A chunk of ice the size of the Isle of Man has started to break away from Antarctica in what scientists say is further evidence of a warming climate.
Satellite images suggest that part of the ice shelf is disintegrating, and will soon crumble away.
The Wilkins Ice Shelf has been stable for most of the last century, but began retreating in the 1990s.
Six ice shelves in the same part of the continent have already been lost, says the British Antarctic Survey (BAS).
Professor David Vaughan of BAS said: “Wilkins is the largest ice shelf on the Antarctic Peninsula yet to be threatened.
“I didn’t expect to see things happen this quickly. The ice shelf is hanging by a thread – we’ll know in the next few days or weeks what its fate will be.”
‘Like an explosion’
BAS researchers were alerted to the break-up by daily monitoring of satellite images. They sent a Twin Otter aircraft on a reconnaissance mission to video what was happening.
Jim Elliott, who was on board the plane, said he had never seen anything like it before.
He said: “We flew along the main crack and observed the sheer scale of movement from the breakage.
“Big hefty chunks of ice, the size of small houses, look as though they’ve been thrown around like rubble – it’s like an explosion.”
A 41-by-2.5km (25-by-1.6 mile) berg appears to be breaking away, with much of the Wilkins Ice Shelf protected only by a thin strip of ice spanning two islands.
Since an ice shelf is a floating platform of ice, the break-up will have no impact on sea level. But scientists say it heightens concerns over the impact of climate change on this part of Antarctica.
Professor Vaughan predicted in 1993 that the northern part of the Wilkins Ice Shelf would be lost within 30 years if climate warming continued. But he said it is happening more quickly than he expected.
He told BBC News: “What we’re actually seeing is a chunk of the ice shelf drop off in a way that suggests it is not just a normal part of iceberg formation.
“This is not a sea level rise issue, but is yet another indication of climate change in the Antarctic Peninsula and how it is affecting the environment.”
Scientists say the Antarctic Peninsula, which juts out into the Southern Ocean towards the tip of South America, has experienced unprecedented warming over the last 50 years.
Several ice shelves have retreated in the past 30 years – six of them collapsing completely.
Other researchers believe the Wilkins Ice Shelf may hang on a little longer, as Antarctica’s summer melt season draws to a close.
Dr Ted Scambos of the National Snow and Ice Data Center at the University of Colorado said: “This unusual show is over for this season. But come January, we’ll be watching to see if the Wilkins continues to fall apart.”