Sudden disappearance of four square mile Antarctic lake raises alarm over stability of critical ice shelves

Water drained through 4,590 foot-thick ice shelf to reach ocean

Harry Cockburn
Environment Correspondent
Tuesday 29 June 2021 14:20
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<p>Before and after images of the lake (L) and subsequent ice-strewn crater (R) captured a few days apart by NASA’s ICESat-2 </p>

Before and after images of the lake (L) and subsequent ice-strewn crater (R) captured a few days apart by NASA’s ICESat-2

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At some point this month, amid the dark Antarctic winter, a large lake high on the Amery Ice Shelf in East Antarctica suddenly disappeared, leaving a four square mile crater 260 feet (80 metres) deep.

The enormous volume of water contained in the lake – estimated to be 21-26 billion cubic feet – is believed to have escaped to the ocean, draining through ice as thick as 4,590 feet (1,400 metres).

The discovery has alarmed scientists, as rising temperatures due to the climate crisis have meant greater levels of surface meltwater accumulating on Antarctic ice shelves. The resultant lakes can drive fractures through the ice to the ocean and, as in this case, cause the shelves to collapse, leading to increased levels of ice loss from the continent.

The ice shelves which ring Antarctica are critical barriers, acting to hold back even greater loss of ice from the glaciers and ice sheets which cover the land mass.

Despite the continent currently being in the depths of winter, the scientists were able to “see” the lake, and the crater it left behind, using images from a radar satellite.

They said that after drainage, in place of the lake, there was a crater-like depression in the ice shelf surface, covering 4.25 square miles (11 sq km).

The surface depression, known as an ice “doline” contained the fractured remains of the thin ice covering the lake.

“We believe the weight of water accumulated in this deep lake opened a fissure in the ice shelf beneath the lake, a process known as hydrofracture, causing the water to drain away to the ocean below,” said study lead author Roland Warner, a glaciologist with the Australian Antarctic Programme Partnership at the University of Tasmania.

Similar hydrofracture processes are believed to have been part of other ice shelf collapses on the Antarctic Peninsula, but the researchers said they had never seen such a deep hydrofracture before, nor in this ice shelf.

The Amery Ice Shelf is the third largest in Antarctica. It is part of a key drainage basin for the eastern side of Antarctica and the sheet is enormous – covering 23,000 square miles (60,000 sq km) with water reaching over 340 miles (550km) beneath it.

The drainage of the large lake was spotted through use of a green-light laser instrument on NASA’s ICESat-2. This satellite tool transmits pulses of photons and accurately locates the reflection point of each photon it receives back from Earth.

“It is exciting to see ICESat-2 show us details of processes that are occurring on the ice sheet at such fine spatial scale,” said co-author Helen Amanda Fricker, a glaciologist at Scripps Institution of Oceanography.

“Since surface meltwater on ice shelves can cause their collapse which ultimately leads to sea-level rise when grounded ice is no longer held back, it’s important to understand the processes that weaken ice shelves.”

While the scientists cautioned that they could not directly attribute the collapse of the lake to the global climate crisis, they suggested the event could inform how large ice sheets could respond to the warming air temperatures.

They said the increased flows of meltwater into deep, ice-covered lakes and the hydrofracture of thick ice shelves should be considered in projections for future warming.

The research into the event is published in the journal Geophysical Research Letters.

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