A new study puts the time of the last melting of Greenland around 400,000 years ago
In 2016, a groundbreaking study of a single rock core drilled beneath the center of the Greenland Ice Sheet suggested that most or all of the ice covering the landmass had molten at least once in the last 1.1 million years. In 2021, a study of another core containing plant-laden subglacial sediments from a site 500 miles away came to a similar conclusion. The pair of studies helped overturn an earlier view that the ice sheet had been stable for millions of years, even during naturally warm periods. It also bolstered the prospect that human-induced warming could wipe out the ice sheet, which holds about 23 feet of potential sea level rise.
Researchers now say they have more precise timing for at least one such meltdown event. A new study in the journal Science says that much of Greenland turned into ice-free tundra about 416,000 years ago, roughly 38,000 years ago, which is quite recent in geological time. They calculate that the melt caused a sea level rise of at least five feet – and possibly as much as 20 feet – at a time when temperatures were only slightly warmer than today, even though atmospheric levels of heat-trapping carbon dioxide were much lower. This indicates that Greenland’s ice may be more susceptible to human-caused climate change than previously thought, and may be vulnerable to rapid and irreversible melting over the coming centuries.
The scientists, from the University of Vermont, Columbia University and other institutions, used sediment from the bottom of a long-lost ice core, collected from 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.
“A big question remaining after (previous studies) was when was the most recent exposure?” said the study’s co-author Sidney Hemminggeochemist at Columbia Climate School Lamont-Doherty Earth Observatory. “It’s a solid case. It was a serendipitous opportunity to probe the history recorded in the sediments.
The study site, called Camp Century, is in northwest Greenland, 138 miles inland from the coast and just 800 miles from the North Pole. One of the Cold War camp’s goals was to secretly station hundreds of nuclear missiles near the Soviet Union. As a cover, the military claimed it was a science station.
The missile’s mission was a failure, but a science team carried out significant research there, including drilling an ice core 4,560 feet deep. Then they continued, to extract a 12-foot-long tube of dirt and rock from under the ice. Scientists at the time had little interest in sediments; the core was moved in the 1970s from a military freezer at the University at Buffalo, then to a freezer in Denmark in the 1990s. There it was forgotten, until it was examined in 2017. The results, published in 2021, showed that it contained not only sediment, but also leaves, moss and other detritus from living things on the surface – remnants of a landscape without ice, possibly a boreal forest.
But how long ago did these plants grow there, where today there is an ice cap three times the size of Texas and up to two miles thick? The new study presents evidence that sediments just below the ice sheet were deposited by water flow in an ice-free environment during a period of moderate warming called Marine Isotope Stage 11 that lasted from 424,000 to 374,000 years ago, when temperatures were slightly warmer than today.
“This is really the first bulletproof evidence that much of the Greenland ice sheet disappeared when it warmed up,” says University of Vermont scientist Paul Bierman, who co-led the new study with Drew Christ, a post-doctoral geoscientist who worked in Bierman’s lab.
Jörg Schäfer, a Lamont-Doherty Earth Observatory geochemist who helped lead the previous two studies but was not involved in the current paper, said he was not surprised the researchers focused on this time period. “We clearly looked at MIS 11 as a contender because it was one of the hottest times,” he said. Still, he thinks more work is needed to really prove the case. He currently helps lead GreenDrill Project, a massive effort funded by the US National Science Foundation to do just that. The project will drill four new bedrock cores around Greenland, which will be studied intensively to better document Greenland’s recent melting history. The team set up its first nucleus this summer.
Understanding Greenland’s past is key to predicting how its giant ice cap will respond to global warming in the future. “Greenland’s past, preserved in 12 feet of frozen ground, suggests a warm, humid and largely ice-free future for planet Earth unless we can significantly reduce the concentration of carbon dioxide in the atmosphere,” Bierman said.
The sediment core in the new study was examined for a so-called luminescence signal in the lab of Tammy Rittenour at Utah State University. As pieces of rock and sand are carried by wind or water, they can be exposed to sunlight, which basically cancels out any previous luminescence signal, and then reburied under the rock or ice. In the dark, over time, the quartz and feldspar minerals in the sediment accumulate liberated electrons in their crystals.
Rittenour’s team took pieces of sediment from the ice core and exposed them to blue-green or infrared light, releasing the trapped electrons. The number of electrons released forms a kind of clock, accurately revealing the last time these sediments were exposed to sunlight.
The new luminescence data was combined with data from Bierman’s lab. There, scientists studied quartz from the core. Inside this quartz, rare isotopes of the elements beryllium and aluminum accumulate when the ground 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 on the surface have been exposed compared to those buried under layers of ice. This data helped scientists show that the Camp Century sediments were exposed to the sky less than 14,000 years before being deposited under the ice, narrowing the time window during which this part of Greenland had to be ice-free.
Although the period when this happened is thought to have been only slightly warmer than today, there was much less carbon dioxide in the atmosphere then – 280 parts per million or less, compared to 420 parts per million today and rising. The results confirm the fragile nature of the entire Greenland Ice Sheet, the scientists said.
“Forward modeling melting rates and the response to high carbon dioxide, we’re looking at meters of sea level rise, probably tens of meters,” Rittenour said. “So look at the elevation of New York, Boston, Miami, Amsterdam. Look at India and Africa – most of the world’s population centers are near sea level.”
“Four hundred thousand years ago there were no cities on the coast,” Bierman said. “Now there are towns on the coast.”
Based in part on a University of Vermont press release.