The scientists prepared for the descent into the Greenland ice sheet by drilling deep into the ice. They created two intersecting holes in the bed of a now frozen ice river that ran a rope through them in the shape of a V to anchor their lines. It would be more than strong enough to carry their weight, but they also drilled another anchor – just in case.
Then Matt Covington, a geologist and cave researcher who has spent more than a year of his life underground, was ready. He began to sink into the vertical cave, which in summer is filled with the chaos of a waterfall – a moulin. The sharp crampons on his boots gripped the ice. The fact that it was October made the mill a little safer, but Covington could still hear running water somewhere.
As he backed over the edge and looked down the shaft, he saw white ice, then bluer ice and then darkness. The hole, scientists believe, eventually penetrates more than half a mile into the ice and connects with a network of canals that extend all the way to the bottom of the ice sheet.
“When you dangle the wall on this deep shaft and you hear ice breaking and falling, it brings your heart up to your throat,” recalled Covington, a professor of geology at the University of Arkansas.
Covington and his colleague Jason Gulley, an expert on ice caves at the University of South Florida, were motivated by a scientific question with huge implications as the climate warms. How big are these moulins, these ice caves, which thousands are found above the surface of Greenland? How much do they undermine the integrity of the second largest ice sheet on the planet? And how much worse will it get when melting and moulins begin to stretch further and further towards the airy center of Greenland, where the ice is well over a kilometer thick?
As Covington noted, he and Gulley operate on an ancient science model – one in which, as in the 18th and 19th centuries, exploration and research were the same. Darwin and other scientists just had to travel around the surface of the world to gain crucial insight. But today you have to get a little more extreme to find unexplored places.
In the summer, Greenland’s moulins swallow an ever-growing abundance of meltwater that spills violently into them. Because the Greenland ice sheet extends high into the air and has a complicated topography, the meltwater cannot just run from the edge to the sea. Rather, it runs under the ice through moulins and furtively towards the sea – where it spills out and raises the sea surface.
“These are big giant scary holes in the ice,” Gulley said. “Places that people have historically avoided. During the summer, you have whole lakes disappearing down these things, sometimes overnight. Almost every river on the Greenland ice sheet disappears in these holes. ”
As the climate warms – with the Arctic warming fastest of all – more and more of Greenland’s surface is melting in the summer. More lakes are formed and at higher, colder heights on the inland ice. The ice has lost about 4 trillion tons of mass since 1992, and scientists estimate that surface melting amounts to approx. half of these losses, while the rest are driven by huge icebergs that calve into the sea.
Researchers have long known that Greenland’s meltwater uses thousands of mills to escape, and deduced that these holes must be the surface’s entry points to a huge sub-network of drainage channels. And they may destabilize Greenland in the process.
“If you keep the crack full of water, the water is denser, it is able to exert a force, and it basically cracks the crack all the way up to the bottom of the ice sheet,” Gulley explained. “The water acts like a wedge that you taste with a hammer to split firewood. So once you have established the hydraulic connection, the water has access to the bottom of the ice sheet and it starts spraying out everywhere. ”
There has been a fear that as more and more water pumps enter this network, the inland ice itself may move faster towards the sea. This is because the water pressure pushes outwards on the ice, even though water at the bottom of the inland ice lifts it up and helps it slide forward. It is already clear that Greenland’s ice every summer melting season accelerates its movement towards the sea.
Addressing this fear, however, has been the idea that as more water spills into these ice caves, deeper channels in the ice will open up, releasing some of the hydraulic pressure and reducing slippage. As a result, scientists are discussing how much moulins and their accompanying water channels will aggravate sea levels.
This is where the research from Covington and Gulley comes in.
In 2018 and 2019, the researchers, first supported by the sports drink producer Red Bull and later the National Science Foundation, traveled to a western sector of the Greenland ice sheet and rappelled into these frightening features of the ice. While moulins are found in many glaciers in the world, Greenland hosts the largest known examples.
“There are cracks in the walls, and one of the things with ice, it’s a pretty fragile substance,” Gulley said. “So if you have these big cavities with lots of unsupported ice, it starts to crack. . . when we descend, we check the walls and make sure we will not rig in at any point [something that could collapse]. ”
When the researchers dangled and made measurements using a device called a laser rangefinder, which reflects a laser beam to measure the distance, they also had to struggle with the elements.
“There was nowhere to really get off the rope,” Covington said. “For hours at a time, we hung on our braces in these ropes. It was definitely cold and miserable. ”
The researchers had experience in both exploring caves and also caves in ice – and Will Gadd, a famous ice climber, was part of the first Red Bull-sponsored trip. Yet Gulley admits that there was an element of danger involved that could not be fully controlled, even by his vast experience.
“There are a number of things that can go wrong when driving your car,” he said. “It’s the same when we explore glacier caves, except that when we started doing this 20 years ago, no one knew what was dangerous. Now we know a great deal about what is dangerous. . . . So I feel like we’ve done a good job of managing all the risks, but it’s never going to be as safe as sitting on your couch. ”
In a few tragic cases, scientists working in polar extremes to study climate change have been killed in accidents. Most recently this summer, the famous Swiss glaciologist Konrad Steffen died when he fell into a crack in Greenland.
Human exploration is necessary in the case of the mills, Gulley said, because it is extremely difficult to navigate drones in such small and unpredictable spaces. And beneath the surface of the ice, you can not use GPS to guide them. Plus, he noted that sending a drone is not the same as being in a cave in person, seeing an interesting feature, and getting closer to trying to study and understand it.
The first results of these moulin descents have been published in the journal Geophysical Research Letters. They combine observations of the breathtaking extent of the caves with scientific modeling of the water level inside them and what it can mean.
Most important finding: Moulins can be huge.
In particular, the Phobos mill in western Greenland, which Gulley and Gadd explored in 2018, was not just a narrow hole that penetrated downwards. Instead, it opened into a large cave that reached almost 100 meters in depth before the water level began, and also stretched horizontally outward. The group calculated that the cavity on the water surface was about 5,000 square feet or the size of several houses next to each other.
This volume is much larger than previous models assumed. And that suggests that the mill may store much more water than previously thought. This in turn can mean that the water in the mills can exert more pressure on the surrounding ice and make it slide faster – which would be bad for the sea surface and Greenland’s future.
“If the water level does not fluctuate that much and they remain high, it means that the ice sheet can continue to move,” Gulley said.
As climate change continues, more melting is expected on top of Greenland along with the formation of more mills – including possibly even deeper ones. This is because the melt extends further inland towards the most elevated and traditionally the coldest parts of the ice sheet. Understanding how moulins work to bring water to the ocean and how they affect the speed of ice movement will be critical, researchers say.
“There are signs that they are forming further and further away from the margins,” Covington said. “Not only is the melt moving inland, but it seems that the processes that develop new mills are marching inward.”
Granted, the researchers have so far only explored two mills to a large extent, which makes it difficult to generalize about thousands more who appreciate the Greenland ice sheet.
“As this study is only able to study two mills in a region on the ice sheet, I think it is too early to say whether this finding could change our perspective on future variation in ice movement,” said Twila Moon, a Greenlandic expert. at the National Snow and Ice Data Center in Colorado who were not involved in the research.
“The discovery that there are larger than expected mills in Greenland is significant, as these mills provide a buffer between surface melting and the subglacial drainage system,” added Ádam Ignéczi, who studies mills at Sheffield University in the UK and was also not involved in the latest research.
But Ignéczi added that scientists do not know if “systematic differences” in the size of moulins can be found across the vast ice sheet.
This may mean that researchers will have to study more moulins in the coming years – of course carefully.
“These are really beautiful places,” Gulley said. “When you enter, you are in this intense blue chamber. And you can see the whole layering of the ice. You basically see the inside of the Greenland ice sheet. ”