Ice sheets are large and complex things. Figuring out how quickly—and where—they’ll melt as the world warms is a monumental task. We worry about some portions (like the vulnerable West Antarctic Ice Sheet) collapsing entirely, but we know some other parts will be disappearing in the foreseeable future. Records from past periods of climate change are important guides here. What better way to figure out what will happen than to see what has happened before?

For the Greenland Ice Sheet, there has been some debate about how small it has gotten in past warm periods where we know sea level was higher than it is today. The problem is that Greenland's ice doesn’t go nearly so far back in time as Antarctica’s. Snowfall is greater here, and ice flows more quickly to the edges of the continent where it disappears from the pages of the history we read from ice cores. Few Greenland cores go back more than about 110,000 years, failing to tell us about the last interglacial warm period.

But at the bottom of a couple of ice cores from the thickest parts of the ice sheet, there is some messed up ice we know could be a lot older. Figuring out how old is another matter. Without an orderly stack of annual layers to count back through, there aren’t many reference points preserved in the ice. To make things worse, water can refreeze to the underside of glaciers, so it might not have even been glacial ice in the first place.

Nonetheless, between studying that ice and using models to simulate the Greenland ice sheet in warmer climates, researchers have concluded that at least some portion of Greenland has remained ice-covered during warm periods in the last million years. That basal ice, they have figured, could even include some remnants from the birth of the modern Greenland Ice Sheet 2.4 million years ago, long buried and trapped in these spots.

To get some firmer numbers, a team led by Princeton’s Audrey Yau took advantage of the fact that any real glacial ice down there will still contain samples of the atmosphere from the time it fell as snow. There are several isotopes of argon in the atmosphere, and most are doing their noble gas thing—staying out of pretty much everything and maintaining fairly constant concentrations through time. Argon-40, however, is different. It is constantly being produced by the decay of radioactive potassium-40 within the Earth, and so it is very (very) slowly accumulating in the atmosphere. The older the sample of air, the less argon-40 you’ll find in it.

Using some isotopes of other gases, the researchers determined that the clear sections of basal ice, at least, really are made of ancient snow. Some muddy sections of ice, on the other hand, look like refrozen water lacking samples of atmospheric gas.

The argon-40 measurements might include molecules of argon-40 that came from that mud beneath the ice sheet, so they can only tell you that something is at least a certain age. (Like, say, the apparent age of someone who has had plastic surgery.) Clear ice from the Dye-3 core in southern Greenland was estimated to be at least 400,000 (±170,000) years old. Ice from the GRIP ice core in central Greenland was even older, coming back at 970,000 (±140,000) years old.

Meanwhile, some muddy Dye-3 ice was dated to 210,000 years… in the future. You needn’t rethink the fundamental reality of the cosmos, though—that just shows you the importance of using good, uncontaminated samples. The minerals in the mud are a source of argon-40, so some extra built up in the surrounding ice.

So what do these (real) numbers mean? They confirm that the Greenland Ice Sheet didn’t completely melt away during a pair of key warm interglacial periods about 130,000 and 430,000 years ago. At both times, sea level was probably at least six meters higher than it is today, either because the Arctic was several degrees Celsius warmer (130,000 years ago) or because the interglacial period was warm and especially long-lasting (430,000 years ago). Because several cycles in Earth’s orbit change independently, not all interglacial periods are the same.

As researchers work to understand just how sensitive the Greenland Ice Sheet is to warming, datapoints like these—which fall in line with previous estimates—help them evaluate ideas and models. How much warming does it take to melt Greenland clear of ice, raising sea level about seven meters? More than it has seen in the last million years or so, apparently.

Earth and Planetary Science Letters, 2016. DOI: 10.1016/j.epsl.2016.06.053 (About DOIs).