A gigantic cavity – two-thirds of the Manhattan surface and a height of nearly 300 meters – is growing at the bottom of the Thwaites Glacier in western Antarctica, is one of several disturbing discoveries published in a new NASA-led exploration of the disintegrating glacier.
The findings underline the need for detailed observations of the downstream Antarctic glaciers in calculating how rapidly global sea levels will rise in response to climate change.
The researchers expect to find some gaps between the ice and the foundation of the bottom of the Tvights, where ocean water could enter and melt the glacier from below. However, the size and explosive growth rate of the newly discovered hole surprised them. It's enough to collect 14 billion tonnes of ice, and the bulk of that ice has melted in the last three years.
"For years we have been suspected that the Thwaites are not firmly attached to the foundation underneath it," says Eric Rigot of the University of California, Irvine, and the Laboratory for the NASA Cooling Laboratory. Rignot is co-author of the new study, which was published today in Science Advances. "Thanks to the new generation of satellites, we can finally see the details," he adds.
The cavity was discovered by an ice-breaker radar in NASA's IceBridge, an air campaign that began in 2010, which explores the links between the polar regions and the global climate. The researchers also used data from the constellation of the radars of Italian and German spaceborne synthetic vents. These very high resolution data can be processed by a technique called radar interferometry to reveal how the surface of the floor below is moved between the images.
"[The size of] the glacier below the glacier plays an important role in melting, "says lead author of the study, Pietro Milillo of JPL." As more heat and water are below the glacier, it melts faster. "
Numerical patterns of ice plates use a fixed form to represent a cavity under ice, rather than allowing the gap to change and grow. The new discovery implies that this limitation probably causes these models to underestimate how fast Thwaites are losing ice.
For the size of Florida, Thwaites Glacier is currently responsible for approximately 4 percent of the global sea-level rise. There is enough ice to raise the world's ocean slightly more than 2 feet (65 centimeters) and the rear walls of adjacent glaciers that will increase sea levels by an additional 2.4 meters if the entire ice is lost.
Thwaites is one of the hardest places to reach Earth, but it's about becoming better known than ever before. The National Science Foundation of the United States and the British National Council for Environmental Research are preparing for a five-year outreach project to respond to the most critical issues related to its processes and features. Thwaites Glacier's international collaboration will begin experiments on the field in the southern hemisphere summer 2019-20.
How do scientists measure the loss of ice
There is no way to track Antarctic glaciers from land in the long run. Instead, scientists use data about satellite or air data to observe characteristics that change like melting glacier, such as its flow velocity and surface height.
Another change in characteristic is the glacier grounding line – a place near the edge of the continent, where it raises its bed and starts sailing on seawater. Many Antarctic glaciers extend for miles beyond their ground lines, which float above the open ocean.
Just as the ground ship can sail again, when the weight of its cargo is removed, a glacier that loses the weight of the ice can hover over the land where it was held. When this happens, the earthing line retracts into the interior. It exposes more than the underside of the glacier to seawater, increasing the likelihood of its rate of melting to accelerate.
For Thwaites, "we discover different withdrawal mechanisms," says Michilo. Different processes in different parts of a long kilometer long 160 kilometers from the glacier place grounding rates and remove ice from synchronization.
The huge void is beneath the main trunk of the glacier on its west side – the side away from the West Antarctic Peninsula. In this region, when the tide rises and falls, the grounding line retracts and progresses through a zone of approximately 3 km to 5 km. The glacier bounces from the ridge of the rock at a steady rate of 600 to 800 million per year since 1992. Despite this stable grounding rate, it seems that the melting rate on this side of the glacier is extremely high.
"On the eastern side of the glacier, the retreat of the ground continues through small channels, perhaps a mile wide, like fingers beneath the glacier to melt from below," says Milillo. In that region, the earthing rate withdrew from about 600 million per year from 1992 to 2011 at 1.2 kilometers per year from 2011 to 2017. Even with this acceleration, however, the melting rates on this side of the glacier are lower than on the west side.
These results emphasize that the interactions of the ice-ocean are more complex than previously understood.
Milillo hopes that the new results will be useful for researchers at the International Thwaites Glacier Collaboration, while preparing for fieldwork. "Such data are essential for the field parties to focus on areas where the action is, because the earthing line is quickly retreating with complex spatial models," he says.
"Understanding the details of how the ocean melts from this glacier is essential to project its impact on sea-level rise over the coming decades," adds Rignot.
Milillo's newspaper and his co-authors in the Science Advances magazine are titled "Heterogeneous retreat and melting of ice from Thwaites Glacier, West Antarctica". The co-authors were from the University of California, Irvine; German airspace in Munich, Germany; and the Grenoble Alps University in Grenoble, France.
Pictures: Thwaites Glacier.
Credits: NASA / OIB / Jeremy Harbek