Latest Study Pins Time of Greenland’s Last Melting to Some 400,000 Years Ago
In 2016, a groundbreaking study of a singular bedrock core drilled from under the middle of the Greenland Ice Sheet suggested that the majority or all the ice covering the landmass had melted away at the least once in the course of the last 1.1 million years. In 2021, a study of one other core containing sub-ice sediments laden with plants from a site 500 miles away reached the same conclusion. The pair of studies helped overturn a previous view that the ice sheet has been stable for thousands and thousands of years, even during naturally warm periods. It also strengthened the prospect that human-induced warming could eliminate the ice sheet, which holds some 23 feet of potential sea-level rise.
Researchers now say they’ve more precise timing for at the least one such melting event. A recent study within the journal Science says a big portion of Greenland turned to ice-free tundra about 416,000 years ago, plus or minus 38,000 years—quite recent in geologic time. They calculate that the melting caused at the least five feet of sea level rise—and perhaps as much as 20 feet—at a time when temperatures were only barely warmer than today, regardless that atmospheric levels of heat-trapping carbon dioxide were far lower. This means that the Greenland ice could also be more sensitive to human-caused climate change than previously understood, and may very well be vulnerable to irreversible, rapid melting in coming centuries.
The scientists, from the University of Vermont, Columbia University and other institutions, used sediment from the underside of a long-lost ice core, collected at a secret U.S. military base within the Nineteen Sixties, to make the invention. They applied advanced luminescence and isotope techniques to offer direct evidence of the timing and duration of the ice-free period.
The Greenland ice is believed to have largely melted at the least once in recent geologic time. Here, ice melts off a recent pond. (Joshua Brown/University of Vermont)
“An enormous remaining query following [the previous studies] was when was probably the most recent exposure?” said study coauthor Sidney Hemming, a geochemist on the Columbia Climate School’s Lamont-Doherty Earth Observatory. “It is a strong case. It was a serendipitous opportunity to probe the history recorded within 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 purpose of the Cold War camp was to secretly station tons of of nuclear missiles near the Soviet Union. As cover, the Army claimed it was a science station.
The missile mission was a bust, but a science team there accomplished necessary research, including drilling an ice core 4,560 feet deep. Then they kept going, to drag out a 12-foot-long tube of soil and rock from below the ice. The scientists on the time took little interest within the sediments; the core was moved within the Seventies from a military freezer to the University at Buffalo, then to a freezer in Denmark within the Nineteen Nineties. There it was forgotten, until it was examined in 2017. The findings, published in 2021, showed it held not only sediment but leaves, moss and other detritus of things living on the surface—remnants of an ice-free landscape, perhaps a boreal forest.
Drilling rig that brought up a sediment core at the key Camp Century military installation, 1961. (David Atwood, U.S. Army-ERDC-CRREL, courtesy AIP Emilio Segrè Visual Archives)
But how way back were those plants growing there, where today there may be an ice sheet thrice the dimensions of Texas and as much as two miles thick? The brand new study presents evidence that the 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 lasting from 424,000 to 374,000 years ago, when temperatures were barely warmer than today.
“It’s really the primary bulletproof evidence that much of the Greenland ice sheet vanished when it got warm,” says University of Vermont scientist Paul Bierman, who co-led the brand new study with Drew Christ, a post-doctoral geoscientist who worked in Bierman’s lab.
Joerg Schaefer, a geochemist at Lamont-Doherty Earth Observatory who helped lead the previous two studies but was not involved with the present paper, said he was not surprised that the researchers zeroed in on this time period. “Clearly now we have considered MIS 11 a contender, since it was certainly one of the warmest periods,” he said. Still, he believes more work is required to actually prove the case. He’s currently helping lead Project GreenDrill, a large U.S. National Science Foundation-funded effort aimed toward doing just that. The project will drill 4 recent bedrock cores from around Greenland, which shall be intensively studied in an effort to higher document the recent melt history of Greenland. The team brought up its first core this summer.
Understanding Greenland’s past is critical for predicting how its giant ice sheet will reply to climate warming in the longer term. “Greenland’s past, preserved in 12 feet of frozen soil, suggests a warm, wet, and largely ice-free future for planet Earth unless we will dramatically lower the concentration of carbon dioxide within the atmosphere,” said Bierman.
The sediment core in the brand new study was examined for a so-called luminescence signal in Tammy Rittenour’s lab at Utah State University. As bits of rock and sand are transported by wind or water, they will be exposed to sunlight, which, principally zeros out any previous luminescence signal, after which reburied under rock or ice. Within the darkness, over time, minerals of quartz and feldspar within the sediment accumulate freed electrons of their crystals.
Rittenour’s team took pieces of the ice core sediment and exposed them to blue-green or infrared light, releasing the trapped electrons. The variety of released electrons forms a form of clock, revealing with precision the last time these sediments were exposed to the sun.
The brand new luminescence data were combined with data from Bierman’s lab. There, scientists studied quartz from the core. Inside this quartz, rare isotopes of the weather beryllium and aluminum construct up when the bottom is exposed to the sky and will be hit by cosmic rays. Measurements of the ratios of such isotopes to at least one one other can tell scientists how long rocks on the surface had been exposed versus buried under layers of ice. These data helped the scientists show that the Camp Century sediment was exposed to the sky lower than 14,000 years before it was deposited under the ice, narrowing down the time window when that portion of Greenland will need to have been ice free.
Although the period when this happened is believed to have been only barely warmer than today, there was far less carbon dioxide within the atmosphere then—280 parts per million or less, versus 420 parts per million today and rising. The findings confirm the delicate nature of all the Greenland ice sheet, the scientists said.
“Forward modeling the rates of melt, and the response to high carbon dioxide, we’re taking a look at meters of sea level rise, probably tens of meters,” said Rittenour. “Then take a look at the elevation of Latest York City, Boston, Miami, Amsterdam. Take a look at India and Africa—most global population centers are near sea level.”
“4 hundred thousand years ago, there have been no cities on the coast,” said Bierman. “Now there are cities on the coast.”
Based partly on a press release from the University of Vermont.
