THE latest alarming ice shelf collapse in the Antarctic has been caused by a warming Southern Ocean melting the shelf from below.
The climate-change-induced break-up of the Wilkins Ice Shelf began last February, and has become the only documented collapse to run through the depths of winter.
At least 1350 square kilometres of ice shelf on the western side of the Antarctic Peninsula has broken off so far, and more is expected to go, say US and European scientists.
An ice bridge buttressing a large part of the Wilkins against an island off the peninsula is likely to fail soon, taking another 500 square kilometres. The collapse may not halt until the shelf, once 13,680 square kilometres, is at least halved, said Ted Scambos of the US National Snow and Ice Data Centre in Colorado.
Much of the loss of seven great ice shelves on the vulnerable Antarctic Peninsula has been blamed on surface melting.
With the area having experienced the world's greatest regional temperature rise — 2.5 degrees in 50 years — scientists have seen summer melt ponds form on top of ice shelves, before the water falls hundreds of metres through the ice, splitting it apart.
Dr Scambos said a different culprit had been found in the case of the Wilkins.
"This is the first time that it's been observed in a way that's so clearly attributable to basal melt," he said.
The conclusion was reinforced by a team led by the University of Bonn's Matthias Braun.
"We show that drainage of melt ponds into crevasses was of no relevance for the break-up at Wilkins," Dr Braun concluded in the journal Cryosphere.
He said the ice was over-stressed by buoyancy forces pressing on it from below, breaking open rifts in the shelf.
Changes in wind patterns further north have meant that, at vulnerable places such as the Wilkins, warmer water is being pushed more vigorously beneath the ice front.
"The water below the Wilkins was two, three or four degrees above the melting point of sea water," Dr Scambos said.