An ice sheet more than 1.5 km thick melted in Greenland 416,000 years ago during a period of moderate natural global warming, according to a study published on Thursday, a sign that the country is more vulnerable than previously thought to the current climate change.
The melting of this ice cap then led to a significant rise in the water level, which today threatens the coastal regions.
This scientific discovery challenges the deeply held belief that the world’s largest island was an ice fortress that had existed for 2.5 million years.
“If we want to understand the future, we have to understand the past,” said Paul Bierman, a professor at the University of Vermont (Northeast US) who co-led the study, published in the journal Science.
The latter is based on the study of an ice core extracted 1,390 meters from the surface in northern Greenland by a team of researchers from Camp Century, a secret American military base in the 1960s.
This sample, more than 3 meters long and containing soil and rocks, was forgotten in a freezer and rediscovered in 2017.
The researchers were surprised to find that, in addition to sediment, it also contained remains of leaves and moss – irrefutable evidence of a soil that was once ice-free.
A green Greenland
Although this valuable sample was withheld from scientists for a long time, the oversight was somewhat “precautionary” since methods for dating useful sediments in this case were only recently developed, explains Professor Bierman.
One of these, luminescence dating, now allows scientists to date buried minerals when they were last exposed to the sun.
Examination of the ice core found revealed that some ancient sediments were once exposed to light – meaning the ice currently covering them was non-existent.
“You also need light to have plants,” said Tammy Rittenour of Utah State University, who also participated in the study, of the discovery of leaf and moss debris.
The technique of dating by luminescence made it possible to date the end of the ice-free period, while another method, the determination of quartz isotopes, could decide when it began.
These techniques allowed it to be estimated that the sediments had been exposed to light for less than 14,000 years and Greenland was therefore ice-free during that period.
coastal regions at risk
The sample taken at Camp Century was just 800 miles from the North Pole. The study found that the entire region would have been covered with vegetation.
This happened during a period of natural warming, the so-called interglacial period, when temperatures were similar to those of today, ranging between +1°C and +1.5°C compared to the pre-industrial era.
The models developed by the researchers showed that the sea level rise caused by the melting of this ice cap at the time would have been between 1.5 and 6 meters.
These estimates suggest that all of the world’s coastal regions, where many important population centers are concentrated today, are therefore at risk of flooding in the coming centuries.
Joseph MacGregor, a NASA climate scientist who was not involved in the study, notes that the interglacial period that led to this previously unknown melt lasted tens of thousands of years.
While humans, he points out, have managed to exceed “greenhouse gas emissions released at the time” in a much shorter time.
Atmospheric CO2 levels, which trap heat on the planet, are currently 420 parts per million (ppm), up from 280 ppm during Greenland’s green days, and will take thousands of years to disappear.
“We’re running a gigantic experiment in the Earth’s atmosphere and we don’t know the results,” warns Mr. Bierman. “I don’t think the sky is falling, we just have to pull ourselves together.”