The fact that over 80 percent of a landmass, more than a fifth the size of the United States, is covered in ice and is still called Greenland has been a source of frequent puzzlement for many, who wonder at the misnomer. And while there is evidence that the world’s largest island (smaller than Australia, which counts as a continent) was greener in the past than it is now, a new study shows Greenland could have been completely ice-free 1.1 million years ago.

In the Pleistocene epoch, which lasted from approximately 2.59 million years ago to 11,700 years ago, there was a period of 280,000 years when there was no ice in Greenland, the study, led by Joerg Schaefer, a paleoclimatologist at Columbia University’s Lamont-Doherty Earth Observatory, suggests. According to Schaefer and his co-authors, this period was between 1.4 million and 1.1 million years ago.

They base their conclusions, which they call “the most conservative interpretation,” on the study of a bedrock core that was drilled from the base of the Greenland ice sheet, more than 10,000 feet (about 2 miles) under the surface, in 1993. It is the only sample of its kind, and while the ice from the core has been studied extensively, technology emerged only last year that allowed for detailed study of the 5 feet of rock that was drilled.

The researchers found traces of beryllium-10 and aluminum-26 isotopes, radioactive material produced by miniscule particles from outer space that are always bombarding Earth. Since the isotopes could only be created when the rock face was exposed to the sky (and not covered by ice), and their decay rate is a known constant, it allowed scientists to calculate how long ago the rock was free of ice.

The study is important because of the total amount of ice in Greenland. At 684,000 cubic miles, if all of it melted, global sea levels would rise by about 24 feet, which is about two floors of most buildings. That makes it imperative to understand the stability of the Greenland ice sheet, a matter of much debate and controversy.

“Unfortunately, this makes the Greenland ice sheet look highly unstable. If we lost it in periods of natural forcing, we may lose it again,” Schaefer said in a statement.

Most current models assume that the autonomous country, which is a part of the Danish Realm but is a part of North America, has been continuously covered by ice for the last million years. But the new study throws that into doubt.

Jeff Severinghaus, a paleoclimatologist at Scripps Institution of Oceanography who was not involved in the study, said in the statement: “We can now reject some of the lowest sea-level projections, because the models underpinning them assume continuous ice cover during the last million years.”

The study, published Wednesday in the journal Nature under the title “Greenland was nearly ice-free for extended periods during the Pleistocene,” is clear that the reason behind the loss of ice cover in the period it talks about was natural. But in the face of human-induced climate change and global warming (some estimates suggest Greenland has lost more than 1 trillion tons of ice in the last four years), it is important to understand exactly how the Greenland ice sheet works.

“This study shows we are missing something big about how the system works, and we need to find out what it is, fast,” Schaefer said.