A big part of greenland it was an ice-free tundra landscape, probably covered with trees and wandering woolly mammoths, in the recent geological past (about 416,000 years ago), according to a new study published in the journal ‘Science’. The results help refute the earlier view that much of the Greenland ice sheet persisted for most of the last 2.5 million years. Instead, a moderate warming, from 424,000 to 374,000 years ago, resulted in a spectacular thaw.
At that time, the melting of Greenland demonstrated a sea level rise of at least five feet, despite atmospheric levels of heat-trapping carbon dioxide being much lower than today (280 vs. 420 ppm). This indicates that the Greenland ice sheet may be more sensitive to human-induced climate change than previously thought, and will be vulnerable to rapid and irreversible melting in the coming centuries.
Scientists from the University of Vermont (UVM), Utah State University and fourteen other international institutions used sediment from a long-lost ice core, collected at a US Army military base in the 1960s, to make the discovery. They applied advanced luminescence and isotope techniques to obtain direct evidence of the timing and duration of the ice-free period.
During the Cold War, a secret US Army mission at Camp Century in northwestern Greenland drilled through 4,560 feet of ice on the icy island and then continued drilling to remove a twelve-foot-long tube of soil and rock from beneath the ice. This icy sediment was then lost in a freezer for decades. It was accidentally rediscovered in 2017 and found to contain not only sediment, but also leaves and moss, remnants of an ice-free landscape, perhaps a boreal forest.
Until recently, geologists believed that Greenland was a fortress of ice, most of it unmelted for millions of years. But, two years ago, using Camp Century’s rediscovered ice core, this team of scientists set off that probably melted less than a million years ago. Other scientists, working in central Greenland, obtained data showing that the ice had melted at least once in the last 1.1 million years, but until this study, no one knew exactly when it had disappeared.
Now, using advanced luminescence technology and rare isotope analysis, the team has created a starker picture: Large portions of the Greenland ice sheet melted well over a million years ago. The new study presents direct evidence that sediments just below the ice sheet were deposited by flowing water in an ice-free environment during a period of moderate warming called Marine Isotopic Stage 11, between 424,000 and 374,000 years ago. This thaw is due to a rise in sea level of at least a meter and a half across the planet.
«Is really the first irrefutable evidence that much of the Greenland ice sheet disappeared when it warmed,» says Paul Bierman, a scientist at the University of Vermont who co-led the new study with lead author Drew Christ, a postdoctoral geoscientist who worked in Bierman’s lab, Professor Tammy Rittenour of Utah State University, and 18 other scientists from around the world. With Greenland’s ice accumulating about twenty feet of sea level rise, all of the world’s coastal regions are at risk. The new study provides strong and precise evidence that Greenland is more sensitive to climate change than previously believed, and is at serious risk of irreversible melting.
«Greenland’s past, preserved in twelve feet of frozen ground, suggests a warm, wet, and largely ice-free future for planet Earth,» says Bierman, a geoscientist at UVM’s Rubenstein School of Environment and Natural Resources and a fellow at the Gund Institute for the Environment, «unless we can reduce the concentration of carbon dioxide in the atmosphere.»
The new study, combined with their previous work, is prompting a major and worrying rethink of the Greenland ice sheet history. «We’ve always assumed that the Greenland ice sheet formed about 2.5 million years ago and that it’s been there all this time and that it’s very stable,» said Tammy Rittenour, a Utah State University scientist who co-authored the new study. «It may have melted at the edges, or it may have gotten a little fatter with more snowfall, but it doesn’t go away or melt back in shape. But this work shows that it did.»
In Rittenour’s lab, they will sift through the Camp Century core in search of what is called a «luminescence signal.» When bits of rock and sand are carried by wind or water, they can become exposed to sunlight—which basically nullifies any previous luminescence signals—and then bury themselves back under rock or ice. In the dark, over time, quartz and feldspar minerals in the sediment accumulate released electrons in their crystals.
In a specialized darkroom, Rittenour’s team took chunks of the ice core sediment and exposed them to blue-green or infrared light, releasing the trapped electrons. With some advanced tools and measurements, and many repeated tests, the number of electrons released forms a kind of clock, accurately revealing the last time these exposed sediments were left in the sun.
This powerful new data is combined with insights from Bierman’s UVM lab. There, scientists study the quartz core of Camp Century. Within this quartz, rare forms — called isotopes — of the elements beryllium and aluminum accumulate when the ground is exposed to the sky and can be struck by cosmic rays. Analyzing the ratios of beryllium and other isotopes allowed scientists to learn how long the surface rocks were exposed and how long they were buried under layers of ice. These data helped scientists show that the Camp Century sediments were exposed to the sky less than 14,000 years before they were deposited under the ice, narrowing the time window in which that part of Greenland will need to be ice-free.
Using this information, the team’s models show that, during that period, the ice sheet melted enough to cause a sea level rise of at least enough meters and perhaps as much as twenty feet. The research matches results from two other ice cores collected in the 1990s in central Greenland. Sediment from these cores also suggests that the gigantic ice sheet melted in the recent geologic past. Combining these earlier cores with new information from Camp Century reveals the fragility of the entire Greenland ice sheet, both in the past (at 280 parts per million atmospheric CO2 or less) and today (422 ppm and rising).