A good 400,000 years ago, large parts of Greenland were ice-free and resembled a tundra landscape. This is what an international research team reports after analyzing an ice core from the northwest of the island in the specialist magazine Science. According to the researchers, the result shows how sensitively the Greenland ice sheet – the second largest ice mass on earth – reacts to temperature changes.
A coincidence made a significant contribution to the current finding: In the north-west of Greenland, the USA had built an underground military base during the Cold War and camouflaged it as an arctic research station. This Camp Century proved to be a bad plan and was abandoned in 1967. Shortly before, researchers had removed an ice core that was almost 1400 meters long and thus almost 4 meters below the ice sheet. However, this drill core was lost and was only accidentally rediscovered in Denmark in 2017.
The Camp Century drilling rig.
(Photo: David Atwood/AIP EMIlio Segrè Visual Archives)
One 2021 in the journal PNAS published initial drill core interpretation by a team led by Andrew Christ and Paul Bierman of the University of Vermont at Burlington had found that the Greenland ice sheet had shrunk sharply at least once in the past million years. However, the exact time was unclear.
In the current study, the team led by Christ and Bierman used various methods to date this phase to what is known as Marine Isotope Stage 11 (MIS 11). This interglacial period lasted from 424,000 to 374,000 years ago, making it one of the longest interglacial periods in recent Earth history. Its exact cause has not yet been clarified – long-term changes in the Earth’s orbit or ocean currents are being discussed.
The first analyzed soil sample
(Photo: Paul Bierman/University of Vermont)
Various methods of dating and analysis have shown that the deposits underlying the ice in the drill core, including sand, rocks and plant and animal remains, were last exposed to sunlight roughly 416,000 years ago. “This is the first clear evidence that much of the Greenland ice sheet disappeared as it warmed,” Bierman said in a statement from his university. So far, there has only been indirect evidence for this, for example from analyzes of sediments from the North Atlantic.
In a next step, the team used climate models to simulate how much the Greenland ice sheet had to have melted at that time for the north-west of the island at Camp Century – about 1,300 kilometers south of the North Pole – to be ice-free. And how much that loss contributed to sea level rise at the time.
Researchers look at one of the drill cores taken in 1966.
(Photo: David Atwood/US Army-ERDC-CRREL, courtesy of AIP Emilio Segrè Visual Archives)
According to this, the disappearance of large areas of ice, which is considered certain for the south and north-west of the island, caused the sea to rise by at least 1.4 meters, possibly also by 5.5 meters – the total sea level at that time was six to 13 meters higher than today. According to the simulations, part of the ice on the eastern highlands of the island was probably preserved, possibly also in the centre.
The researchers see the results as evidence of how sensitively the ice sheet reacts to climate fluctuations. Especially since the CO₂ concentration in the atmosphere at that time was far below today’s values: At that time it reached 286 ppm (parts per million; parts per million), today it is significantly more than 400 ppm.
Andrew Christ cutting cores under red light.
(Photo: University of Vermont)
Given today’s higher and rising value, the team believes the current warming will severely impact the ice sheet. “The long residence time of anthropogenic greenhouse gases will prolong current human-caused global warming for many thousands of years,” the group writes. Even in a moderate climate scenario, in which emissions would peak in 2040, it would take tens of thousands of years for CO₂ levels to fall significantly again. This would also have an impact on sea level rise.
However, the consequences of such an increase would be much more drastic this time than then: “400,000 years ago there were no cities on the coast,” says Bierman.