A gigantic cavity – two-thirds of the 300-meter-high Manhattan area that grows at the bottom of Thwaites Glacier in West Antarctica – is one of several disturbing discoveries reported in a new NASA-led study of disintegration. glacier.
The findings underscore the need for detailed underwater observations of Antarctic glaciers to calculate how quickly the global sea level will increase in response to climate change.
The researchers hoped to find some gaps between the ice and the bedrock at the bottom of Thwaites, where ocean water could flow and melt the glacier from below. The size and rate of explosive growth of the new hole, however, surprised them. It's big enough to contain 14 billion tons of ice, and most of that ice has melted in the last three years.
"We suspected for years that the Thwaites were not tightly bound to the rock bed below," said Eric Rignot of the University of California at Irvine and NASA's Jet Propulsion Laboratory in Pasadena, Calif. Rignot is co-author of the new study, published today in Science Advances. "Thanks to a new generation of satellites, we can finally see the detail," he said.
The cavity was revealed by an ice-penetrating radar in NASA's IceBridge Operation, an airborne campaign begun in 2010 that studies the connections between the polar regions and the global climate. The researchers also used data from a constellation of German and Italian synthetic aperture radars. These very high resolution data can be processed by a technique called radar interferometry to reveal how the surface of the ground below has moved between the images.
"[The size of] a cavity under a glacier plays an important role in melting, "said lead study author Pietro Milillo of JPL. As more heat and water under the glacier, it melts faster. "
Numerical models of ice sheets use a fixed shape to represent a cavity under the ice, instead of allowing the cavity to change and grow. The new finding implies that this limitation probably makes these models underestimate the speed with which the Thwaites are losing ice.
About the size of Florida, the Thwaites Glacier accounts for approximately 4% of sea level rise. It contains enough ice to raise the ocean at just over 2 feet and protects neighboring glaciers by raising sea level by an additional 2.4 meters (8 feet) if all ice is lost.
Thwaites is one of the most difficult places to reach on Earth, but it is about to become better known than ever. The US National Science Foundation and the British National Environmental Research Council are setting up a five-year field project to answer the most critical questions about their processes and characteristics. Thwaites International Glacier Collaboration will begin its field experiments in the Southern Hemisphere summer of 2019-20.
How Scientists Measure Ice Loss
There is no way to monitor long-term Antarctic ground-level glaciers. Instead, scientists use data from airborne or satellite instruments to observe characteristics that change as a glacier melts, such as the velocity of the stream and the height of the surface.
Another mutating feature is the glacier ground line – the location near the edge of the continent, where it leaves the bed and begins to float in the sea water. Many Antarctic glaciers stretch for miles beyond their land lines, floating over the open ocean.
Just as a grounded boat can float again when the weight of its cargo is removed, a glacier that loses weight from the ice may float over the land where it used to stick. When this happens, the ground line retreats inland. This exposes more the bottom of a glacier to seawater, increasing the likelihood of its melting rate accelerating.
An Irregular Retreat
For Thwaites, "we are discovering different mechanisms of withdrawal," Millilo said. Different processes at various parts of the front of the 100-kilometer-long (160-km-long) glacier are putting the ground-line retreat and ice-loss rates out of sync.
The huge cavity is under the main trunk of the glacier on its west side – the furthest side of the Western Antarctic Peninsula. In this region, as the tide goes up and down, the ground line retreats and advances through an area of about 2 to 3 miles (3 to 5 kilometers). The glacier has been taking off from a mountain range on the rocky bed at a constant rate of about 0.4 to 0.8 km (0.6 to 0.8 km) per year since 1992. Despite this stable rate of land-line retreat , the melting rate of this side the glacier is extremely high.
"On the eastern side of the glacier, the retreat from the land line follows small canals, perhaps one kilometer wide, like fingers reaching the glacier to melt underneath," Milillo said. In this region, the ground-line retreat rate has doubled from about 0.6 miles per year from 1992 to 2011 to 1.2 kilometers per year from 2011 to 2017. Even with this accelerated retreat, however, melt rates this side of the glacier are smaller than on the west side.
These results highlight that ice-ocean interactions are more complex than previously understood.
Milillo hopes the new results will be useful to researchers at the Thwaites International Glacier Collaboration as they prepare for field work. "Such data are essential for the field parts to focus on the areas where the action is, because the ground line is rapidly receding with complex spatial patterns," he said.
"Understanding the details of how the ocean melts this glacier is essential to projecting its impact on sea-level rise in the coming decades," Rignot said.
Milillo's paper and its co-authors in Science Advances magazine is titled "Heterogeneous Retreat and Melting Ice from the Thwaites Glacier, Western Antarctica." Co-authors were from the University of California, Irvine; the German Aerospace Center in Munich, Germany; and the Grenoble Alpes University in Grenoble, France.
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