Scientists have explored a hard-to-reach ocean cavity under a crucial glacier in Antarctica.
Antarctica’s most vulnerable climate hotspot is a remote and hostile place, a narrow strip of seawater under a sheet of floating ice more than half a kilometer thick. Scientists finally investigated this and discovered something surprising.
“The rate of melting is much slower than we might think given how warm the ocean is,” said Peter Davies, an oceanographer with the British Antarctic Survey in Cambridge who was part of the team that drilled a narrow hole in the nook and lowered instruments into it. The find might sound like good news, but it’s not, he says. “Despite these low melting rates, we’re still seeing rapid retreat” because the ice is disappearing faster than it’s being restored.
Davis and about 20 other scientists conducted the study on Thwaites Glacier, a massive ice conveyor belt about 120 kilometers wide that flows off the coast of West Antarctica. Satellite measurements show that Thwaites is losing ice faster than at any time in the past several thousand years. Since 2000, it has accelerated its flow into the ocean by at least 30 percent, shedding more than 1,000 cubic kilometers of ice, about half the ice lost from all of Antarctica.
Much of the current ice loss is caused by warm, salty ocean currents, which destabilize the glacier at its bedrock, a crucial fulcrum about 500 meters below sea level at the drilling site, where the ice rises from its bed and floats.
Now, this first-ever look at the bottom of a glacier near its calving zone shows that the ocean is attacking it in previously unknown and troubling ways.
Subbelly of the glacier
Thwaites Glacier and most of the West Antarctic Ice Sheet lie on a gravel bed that is hundreds of meters below sea level, making the ice vulnerable to warm, salty ocean currents that sweep the sea floor. Thwaites is particularly vulnerable because parts of its grounding zone (where it rises from the bottom and floats in the ocean) are 1 kilometer below sea level, exposing it to the warmest water.
Anatomy of a glacier
SOURCE: TA SCAMBOS
ET AL/GLOBAL AND PLANETARY CHANGE 2017, JENNIFER MATTHEWS/SCRIPPS INSTITUTION OF OCEANOGRAPHY
When the researchers sent a remotely operated vehicle, or ROV, down a well into the water, they found that most of the melting is concentrated in places where the glacier is already under mechanical stress — massive cracks called basal fissures. These holes cut into the lower part of the ice.
Even a little melting in these weak spots can cause a disproportionate amount of structural damage to the glacier, researchers report in two papers published Feb. 15 in Nature .
The results are “somewhat of a surprise,” said Ted Scambos, a glaciologist at the University of Colorado Boulder who was not part of the team. Thwaites and other glaciers are observed mostly by satellites, which gives the impression that thinning and melting are occurring evenly beneath the ice.
As the world continues to warm due to human-induced climate change, the shrinking glacier alone could raise global sea levels by 65 centimeters over centuries. Its collapse would also destabilize the rest of the West Antarctic Ice Sheet, causing a possible rise of three meters in global sea levels.
With these new results, Scambos says, “we’re seeing much more detailed processes that will be important for modeling” how the glacier responds to future warming and how quickly sea levels will rise.
A cold thin layer protects parts of the lower part of Thwaites Glacier
Simply getting these observations “is like taking a picture of the Moon or even a picture of Mars,” Scambos says. Thwaites, like most of the West Antarctic Ice Sheet, lies on a bed that is hundreds of meters below sea level. The floating front of the glacier, called the ice shelf, extends 15 kilometers into the ocean, forming a roof of ice that makes the area almost completely inaccessible to humans. “This could be the pinnacle of exploration” of Antarctica, he says.
These new results are the result of a $50 million effort—the Thwaites International Glacier Collaboration—led by the United States’ National Science Foundation and the United Kingdom’s Environmental Research Council. The research team, one of eight funded by the collaboration, landed on the snowy flatlands of the Thwaites in the final days of 2019.
The researchers used a hot water drill to drill a narrow hole, not much wider than a basketball, through more than 500 meters of ice. Under the ice lay a column of water only 54 meters thick.
When Davis and his colleagues measured the temperature and salinity of this water, they found that most of it was about 2 degrees Celsius above freezing—potentially warm enough to melt 20 to 40 meters of ice a year. But the lower part of the ice seems to be melting at a rate of only 5 meters per year, researchers report in one of the publications Nature . The team calculated the melting rate based on the water’s salinity, which shows the ratio of seawater, which is salty, to glacial meltwater, which is fresh.
The reason for this slow melting was quickly revealed: just under the ice lay a layer of cold floating water only 2 meters thick, which had formed from the melting ice. “A lot more fresh water accumulates at the base of the ice,” Davis says, and this cold layer protects the ice from the warmer water below.
These measurements provided a snapshot right in the borehole. A few days after the opening of the hole, researchers began a wider study of the uncharted ocean cavity under the ice.
The workers lowered the thin yellow-black cylinder into the well with a winch. This ROV, called Icefin, was developed over the past seven years by a team of engineers led by Brittney Schmidt, a glaciologist at Cornell University.
Schmidt and her crew piloted the ship from a nearby tent, controlling the instruments, while she steered the ship with light nudges on the buttons of a PlayStation 4 controller. The smooth, mirrored ice ceiling scrolled silently past a computer monitor, live video fed through 3.5 kilometers of fiber-optic cable.
As Schmidt steered the Icefin about 1.6 kilometers upstream from the well, the water column gradually narrowed until less than a meter of water separated the ice from the seafloor. A few fish and shrimp-like crustaceans called amphipods flitted among the barren piles of gravel.
According to Schmidt, this new section of the sea floor, which is revealed as the ice thins, rises and floats further inland, has been open for “less than a year.”
From time to time Icefin ran past a dark, gaping crack in the ice ceiling, a root crack. Schmidt steered the ship into several such gaps—often more than 100 meters wide—and there she saw something startling.
The melting of Thwaites’ lower abdomen is concentrated in deep cracks
The vertical walls of the cracks were jagged rather than smooth, indicating a faster rate of melting than a flat ice ceiling. And in these places, the video became blurry, as the light was refracted by the energetically swirling eddies of salt and fresh water. Scientists believe that the turbulent eddying of warm ocean water and cold meltwater breaks up the cold layer that insulates the ice, drawing warm salt water into it.
Schmidt’s team calculated that the walls of the cracks are melting at a rate of up to 43 meters per year, the researchers report in a second paper Nature . The researchers also found rapid melting in other places where the ice ceiling is interrupted by short, steep sections.
Greater turbulence and melting are also caused by rift ocean currents. Each time Schmidt steered the Icefin into the crack, the ROV detected streams of water flowing through it, as if the crack were an inverted ditch. These currents moved twice as fast as the currents outside the cracks.
The fact that the melt is concentrated in the cracks has huge implications, says Peter Vashem, an oceanographer with Schmidt’s team at Cornell: “The ocean expands these structures, melting them faster.”
This can greatly speed up the years-long process by which some of these cracks travel hundreds of meters up through the ice until they break through at the top, chipping away at the drifting iceberg. This could cause the floating ice shelf, which presses on the seamount and supports the ice behind it, to break up faster than predicted. This, in turn, can cause the glacier to spill ice into the ocean more quickly. “This will affect the stability of the ice,” says Washam.
This video, taken by a remotely operated vehicle called Icefin, shows the lower part of the Thwaites Glacier as it flows off the coast of West Antarctica. The horizontal sections of the ice are smooth, indicating slow melting. But on steep ice surfaces—especially along the walls of deep crevasses in the ice—the surfaces are ridged, indicating a much faster rate of melting caused by the turbulent eddying of warm, salty ocean water and cold, fresh meltwater. An example of the difference between these two surfaces is clearly visible from 0:11 to 0:13 in the video, when Icefin captures a ridged vertical surface intersecting with a smooth horizontal one.
The new data will improve scientists’ ability to predict the future retreat of Thwaites and other Antarctic glaciers, said Eric Rignot, a glaciologist at NASA’s Jet Propulsion Laboratory in Pasadena, California, who assisted the team by providing satellite measurements of changes in the glacier. . “You just can’t imagine what the water structure might look like in these areas until you observe it,” he says.
But more work is needed to fully understand Thwaites and how this will change as the world continues to heat up. The glacier is made up of two fast-moving bands of ice side by side—one moving at 3 kilometers per year, the other at about 1 kilometer per year. For safety reasons, the team visited a slower lane, which was still extremely challenging. Rigneault says the scientists should eventually visit the expressway, whose top surface is more cracked, making landing planes and operating field camps even more difficult.
The study reported today “is a very important step, but it must be followed by a second step,” he says. “It doesn’t matter how hard it is.”
