A long-lost ice core reveals that most of Greenland was green 416,000 years ago, and that its melting today risks sea level rises
During the Cold War, a secret US Army mission, at Camp Century in northwestern Greenland, drilled down through 4,560 feet of ice on the frozen island – and then kept drilling to pull out a twelve-foot-long tube of soil and rock from below the ice.
This icy sediment was then lost in a freezer for decades.
It was accidentally rediscovered in 2017 and shown to hold not just sediment but also leaves and moss, remnants of an ice-free landscape, perhaps a boreal forest.
But how long ago were those plants growing – where today stands an ice sheet two miles thick and three times the size of Texas?
An international team of scientists was amazed to discover that Greenland was a green land only 416,000 years ago (with an error margin of about 38,000 years).
Until recently, geologists believed that Greenland was a fortress of ice, mostly unmelted for millions of years.
But, two years ago, using the rediscovered Camp Century ice core, this team of scientists showed that it likely melted less than one million years ago.
Other scientists, working in central Greenland, gathered data showing the ice there melted at least once in the last 1.1 million years – but until this study, no one knew exactly when the ice was gone.
Now, using advanced luminescence technology and rare isotope analysis, the team has created a starker picture: large portions of Greenland’s ice sheet melted much more recently than a million years ago.
The new study presents direct evidence that sediment just beneath the ice sheet was deposited by flowing water in an ice-free environment during a moderate warming period called Marine Isotope Stage 11, from 424,000 to 374,000 years ago.
This melting caused at least five feet of sea level rise around the globe.
Study co-lead, Paul Bierman of the University of Vermont, said: “It’s really the first bulletproof evidence that much of the Greenland ice sheet vanished when it got warm.”
- More detailed picture of Greenland ice sheet emerges
- Global warming reaches central Greenland
- Ice loss from north-eastern Greenland ‘significantly underestimated’
- Global sea level rise to accelerate above 1.8°C
Greenland is more sensitive
Understanding Greenland’s past is critical for predicting how its giant ice sheet will respond to climate warming in the future and how quickly it will melt.
Since about twenty-three feet of sea-level rise is tied up in Greenland’s ice, every coastal region in the world is at risk.
The new study provides strong and precise evidence that Greenland is more sensitive to climate change than previously understood – and at grave risk of irreversibly melting off.
Bierman, a geoscientist in UVM’s Rubenstein School of the Environment and Natural Resources and a fellow in the Gund Institute for Environment, said: “Greenland’s past … suggests a warm, wet, and largely ice-free future for planet Earth, unless we can dramatically lower the concentration of carbon dioxide in the atmosphere.”
Tammy Rittenour, a scientist at Utah State University and co-author on the new study, said: “We had always assumed that the Greenland ice sheet formed about two and a half million years ago – and has just been there this whole time and that it’s very stable.
“Maybe the edges melted, or with more snowfall it got a bit fatter – but it doesn’t go away, and it doesn’t dramatically melt back.
“But this paper shows that it did.”
Rare isotopes
At Rittenour’s lab, sediment from the Camp Century core was examined for what is called a ‘luminescence signal’.
In a specialised dark room, Rittenour’s team took pieces of the ice core sediment and exposed them to blue-green or infrared light, releasing the trapped electrons.
With some advanced tools and measures, and many repeated tests, the number of released electrons forms a kind of clock, revealing with precision the last time these sediments were exposed to the sun.
Rittenour explained: “And the only way to do that at Camp Century is to remove a mile of ice. Plus, to have plants, you have to have light.”
These powerful new data were combined with insight from Bierman’s UVM lab. There, scientists study quartz from the Camp Century core.
Inside this quartz, rare isotopes of the elements beryllium and aluminium build up when the ground is exposed to the sky and can be hit by cosmic rays.
Looking at ratios of beryllium and other isotopes gave the scientists a window into how long rocks at the surface were exposed versus buried under layers of ice.
This data helped the scientists show that the Camp Century sediment was exposed to the sky less than 14,000 years before it was deposited under the ice, narrowing down the time window when that portion of Greenland must have been ice-free.
Metres of sea level rise
The models show that the ice sheet melted enough to cause at least five feet, and perhaps as much as twenty feet, of sea-level rise.
Rittenour said: “If we melt just portions of the Greenland ice sheet, the sea level rises dramatically.
“Forward modelling the rates of melt, and the response to high carbon dioxide, we are looking at metres of sea level rise, probably tens of metres.
“And then look at the elevation of New York City, Boston, Miami, Amsterdam. Look at India and Africa – most global population centres are near sea level.”
The study is published in the journal Science.
Image: A large portion of Greenland melted about 416,000 years ago—perhaps a bit like the modern Greenland landscape shown in this photo—and became ice-free tundra, or boreal forest, a new study in the journal Science shows. The results help overturn a previous view that much of the Greenland ice sheet persisted for most of the last two and a half million years. Instead, moderate warming, from 424,000 to 374,000 years ago, led to dramatic melting. This finding indicates that the ice sheet on Greenland may be more sensitive to human-caused climate change than previously understood—and will be vulnerable to irreversible, rapid melting in coming centuries. Credit: Joshua Brown.
