Recent studies show that the last time concentrations of the greenhouse gas carbon dioxide were as high as they are today, huge chunks of the East Arctic ice sheet melted and raised global sea levels by more than 65 feet higher than they are right now.

Experts have long known that sea levels were higher during the Pilocene, a geological epoch that lasted from 5.3 million to 2.6 million years ago according to NBC News.

During the time period atmospheric carbon dioxide levels were similar to today's 400 parts per million.

"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, said to NBC News.

The ice sheets were completely melted during that time according to Cook, though some of it must have melted to account for the highest global sea levels calculated by some experts of the ancient Earth.

Cook and her colleagues studied the chemical composition of sediments drilled from the ocean floor near East Antarctica where they were able to identify 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 according to NBC News.

"The only way it could have been eroded is by retreating the East Antarctic ice sheet inland, which means it must have melted significantly," Cook said.

Cook and her colleagues have determined that a lot 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 up, the ice was able to melt away.

"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 to NBC News.

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