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Since the mid-1990s, the Greenland ice sheet has been losing mass, leaving only three floating tongues remaining. One of these, Nioghalvfjerdsbræ or the 79°N Glacier, is already showing the first signs of instability.

In a new study, researchers from the Alfred Wegener Institute investigated how—caused by global warming—a 21 km² large meltwater lake formed and developed on the surface of the 79°N Glacier. They observed that over the years, this lake has caused gigantic cracks and the outflowing water is lifting the glacier. Their findings have been in the journal The Cryosphere.

The lake first appeared in the observation data of the year 1995. "There were no lakes in this area of the 79°N Glacier before the rise in atmospheric temperatures in the mid-1990s," as Prof. Angelika Humbert, glaciologist at the Alfred Wegener Institute Helmholtz Center for Polar and Marine Research (AWI) stated.

"From the time of its formation in 1995 until 2023, the lake's water repeatedly and abruptly drained through channels and cracks in the ice, causing massive amounts of fresh water to reach the edge of the glacier tongue towards the ocean." There were a total of seven such drainage events, four of which took place in the last five years.

"During these drainages, extensive triangular fracture fields with cracks in the ice formed from 2019 onwards, which are shaped differently from all lake drainages I have seen so far," Humbert says.

Some of these cracks form channels with openings several dozen meters wide (moulins). Water flows through these moulins also after the main drainage of the lake, meaning that within hours, a huge amount of water reaches the base of the ice sheet.

"For the first time, we have now measured the channels that form in the ice during drainage and how they change over the years," says Humbert.

After the lake formed in 1995, its size decreased over time with the first cracks appearing. In recent years, the drainage has occurred at increasingly shorter intervals. "We suspect that this is due to the triangular moulins that have been reactivated repeatedly over the years since 2019," says Humbert.

The material behavior of the glacier plays a role here: on the one hand, the ice behaves like an extremely thick (viscous) fluid that flows slowly over the substrate. At the same time, however, it is also elastic, allowing it to deform and return to its original shape, similar to a rubber band. The elastic nature of the ice is what allows cracks and channels to form in the first place.

On the other hand, the creeping nature of the ice helps channels inside the glacier to close again over time after the drainage has taken place. "The size of the triangular moulin fractures on the surface remains unchanged for several years. Radar images show that although they change over time inside the glacier, they are still detectable years after their formation." This data also reveals that there is a network of cracks and channels, meaning that there is more than one way for the water to escape.

Meltwater is lifting the glaciers

The researchers were able to see shadows along the cracks in some aerial photographs. "In some cases, the ice at the fracture surfaces has also shifted in height, as if it were raised more on one side of the moulin than on the other," Humbert explains.

The largest shift is encountered directly in the lake, which is due to the enormous masses of water that have entered the cracks beneath the glacier and formed a subglacial lake there. Radar images from inside show that a blister has apparently formed on this lake beneath the ice, pushing the glacier upwards at this point. Even more than 15 years after the first drainage, the cracks are still visible on the surface.

In conducting their study, the researchers analyzed data from various measurements. Using satellite remote sensing data and data from airborne surveys, they were able to investigate how the lake fills and drains and the paths of the water within the glacier. Viscoelastic modeling enabled them to determine whether and how drainage paths close over time.

The results raise a crucial question: Have the frequent drainages forced the glacier system into a new state, or can the system (still) return to a normal winter state in spite of these extreme amounts of water?

"In just ten years, recurring patterns and regularity have developed in the drainage, with massive and abrupt changes in meltwater inflow on a timescale of hours to days," says Humbert. "These are extreme disturbances within the system, and it has not yet been investigated whether the glacial system can absorb this amount of water and is able to influence the drainage itself."

The study provides important data for integrating cracks into ice sheet models and researching as to how they form and influence the glacier. AWI researchers are working closely with scientists from TU Darmstadt and the University of Stuttgart on the modeling.

Understanding and taking the behavior and effects of cracks in the glacier into account is particularly important when regarding the development of the lake on the 79°N Glacier: due to the advancing warming of the atmosphere, the fracture surfaces have been occurring further and further up the slope, impacting on an increasingly larger area of the glacier.