Scientists prepared for their descent into the mouth of the Greenland ice sheet by drilling deep into the ice. They created two holes that cross in the bed of a now frozen river of ice, passing a V-shaped rope through them to anchor their lines. It would be more than strong enough to support his weight, but they drilled a second anchor too – just in case.
So Matt Covington, a geologist and cave explorer who spent more than a year of his life underground, was ready. He began to descend into the vertical cave, which in the summer is filled with the chaos of a waterfall – a moulin. The sharp cleats on his boots grabbed the ice. The fact that it was October now made the moulin a little safer, but Covington could still hear running water somewhere.
Below him, as he retreated over the edge and looked down from the well, he saw white ice, then more blue ice, then darkness. The hole, scientists believe, ends up penetrating more than half a kilometer in the ice, joining a network of channels that extends to the base of the ice sheet.
“When you hang on the wall of this deep well and hear the ice breaking and falling, your heart goes up to your throat,” recalled Covington, professor of geology at the University of Arkansas.
Covington and his colleague Jason Gulley, an ice cave expert at the University of South Florida, were motivated by a scientific issue with huge implications as the climate warms up. How vast are these moulins, these ice caves found by the thousands across the surface of Greenland? How much are they undermining the integrity of the second largest ice sheet on the planet? And how much worse will it be when the melting and moulins start to extend further and further into the airy center of Greenland, where the ice is over a kilometer thick?
As Covington noted, he and Gulley operate from an old model of science – one in which, as in the 18th and 19th centuries, exploration and research were the same. Darwin and other scientists simply had to travel the surface of the world to obtain essential insights. But today, to find unexplored places, you need to be a little more radical.
In the summer, Greenland moulins swallow an increasing profusion of melt water, which spills violently over them. Since the Greenland ice sheet extends high in the air and has a complicated topography, water from melting water cannot simply drain from its edge into the sea. Instead, it runs under the ice through moulins and stealthily into the ocean – where it spills and sea level rises.
“These are gigantic and scary holes in the ice,” said Gulley. “Places that people historically avoid. During the summer, you have entire lakes that disappear from these things, sometimes during the night. Almost all of the Greenland ice sheet rivers disappear into these holes. “
Matt Covington climbs into a moulin in the Greenland ice sheet. (Jason Gulley / University of Arkansas)
As the climate warms up – with the fastest Arctic warming of all – more and more of Greenland’s surface melts in the summer. More lakes are forming and at higher and colder elevations of the ice sheet. The ice sheet has lost about 4 trillion tons of mass since 1992 alone, and scientists estimate that surface melting accounts for about half of those losses, with the rest being driven by huge icebergs that fall into the sea.
Scientists have long known that the Greenland thaw uses thousands of moulins to escape and inferred that these holes must be the entry points of the surface for a huge network of sub-ice drainage channels. And they may be destabilizing Greenland in the process.
“If you keep the crack full of water, the water gets denser, is able to exert a force and basically tears the crack down to the base of the ice sheet,” explained Gulley. “The water is acting like a wedge, which you hit with a hammer, to break the wood. Then, once you establish that hydraulic connection, the water has access to the base of the ice sheet and begins to flow in all directions. “
It is feared that as more and more water is pumped into this network, the ice layer itself will move faster towards the sea. This is because the water pressure pushes the ice out, even when the water at the base of the ice sheet lifts it up and helps it slide forward. It is already clear that, with each summer melting season, Greenland’s ice accelerates in its movement towards the sea.
Contrary to this fear, however, there is the idea that, as more water is poured into these ice caves, deeper channels in the ice open up, releasing some of the hydraulic pressure and decreasing the slip. As a result, scientists are debating how much the moulins, and the water channels that accompany them, will make sea level rise worse.
That’s where Covington and Gulley research comes in.
In 2018 and 2019, scientists, initially supported by sports drink maker Red Bull and later by the National Science Foundation, traveled to a western sector of the Greenland ice sheet and rappelled into these scary areas of ice. While moulins are found on many glaciers in the world, Greenland is home to the greatest known examples.
“There are cracks in the walls, and one of the things with ice, is a very fragile substance,” said Gulley. “So, if you have these big voids with a lot of unsupported ice, it starts to crack. . . when we are going down, we check the walls and make sure that in no time we will mount [something that could collapse]. “
While scientists hung on and took measurements using a device called a laser rangefinder, which reflects a laser beam to measure distance, they also had to face the elements.
“There was nowhere to actually get off the rope,” said Covington. “For hours on end, we hung from our harness on those ropes. It was cold and horrible, for sure. “
Scientists were experienced in exploring caves as well as caves in the ice – and Will Gadd, a famous climber, was part of the first Red Bull-sponsored trip. Still, Gulley admits that there was an element of danger involved that could not be fully controlled even by his considerable experience.
“There are a number of things that can go wrong when you are driving your car,” he said. “It’s the same when we explore glacial caves, except that when we started doing this 20 years ago, nobody knew what was dangerous. Now, we know a little about what is dangerous. . . . So, I feel like we’ve done a good job of managing all the risks, but it will never be as safe as sitting on the couch. “
In some tragic cases, scientists working at polar extremes to study climate change have died in accidents. More recently, this summer, renowned Swiss glaciologist Konrad Steffen died from falling into a crack in Greenland.
Human exploration is necessary in the case of moulins, said Gulley, because it is extremely difficult to navigate drones in such small and unpredictable spaces. And below the surface of the ice sheet you cannot use GPS to guide them. Furthermore, he noted, sending a drone is simply not the same as being in a cave in person, seeing an interesting feature and approaching to try to study and understand it.
The first results of these moulin descents were published in the journal Geophysical Research Letters. They combine observations of the impressive extent of the caves with scientific modeling of the water levels contained in them, and what that can mean.
The main discovery: Moulins can be huge.
In particular, the Phobos moulin in western Greenland, which Gulley and Gadd explored in 2018, was not simply a narrow hole that penetrated downward. Instead, it opened up into a vast cavern that reached nearly 100 meters deep before the water level started, and extended horizontally outward as well. The group calculated that, on the surface of the water, the space area of the cave was about 5,000 square feet, or the size of several houses next to each other.
This volume is much higher than the previous assumed models. And it suggests that the moulin can store much more water than previously thought. That, in turn, could mean that the water in the moulins can put more pressure on the surrounding ice and cause it to slide faster – which would be bad for rising sea levels and for Greenland’s future.
“If the water levels are not fluctuating so much and remain high, it means that the ice sheet can continue to move,” said Gulley.
As climate change continues, more thaw is expected at the top of Greenland, along with the formation of more moulins – including, possibly, even deeper ones. This is because the melting will extend further inland, towards the higher and, traditionally, cooler parts of the ice sheet. Understanding how moulins work to bring water into the sea and how they affect the speed of ice movement will be critical, say the scientists.
“There is evidence that they are forming farther and farther from the margins,” said Covington. “Not only is the melt moving inland, but it appears that the processes that develop new moulins are marching inland.”
It is true that researchers have extensively explored only two moulins so far, making it difficult to generalize about another thousands that mark Greenland’s ice sheet.
“Since this study is able to examine only two moulins in an ice sheet region, I think it is too early to say whether this discovery could change our perspective on the future variability of ice movement,” said Twila Moon, an expert from Greenland at the National Snow and Ice Data Center in Colorado, which was not involved in the research.
“The finding that larger-than-expected moulins are present in Greenland is significant, as these moulins provide a buffer between the surface melt and the subglacial drainage system,” added Ádam Ignéczi, who studies moulins at Sheffield University in the Kingdom United and was also not involved in the latest research.
But Ignéczi added that scientists do not know whether “systematic differences” in the size of the moulins can be found in the huge ice sheet.
This could mean that scientists will have to study more moulins in the coming years – carefully, of course.
“These are really beautiful places,” said Gulley. “When you enter, you are in this intensely blue chamber. And you can see all the layers of the ice. Basically, you are looking into the Greenland ice sheet. “