The Greenland ice sheet contains enough water to raise global sea levels 24 feet should it all melt. And a massive melt-out is exactly what seems to have happened about a million years ago, according to a groundbreaking analysis of a unique geologic sample from Greenland’s rocky underbelly.
Well over half of the Greenland ice sheet appears to have melted to bedrock in the not-too-distant past, when temperatures weren’t much warmer than they are today, according to new research led by paleoclimatologist Joerg Schaefer of Columbia University’s Lamont Observatory. Schaefer’s analysis, which is published today in the journal Nature, casts doubt on long-held assumptions about the stability of Greenland’s 684,000 cubic mile mantle of ice, suggesting there are unknown mechanisms that can trigger rapid melting.
Naturally, this has scientists more concerned than ever about future sea level rise, as our planet continues to warm and Greenland sheds ice at an ever-increasing clip.
This story begins in the late 1980s, when two teams of geologists, one American and one European, embarked on unprecedented drilling expeditions to acquire ice cores representing the entire length of the Greenland ice sheet from its thickest section, a site in central Greenland where the ice is piled up about 10,000 feet (3,000 meters). It took each team five summers to drill through the ice, but the prize was a series of cores that have allowed Earth scientists to reconstruct a world-class record of Arctic climate history.
“This rock is probably the most unique terrestrial rock sample ever,” Schaefer said. “It’s as precious as the moon rock that was retrieved in the 1970s.”
The GISP2 bedrock samples, as they’re called, have been sitting in storage for nearly a quarter century. Only very recently, with the advent of high-resolution accelerator mass spectrometers, have scientists been able to probe these rocks for trace quantities of atoms that can tell us about the land surface’s history.
The cosmogenic nuclides in the GISP2 bedrock tell us something astonishing. This region of Greenland, which remember, is today covered in 10,000 feet of ice, was exposed and ice-free as recently as 1.1 million years ago, for at least one chunk of time, and perhaps many, spanning 280,000 years over the Quaternary (the last 2.6 million years of Earth history).
It’s worth noting that the analysis is based on a single sample of rock. But that sample was taken from underneath the thickest part of the Greenland ice sheet, and most models agree that this would have been one of the last places to melt out were the ice sheet to retreat. The implication, therefore, is that most of Greenland was ice-free when beryllium-10 was raining down on GISP2.
“I would say that the leading hypothesis [now] is that not too much warming is required to remove much or most of Greenland’s ice to expose rock at GISP2,” study co-author Richard Alley told Gizmodo in an email. Alley added, however, that scientists don’t think Greenland melted completely in recent history.
Patches of Greenland’s ice may be quite stable. Nevertheless, the conclusion of the Schaefer’s study, that the majority of the ice sheet melted rapidly in recent history, is certainly cause for concern.
“I think it’s unquestionably one of the most important papers on Greenland to come out in a long time,” Eric Steig, a climate scientist at the University of Washington who was not involved with the study told Gizmodo. “[It] underscores the already strong view that Greenland is going to gradually melt and contribute significantly to sea level rise in the future, as is already happening.”