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.