Why has the ‘ice tongue’ from the Antarctic Pine Island Glacier shrunk?

Why has the ‘ice tongue’ from the Antarctic Pine Island Glacier shrunk?
A team of researchers on the German research icebreaker Polarstern have successfully mapped an area of the seafloor previously covered by ice.

The Antarctic Pine Island Glacier is one of the fastest-flowing glaciers and over the last 11 years, four icebergs have broken away from its floating ice tongue.

A team of researchers on the German research icebreaker Polarstern have successfully mapped an area of the seafloor previously covered by ice. A comparison of these new maps with satellite images of the Pine Island Glacier reveal why it suddenly retreated toward the coast.

The comparison between the two revealed that at important points the glacier tongue had lost contact with the ground, the team reports in the study published in Cryosphere.

The Pine Island Glacier is one of the fastest-flowing in Antarctica. Between it and the neighbouring glaciers they transport more than 300 gigatonnes of ice from its interior to the Amundsen Sea. It alone is responsible for between 5-10% of global sea-level rise.

Identifying ice loss

Scientists have already identified the cause of this rapid loss of ice. Since the 1940s, warm water masses, which branch off from the Antarctic Circumpolar Current, have travelled beneath the floating part of the glacier, melting what is referred to as its ice shelf from below.

As a result, the ice tongue, which is currently ca. 55 kilometres long, has been losing roughly five metres of thickness per year over the last 25 years.

The British Antarctic Survey have said that what remained unclear was why despite this sustained melting, the Pine Island Glacier’s calving front had barely retreated since the start of observations. In 2015, a significant calving event shifted the edge of the ice shelf 20 kilometres nearer the coast and reduced the floating ice tongue’s total area to roughly 470 square kilometres.

With the help of multibeam echo sounders, the team was able to precisely map the seafloor.

What held the ice back?

The newly created maps of the ocean floor reveal a previously unmapped submarine ridge and two mountains, the peaks of which reach up to a water depth of 370 metres.

The Pine Island Glacier’s 400-metre thick ice shelf must have therefore been grounded on the ridge for several decades as the satellite images of the glacier gathered by the researchers confirm.

When an ice shelf loses contact with such ‘pinning points’, the glacier reacts as if someone had suddenly released a giant brake.

The maps of the sea floor can now be fed into a computer model to work to produce more accurate simulations for future change and the glacier’s contribution to sea-level rise.

The full study is available here.

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