A new study dates Greenland’s last melt to about 400,000 years ago
In 2016, a groundbreaking study of a unique rock core drilled from beneath the center of the Greenland ice sheet indicated that most or all of the ice covering the landmass had melted at least once in the past 1.1 million years. In 2021, a study of another core containing vegetation-laden ice deposits from a site 500 miles away came to a similar conclusion. These two studies helped overturn the previous view that the ice sheet has been stable for millions of years, even during naturally warm periods. It also raised the possibility that human-caused warming could wipe out the ice sheet, which holds about 23 feet of potential sea level rise.
The researchers now say they have a more accurate timing for at least one melting event. A new study in the journal Sciences He says that much of Greenland switched to ice-free tundra about 416,000 years ago, plus or minus 38,000 years—very recent in the geologic era. They calculated that the melt caused sea levels to rise by at least five feet—and possibly as much as 20 feet—at a time when temperatures were only slightly warmer than they are today, even though atmospheric levels of heat-trapping carbon dioxide were much lower. This suggests that Greenland’s ice may be more sensitive to human-caused climate change than previously understood, and could be subject to rapid and irreversible melting in the coming centuries.
The scientists, from the University of Vermont, Columbia University and other institutions, used deposits from the bottom of a long-lost ice core, collected at a secret US military base in the 1960s, to make the discovery. They applied advanced luminescence and isotope techniques to provide direct evidence of the timing and duration of the ice-free period.
Greenland’s ice is thought to have melted considerably at least once in the recent geological era. Here, ice is melting from a contemporary pond. (Joshua Brown/University of Vermont)
“The big remaining question that followed[the previous studies]was when was the most recent exposure?” said study co-author Sydney Hemming, a geochemist at Columbia Climate School’s Lamont-Doherty Earth Observatory. “This is a strong case. It was a serendipitous opportunity to explore the history recorded in the sediments.”
The study site, called Camp Century, is in northwest Greenland, 138 miles inland from the coast and only 800 miles from the North Pole. One of the goals of the Cold War camp was to secretly place hundreds of nuclear missiles near the Soviet Union. As a cover, the military claimed it was a science station.
The rocket mission was a bust, but a science team there completed important research, including drilling an ice core 4,560 feet deep. Then they continued to pull a 12-foot tube of soil and rock out from under the ice. Scientists of the time were not interested in the sediment; The pulp was moved in the 1970s from a military freezer to the University at Buffalo, and then to a freezer in Denmark in the 1990s. It was forgotten there, until it was examined in 2017. The results, published in 2021, show that it contained not only sediment but leaves, moss and other remnants of things that lived on the surface – the remnants of an ice-free landscape, perhaps a boreal forest.
The dredger fetching a sediment core at a secret military facility at Camp Century, 1961. (David Atwood, US Army-ERDC-CRREL, courtesy of AIP Emilio Segre Visual Archives)
But how long have those plants been growing there, where today there is an ice sheet three times the size of Texas and up to two miles thick? The new study provides evidence that sediments beneath the ice sheet were deposited by flowing water in an ice-free environment during a moderate warming period called Marine Isotope Stage 11 that lasted from 424,000 to 374,000 years ago, when temperatures were slightly warmer than today.
“It’s really the first bulletproof evidence that much of the Greenland ice sheet disappeared when it got warm,” says University of Vermont scientist Paul Berman, who co-led the new study with Drew Christ, a postdoctoral geologist who worked in Berman’s lab.
Joerg Schaefer, a geochemist at the Lamont-Doherty Earth Observatory, who helped lead the two previous studies but was not involved in the current paper, said he wasn’t surprised that the researchers focused on this time period. “Obviously, we considered MIS 11 a competitor, because it was one of the warmest periods,” he said. However, he believes more work is needed to really prove this case. He is currently helping lead the GreenDrill Project, a massive effort funded by the US National Science Foundation with the goal of doing exactly that. The project will excavate four new rock cores from across Greenland, which will be studied intensively in order to better document the history of recent melting in Greenland. The team rolled out its first core this summer.
Understanding Greenland’s past is critical to predicting how the giant ice sheet will respond to a warming climate in the future. “Greenland’s past, preserved in 12 feet of permafrost, suggests a warm, wet and largely ice-free future for the planet unless we can significantly reduce the concentration of carbon dioxide in the atmosphere,” Berman said.
The sediment core in the new study was examined for a so-called luminescence signal in Utah State University’s Tammy Ritnor lab. When bits of rock and sand are carried away by wind or water, they can be exposed to sunlight, which, basically, removes any previously shiny sign, and then they are reburied under the rock or ice. In the dark, over time, the quartz and feldspar minerals in the sediment accumulate freed electrons in their crystals.
Ritnor’s team took chunks of icy sediment and exposed them to blue-green or infrared light, releasing trapped electrons. The number of liberated electrons forms a kind of clock, accurately revealing the last time these deposits were exposed to the sun.
New luminescence data were combined with data from Berman’s lab. There, scientists studied quartz from the heart. Inside this quartz, rare isotopes of the elements beryllium and aluminum accumulate when the Earth is exposed to the sky and can be hit by cosmic rays. Measurements of the ratios of these isotopes to each other can tell scientists how long rocks have been exposed at the surface versus how long they have been buried under layers of ice. This data helped scientists show that the camp’s sediment was exposed to the sky less than 14,000 years before it was deposited under the ice, narrowing the time window for when this part of Greenland should have been ice-free.
Although the period when this happened is thought to have been slightly warmer than it is today, CO2 was much lower back then – 280 ppm or less, versus 420 ppm today and it’s rising. Scientists said the findings confirm the fragile nature of the entire Greenland ice sheet.
“Modeling rates of melt, and in response to rising carbon dioxide, we’re looking at sea level rise in meters, possibly tens of metres,” Ritnor said. Then look at the height of New York City, Boston, Miami, and Amsterdam. Look at India and Africa – most of the world’s population centers are close to sea level. “
“Four hundred thousand years ago, there were no cities on the coast,” Berman said. “Now there are cities on the coast.”
Based in part on a press release from the University of Vermont.