New study dates Greenland’s last melt to about 400,000 years ago
In 2016, a pioneering study of unique bedrock cores drilled from beneath the center of the Greenland ice sheet showed that most or all of the ice covering the landmass was covered. melted Occurred at least once in the past 1.1 million years. In 2021, a study of another core containing subglacial sediments overgrown with plants A site 500 miles away came to similar conclusions. The two studies overturn previous ideas that ice sheets have been stable over millions of years even during periods of natural warmth. It also reinforces the prospect that human-caused warming could remove the ice sheet, which could raise sea levels by about 23 feet.
Researchers now say they have a more precise timing of at least one such melting event. A new research in the journal science Much of Greenland became ice-free tundra about 416,000 years ago (plus or minus 38,000 years), which is fairly recent in geological time. They calculated that melting caused sea levels to rise by at least 5 feet, and possibly as much as 20 feet, at a time when air temperatures were only slightly warmer than they are now, even though the atmosphere was much lower in heat-trapping carbon dioxide. This suggests that Greenland’s ice may be more sensitive to human-caused climate change than previously understood, and could suffer irreversibly rapid melting in the coming centuries.
Scientists from the University of Vermont, Columbia University and other institutions made the discovery using sediment from the bottom of long-lost ice cores collected in the 1960s at a secret U.S. military base. They applied advanced luminescence and isotopic techniques to provide direct evidence of the timing and duration of ice-free periods.
Greenland’s ice is thought to have melted at least once in recent geological times. Here, the ice of the modern pond melts. (Joshua Brown/University of Vermont)
“The next big problem [the previous studies] When was the last exposure? said the study co-authors Sidney Hemminga geochemist at the Columbia Climate Institute Lamont-Doherty Earth Observatory. “It’s a strong case. It’s a fortuitous opportunity to explore the history recorded in the sediment.”
The study site, called Camp Century, is located in northwest Greenland, 138 miles inland from the coast and just 800 miles from the North Pole. One of the purposes of the Cold War camp was to secretly deploy hundreds of nuclear missiles near the Soviet Union. As a cover, the Army claimed it was a scientific station.
The missile mission failed, but a science team there completed important research, including drilling a 4,560-foot-deep ice core. They then moved on, pulling a 12-foot pipe of dirt and rock from beneath the ice. Scientists at the time were not very interested in sediments. Cores were transferred from a military freezer to UB in the 1970s, and then to a freezer in Denmark in the 1990s. It was forgotten there until it was inspected in 2017. The results, published in 2021, suggest that it contains not only sediment, but also leaves, moss and detritus of other surface-dwelling organisms—remnants of an ice-free landscape, possibly boreal forest.
Drilling a sediment core at the Camp Century secret military facility, 1961. (David Atwood, US Army-ERDC-CRREL, courtesy of AIP Emilio Segrè Visual Archives)
But how long ago did these plants grow there? Today, there is an ice sheet three times the size of Texas and two miles thick? The new study provides evidence that the sediments directly beneath the ice sheet were deposited by flowing water in ice-free environments during a period of moderate warming known as Ocean Isotope Phase 11, between 424,000 and 374,000 years ago, when temperatures were slightly warmer than today.
“This is really the first conclusive evidence that a large portion of the Greenland ice sheet disappeared as it warmed,” said University of Vermont scientist Paul Bierman, who co-led the new study with postdoctoral geoscientist Drew Christ working in Bierman’s lab.
Jörg SchaeferA geochemist at the Lamont-Doherty Earth Observatory who helped lead the first two studies but was not involved in the current paper said he wasn’t surprised researchers were focusing on this period. “Obviously, we see MIS 11 as a contender because that’s one of the warmest periods,” he said. Still, he thinks more work needs to be done to actually prove it.He is currently helping to lead Green Drilling Programa large-scale study funded by the National Science Foundation effort designed to do just that. The project will drill four new bedrock cores from around Greenland that will be studied in depth to better document Greenland’s recent melting history. The team rolled out the first core this summer.
Understanding Greenland’s past is critical to predicting how its massive ice sheet will respond to future warming. “Greenland’s past, preserved in 12 feet of frozen soil, suggests that Earth’s future will be warm, wet, and largely ice-free, unless we can drastically reduce the concentration of carbon dioxide in the atmosphere,” Bierman said.
Utah State University’s Tammy Ritenur’s lab examined sediment cores from the new study for so-called luminescence signatures. When rocks and sand are transported by wind or water, they can be exposed to sunlight, which essentially erases any previous luminous signature, and then re-buried under rock or ice. In the dark, the quartz and feldspar minerals in the sediment accumulated free electrons in their crystals over time.
Ritnur’s team took fragments of the ice core deposits and exposed them to blue-green or infrared light, releasing trapped electrons. The number of electrons released forms a kind of clock that shows exactly when the deposits were last exposed to sunlight.
The new luminescence data were combined with those from Biermann’s lab. There, scientists studied the quartz in the rock core. When the ground is exposed to the sky and possibly cosmic rays, rare isotopes of the elements beryllium and aluminum accumulate inside the quartz. Measuring the ratios between these isotopes can tell scientists how long surface rocks have been exposed to ice compared to how long they have been buried. The data helped scientists demonstrate that Camp Century sediments were exposed to the sky less than 14,000 years before they were deposited under the ice, shortening the window in which that part of Greenland must be ice-free.
Although the period when this was thought to have occurred was only slightly warmer than today, there was much less carbon dioxide in the atmosphere — 280 ppm or less, compared with 420 ppm today and rising. The findings confirm the vulnerability of the entire Greenland ice sheet, the scientists said.
“Forward modeling the rate of melting and the response to high carbon dioxide, we’re looking at sea level rise of meters, possibly tens of meters,” Ritnur said. “Then look at the elevations of New York City, Boston, Miami, Amsterdam. Look at India and Africa — most of the population centers around the world are near sea level.”
“Fourty thousand years ago, there were no cities on the coast,” Biermann said. “Now there are cities along the coast.”
Based in part on a press release from the University of Vermont.
