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The glacier cavity indicates that the ice melts faster than we thought

The huge cavity – two-thirds of the Manhattan area and nearly 300 m high – growing at the bottom of the Thwaites glacier in the western Antarctic is one of the few disturbing discoveries reported in a new NASA study of a decayed glacier.

The findings highlight the need for detailed observations of the underside of the Antarctic glaciers in calculating global sea-level speeds in response to climate change.

Scientists expected to find some gaps between ice and subsoil at Thwaites' bottom, where ocean water could flow and melt the glacier down. However, the size and speed of the explosion of the newly found hole surprised. It's big enough to contain 14 billion tons of ice, and most of this ice melt in the last three years.

"We have years of suspicion that the Thwaites were not firmly connected to the subsoil beneath it," says Eric Rignot of the University of California, Irvine and the NASA Jet Propulsion Laboratory. Rignot co-authored a new study published today in the Science Advances journal. "Thanks to the new generation of satellites, we can finally see the details," he adds.

The cavity was unveiled by NASA's icebreaking icebreaker IceBridge, an air campaign launched in 2010 to explore the link between the polar regions and the global climate. Researchers also used data from the constellations of Italian and German space-based radar satellites. This high-resolution data can be processed by a technique called radar interferometry, which reveals how the underground area moves between the images.

"[The size of] the cavity under the glacier plays an important role in melting, "says lead author Pietro Milillo of JPL." When he gets more heat and water under the glacier, he's spinning faster. "

Numerical models of ice tables use a solid shape to represent the cavity beneath the ice, rather than allowing the cavity to change and grow. A new discovery suggests that this restriction is most likely to cause these models to underestimate how quickly Thwaites lose ice.

Of the size of Florida, Thwaites Glacier is currently responsible for about 4% of the global sea level rise. It has enough ice to lift the world's ocean just over two feet (65 centimeters) and the back of the adjacent glaciers that would raise the sea level by an additional 2.4 m if the whole ice was lost.

Thwaites is one of the hardest places to land on Earth, but it is about to be more famous than ever before. The American National Science Foundation and the British National Environmental Research Council are preparing a five-year field project to answer the most critical questions about its processes and properties. International Thwaites Glacier Collaboration will begin its field experiments in the summer of the Southern Hemisphere 2019-20.

How do scientists measure ice losses

You can not watch the Antarctic glaciers from the terrain level in the long run. Instead, scientists use data from satellites or aerospace devices to track the properties that change as the glacier fuses, such as flow rate and surface height.

Another variable element is the grounding of the glacier – a place near the edge of the continent, where it rises out of bed and begins to swim on the seawater. Many Antarctic glaciers stretch for miles beyond their ground lines and float across the open ocean.

Just as the grounded ship can swim again when the weight of its cargo is removed, the glacier that loses the weight of ice can swim above the ground where it is held. When this happens, the ground line will retreat into the interior. This exposes the bottom of the glacier to the bottom of the seawater, thereby increasing the likelihood that its melting speed will accelerate.

Irregular retreat

For Thwaites: "We are discovering different mechanisms of retreat," says Millilo. Different processes in various parts of the 100 km long (160 kilometers long) front of the glacier cause the degree of earthing loss and ice loss not synchronized.

The huge cavity is below the main glacier trunk on its west side – from the western Antarctic peninsula. In this area, when the tide rises and falls, the ground line recedes and passes through a zone of about 3 km to 5 km. Since 1992, the glacier has been getting from the ridge in the base from 1992 to 600 meters to 800 meters. Although this steady rate of steady leaks is, the melting speed on this side of the glacier is extremely high.

"On the eastern side of the glacier, the receding opening line runs through small channels, perhaps a kilometer wide, like fingers that get under the glacier to melt it from beneath," says Milillo. In this region, the rate of steady decline has doubled from about 600 meters per year from 1992 to 2011 to 1.2 km per year from 2011 to 2017. Even with this rapid decline, melting temperatures on this side of the glacier are lower than on the west side .

These results emphasize that ice-ocean interactions are more complex than before.

Milillo hopes that the new results will be useful to Thwaites Glacier Collaboration researchers in preparing for their field work. "Such data are vital for political parties to concentrate on areas where action is taking place because the grounding line is rapidly receding with complex spatial patterns," he says.

"Understanding how this glacier melts through the ocean is essential to show its impact on sea level rise over the coming decades," adds Rignot.

The article Mililla and his co-authors in Science Advances is called "Heterogeneous retreat and ice melt Thwaites Glacier, West Antarctica." Co-authors were from the University of California, Irvine; German Air Force Center in Munich, Germany; and the University of Grenoble Alpes in Grenoble, France.

Pictured: Thwaites Glacier.
Credits: NASA / OIB / Jeremy Harbeck

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