On the day President Donald Trump is to announce whether or not the United States will withdraw from the Paris Agreement to limit climate change, satellite data showed that the West Antarctic Larsen C shelf is poised to shed an ice block measuring about 5,000 square kilometres (1,900 square miles).
"The rift in Larsen C is likely to lead to one of the largest icebergs ever recorded," said Swansea University in Wales, whose scientists are monitoring the creeping crack.
The rift, which is threatening to carve off a finger-shaped iceberg about 350 metres thick, expanded by 17 kilometres (11 miles) in six days, leaving just a 13-kilometre thread attaching it to the main ice sheet.
"The timing of calving is probably very close," a university statement said.
"There appears to be very little to prevent the iceberg from breaking away completely."
An ice shelf is a floating extension of a land-covering ice sheet.
Larsen C is the most northerly of the Antarctic ice shelves, as well as the largest. The calving of the iceberg would see it lose about a tenth of its total area, shrinking it to its smallest size on record.
On its own, the huge ice cube would not add to sea level rise.
But its detachment may render the remainder of the Larsen C shelf unstable and vulnerable to collapse, which would release vast amounts of water, scientists say.
Latest shelf to fall
If all the glaciers held back by Larsen C were to run into the ocean, the global water mark would increase by about 10 centimetres (four inches), the researchers said.
The West Antarctic ice sheet holds enough frozen water to raise the average sea level by about six metres (20 feet).
Two smaller shelves on the eastern side of the Antarctic peninsula have already collapsed.
The first, Larsen A, was lost in 1995. Seven years later, the Larsen B shelf followed -- at 3,250 square kilometres, it was the size of Rhode Island.
The Larsen B event had no precedent since the end of the last Ice Age some 12,000 years ago, according to glaciologists.
"It is widely accepted that warming ocean and atmospheric temperatures have been a factor in earlier disintegrations of ice shelves elsewhere on the Antarctic Peninsula," the Swansea team said.
Antarctica is one of the fastest warming places on the planet, they added, "a feature which will certainly not have hindered the development of the rift."
Ice shelves break up naturally, but global warming is thought to have accelerated the process, though this has not been scientifically proven.
Scores of countries committed to the 2015 Paris Agreement, which calls for efforts to cap global warming at "well under" two degrees Celsius (3.6 degrees Fahrenheit) over pre-industrial era levels.
Temperatures have already increased by 1.1 degrees Celsius from those levels.
On the campaign trail, Trump threatened to "cancel" the global pact, and he has said he will announce his final decision later Thursday.
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