland.

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“Each terrain has a distinct fingerprint,” Carlson said. “They also have different tectonic histories and so changes between the terrains allow us to predict how old the sediments are, as well as where they came from. The sediments are only deposited when there is significant ice to erode the terrain. The absence of terrestrial deposits in the sediment suggests the absence of ice.

“Not only can we estimate how much ice there was,” he said, “but the isotopic signature can tell us where ice was present, or from where it was missing. This may give us a better sense of what may happen in the future as temperatures continue rising."

The study was funded by the National Science Foundation and published this week in the journal Nature.

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