An ice sheet more than 1.5 km thick melted in Greenland 416,000 years ago during moderate natural climate warming, indicating it is more vulnerable than expected to current climate change, according to a study published on Thursday that cites AFP.

The melting of this ice sheet has led to a significant rise in water levels, which is now threatening coastal regions.

This scientific discovery refutes the long-held belief that the largest island in the world was an ice fortress that existed for 2.5 million years.

“If we want to understand the future, we have to understand the past,” Paul Bierman, a professor at the University of Vermont (northeastern United States), who was one of the leaders of the study published in the journal Science, told AFP.

The study is based on the study of an ice core extracted at a distance of 1,390 meters from the surface in northern Greenland by a group of researchers from Camp Century, a secret US military base in the 1960s.

This 3-meter-long sample, which contained soil and rocks, was forgotten in a freezer and was rediscovered in 2017.

The researchers were surprised to find that, in addition to the sediment, it contained the remains of leaves and moss, irrefutable evidence that the soil was once free of ice.

Green Greenland

Although scientists have been deprived of this precious specimen for a long time, this omission was somewhat “providential” because dating techniques useful in this case were only recently developed, explains Professor Bierman.

One of these, luminescence dating, now allows scientists to determine the date when buried minerals were last exposed to the sun.

Examination of the recovered ice core has shown that some of the ancient sediments were once exposed to light, meaning that the ice that covers them today was not there to begin with.

“In addition to having plants, you need light,” says Tammy Rittenor of the University of Utah, who also participated in the study, in connection with the discovery of the remains of leaves and moss.

A luminescent dating technique was able to date the end of the ice-free period, while another procedure that evaluated quartz isotopes was able to determine its beginning.

These techniques established that the sediments had been exposed to light for less than 14,000 years, and that Greenland was therefore ice-free during this period.

Coastal regions are under threat

The sample taken at Camp Century was taken just under 1,300 kilometers from the North Pole, and the study shows that the entire region would have been covered in vegetation.

This occurred during a period of natural warming known as the interglacial period, when temperatures were similar to today’s and between +1°C and +1.5°C higher than in pre-industrial times.

Models developed by the researchers showed that the rise in sea level caused by the melting of this ice sheet at the time was between 1.5 and 6 meters.

These estimates indicate that all coastal regions of the world, where many large settlements are concentrated today, are at risk of being flooded in the coming centuries.

Joseph McGregor, a NASA climate scientist who was not involved in the study, notes that the interglacial period that led to this previously unknown melting lasted tens of thousands of years.