By JOSHUA BROWN
University of Vermont
Published July 25, 2023
Until recently, geologists believed that Greenland was a fortress of ice, mostly unmelted for millions of years. But, two years ago, using soil once buried a mile beneath the island’s ice sheet and later stored at UB, a team of scientists showed that it likely melted less than one million years ago.
Now, further analysis of the sediment has created a starker picture: Greenland was a green land only 416,000 years ago.
The team’s new study — published July 21 in the journal Science and co-authored by Elizabeth Thomas, associate professor of geology, UB College of Arts and Sciences — presents direct evidence that the sediment was deposited by flowing water in an ice-free environment during a moderate warming period from 424,000 to 374,000 years ago.
“It’s really the first bulletproof evidence that much of the Greenland ice sheet vanished when it got warm,” says University of Vermont geoscientist Paul Bierman, who co-led the study with lead author Drew Christ, a postdoctoral geoscientist who worked in Bierman’s lab.
Understanding Greenland’s past is critical for predicting how its giant ice sheet will respond to climate warming in the future. The new study provides strong and precise evidence that Greenland is more sensitive to climate change than previously understood — and at grave risk of irreversibly melting off.
Since about 23 feet of sea level rise is tied up in Greenland’s ice, every coastal region in the world is at risk.
Camp Century was a U.S. Army base in Greenland in the 1960s. It was the site of a secret operation, called Project Iceworm, to hide nuclear missiles under the ice near the Soviet Union.
The missile mission was a bust, but the science team did complete first-of-its-kind research, including drilling a nearly mile-deep ice core. The Camp Century scientists were focused on the ice itself, so they took little interest in the 12 feet of sediment gathered from beneath their ice core.
Then, in a bizarre story, the ice core was moved in the 1970s from a military freezer to UB by then-UB geology professor Chester “Chet” Langway, who was one of the leads of the Camp Century drilling expedition. He kept the ice core at a building on Ridge Lea Road in Amherst that housed UB’s Department of Geology while the North Campus was being built.
When he retired from UB in the 1990s, Langway gave the ice core to colleagues in Denmark. There it was lost for decades — until it was found again when the cores were being moved to a new freezer in 2017.
In 2019, Christ looked at the sediment through his microscope and found leaves and moss. That suggested the area had been free of ice in the recent geologic past and that a vegetated landscape stood where today stands an ice sheet two miles thick and three times the size of Texas.
The team, including Christ and UB’s Thomas, published their findings in 2021 in the Proceedings of the National Academy of Sciences.
Other scientists, working in central Greenland, have gathered data showing the ice there melted at least once in the last 1.1 million years — but no one knew exactly when.
For this latest study, the team examined the Camp Century sediment for what is called a “luminescence signal.”
When bits of rock and sand are exposed to sunlight, any previous luminescence signal is zeroed out. When reburied in the darkness under rock or ice over time, minerals of quartz and feldspar in the sediment accumulate freed electrons in their crystals.
At the Utah State University lab of study co-author Tammy Rittenour, pieces of the ice core sediment were exposed to blue-green or infrared light, releasing the trapped electrons. The number of released electrons forms a kind of clock, revealing with precision the last time these sediments were exposed to the sun.
This powerful new data was combined with insight from Bierman’s University of Vermont lab, which studied the quartz from the Camp Century core. Ratios of beryllium and other isotopes inside the quartz — which build up when exposed to the sky and hit by cosmic rays — demonstrate how long rocks at the surface were exposed vs. buried under layers of ice.
This data helped the scientists show that, 400,000 years ago, the Camp Century sediment was exposed to the sky less than 14,000 years before it was deposited under the ice, narrowing down the time window when that portion of Greenland must have been ice-free.
Thomas conducted chemical analysis that enabled the team to understand the types of plants that were on site. The team is doing similar analyses to understand what the temperature and water cycle were like when those chemicals were produced.
The team’s new study, combined with their earlier work, is causing a worrisome rethinking of the history of Greenland’s ice sheet and reveals its fragile nature today.
The last time the ice sheet melted, the team’s models show, it caused at least five feet of sea level rise. Temperatures during that time, an interglacial called Marine Isotope Stage 11, were similar to or slightly warmer than today, and atmospheric carbon dioxide was at least a third less than it is today.
“Forward modeling the rates of melt, and the response to high carbon dioxide, we are looking at meters of sea level rise, probably tens of meters,” Rittenour says. “And then look at the elevation of New York City, Boston, Miami, Amsterdam. Look at Bangladesh, India and Africa — most global population centers are near sea level.”
“Four-hundred-thousand years ago there were no cities on the coast,” Bierman adds, “and now there are cities on the coast.”
The research was supported by the National Science Foundation.