Earth continues to hit temperature and greenhouse gas milestones—just a couple of months ago, multiple stations measured carbon dioxide concentrations in the atmosphere of 400 parts per million, the highest in several million years. Many studies have tried to estimate how much and how rapidly the two great ice sheets covering Greenland and Antarctica might melt—and the one reassuring point has been the apparent relative stability of the eastern (and, by far, larger) half of the Antarctic Ice Sheet. Now, a new study of past melting in East Antarctica suggests that over the long haul, the “stable” ice sheet may be more vulnerable to warming than thought.

To study possible future melting of the ice sheets, many scientists look to the past. Current warm temperatures and high greenhouse gas conditions are reminiscent of the warm Pliocene Epoch that lasted from 5.3 million to 2.6 million years ago. “Early and middle Pliocene global temperatures and greenhouse gas concentrations are probably the closest analog in Earth’s history to the present climate on this planet and the climate conditions we will encounter before the end of this century,” says Claus-Dieter Hillenbrand, a sedimentologist at the British Antarctic Survey in Cambridge, U.K., who was not involved in the study.