Satellite measurements show that Thwaites is losing ice more quickly than at any time in the last few thousand years.
In order to investigate the small cavity of seawater that is melting the glacier’s grounding zone, where the ice lifts off the seafloor and begins to float, researchers camped on Antarctica‘s Thwaites Glacier melted a hole through more than 500 metres of ice.
The remote and hostile region of a narrow sliver of seawater beneath a slab of floating ice more than half a kilometre thick is where Antarctica’s most vulnerable climate hot spot is located. Finally, after investigating it, scientists found something unexpected.
Peter Davis, an oceanographer at the British Antarctic Survey in Cambridge, and about 20 other scientists conducted research at Thwaites Glacier, a massive conveyor belt of ice about 120 kilometers wide that flows off the coastline of West Antarctica.
Satellite measurements show that Thwaites is losing ice more quickly than at any time in the last few thousand years. The glacier has accelerated its flow into the ocean by at least 30% since 2000, accounting for roughly half of the ice lost from all of Antarctica.
This is driven by warm, salty ocean currents that are destabilizing the glacier at its grounding zone, 500 meters below sea level. A first-ever look at the glacier’s underbelly near the grounding zone shows that the ocean is attacking it in previously unknown and troubling ways.
Thwaites Glacier and the majority of the West Antarctic Ice Sheet are exposed to warm, salty ocean currents that hug the seafloor because they are situated on a gravelly bed hundreds of meters below sea level.
Thwaites is particularly at risk because portions of its grounding zone, where it rises from the ocean’s floor and floats on top of it, are up to one kilometer below sea level, placing it in contact with the warmest water.
The majority of the melting is concentrated in areas where the glacier is already under mechanical stress, such as within large cracks known as basal crevasses, the researchers discovered after sending a remotely operated vehicle, or ROV, down the borehole and into the water below. These slits pierce up into the ice’s underside.
The researchers report their findings in two papers that were published on February 15 in Nature. Even a small amount of melting at these weak spots could cause a disproportionately large amount of structural damage to the glacier.
According to Ted Scambos, a glaciologist at the University of Colorado Boulder who was not on the team, these findings come as “a bit of a surprise.” Thwaites and other glaciers are primarily observed by satellites, which gives the impression that melting and thinning are occurring uniformly beneath the ice.
The shrinking glacier Thwaites has the potential to raise global sea level by 65 centimetres over a period of centuries, destabilising the remainder of the West Antarctic Ice Sheet and triggering an eventual three-meter rise in global sea level.
The floating front of the glacier, called an “ice shelf,” extends 15 kilometers out onto the ocean, creating a roof of ice that makes this spot almost entirely inaccessible to humans. These new results are important for modelling how the glacier responds to future warming and how quickly sea level will rise.
The International Thwaites Glacier Collaboration, funded by the US National Science Foundation and the UK’s Natural Environment Research Council, conducted a $50 million effort to melt 500 metres of ice on the snowy, flat expanse of Thwaites.
The researchers used a hot water drill to melt a narrow hole, not much wider than a basketball, through a water column that was only 54 meters thick. They measured the temperature and salinity of the water and found that most of it was about 2 degrees Celsius above freezing, potentially warm enough to melt 20 to 40 meters of ice per year.
However, the underside of the ice seems to be melting at a rate of only 5 meters per year. The team calculated the melt rate based on the water’s salinity, which reveals the ratio of seawater to glacial meltwater.
The researchers found that a layer of buoyant, cold water, only 2 metres thick, derived from melted ice, protected the ice from warmer water below. They used an ROV, Icefin, developed by Britney Schmidt, a glaciologist at Cornell University, to explore the unmapped ocean cavity under the ice.
Icefin, a remote-operated vehicle, was lowered down a borehole to measure ocean currents and ice melting rates under Thwaites Glacier. Schmidt and her team piloted the craft from a nearby tent, monitoring instruments while she steered it with gentle nudges to the buttons of a PlayStation 4 controller.
As Schmidt guided Icefin about 1.6 kilometers upstream from the borehole, the water column gradually tapered until less than a meter of water separated the ice from the seafloor below. A few fish and shrimplike crustaceans called amphipods flitted among otherwise barren piles of gravel. This new section of seafloor had been exposed for less than a year.
Icefin skimmed past a dark, gaping cleft in the icy ceiling, a basal crevasse. Schmidt steered the craft into several of these gaps and noticed that the vertical walls of the crevasses were scalloped rather than smooth, suggesting a higher rate of melting than that of the flat icy ceiling.
The researchers also found rapid melting in other places where the level ceiling of ice is punctuated by short, steep sections. This turbulent swirling of warm ocean water and cold meltwater is breaking up the cold layer that insulates the ice, pulling warm, salty water into contact with it. Schmidt’s team calculated that the walls are melting at rates of up to 43 metres per year.
The greater turbulence and higher melting in crevasses are driven by ocean currents within the crevasses, which can move up to twice as fast as the currents outside of crevasses.
This has huge implications, as it could accelerate the years-long process by which some of these cracks propagate hundreds of meters up through the ice until they break through at the top, calving off an iceberg. It could also cause the floating ice shelf to break apart more quickly than predicted.
This new data from NASA’s Jet Propulsion Laboratory in Pasadena, Calif., will help scientists better predict the future retreat of Thwaites and other Antarctic glaciers. It shows that horizontal sections of the ice are smooth, indicating slow melting, but on steep ice surfaces, the surfaces are scalloped, suggesting a much higher rate of melt.
This could cause the glacier to spill ice into the ocean more quickly, which could have an impact on the stability of the ice.
More work is needed to understand Thwaites and how it will change as the world continues to warm. The glacier consists of two side-by-side fast-moving lanes of ice, one moving 3 kilometers per year and the other about 1 kilometer per year.
Scientists must eventually visit the fast lane, whose upper surface is more cracked up with crevasses, making it harder to land aircraft and operate field camps.