AS IF on cue, an ice wall damming the endangered Wilkins ice shelf against the Antarctic Peninsula has shattered just as scientific alarms ring about the region's rapid warming.
The 40-kilometre bridge held out for more than a year while ice behind it broke up, but European Space Agency images show it finally failed on Sunday night, Australian glaciologist Neal Young said yesterday.
"Now it looks like a laminated windscreen hit by a stone," said Dr Young, of the Antarctic Climate and Ecosystems Co-operative Research Centre in Hobart.
The loss opens the way for thousands of square kilometres of ice behind to float away. "All that mess will eventually be flushed out," he said. "If that clears, I would anticipate more fracturing around the Wilkins."
American scientist Ted Scambos has calculated about half of the 13,680-square-kilometre Wilkins is in danger of disappearing.
It is the latest of seven great ice shelves afloat along the peninsula to let go — and the farthest south — as warming heads towards the globally important West Antarctic ice sheet.
The peninsula has experienced the world's greatest air temperature rise in the past 50 years: 2.5 degrees. Scientists have seen waterfalls plunge from the top of the shelves in frightening evidence of climate change at work.
But Dr Scambos, of the National Snow and Ice Data Centre in Colorado, said the Wilkins losses are the first to be clearly attributable to a slightly warming ocean, melting the shelf from below.
Just as a melting ice block does not raise the level of water in a glass, sea level is not affected by ice-shelf break-up. Of greater concern is the land-bound ice held behind these shelves, which speeds up its flow to the sea.
Substantial coastal change is now happening in all parts of the Antarctic Peninsula, according to a new study by the US Geological Survey and British Antarctic Survey that mapped 174 ice coastlines and counted 142 in retreat.
"The changes in the map area are widely regarded as among the most profound, unambiguous examples of the effects of global warming on Earth," Dr Scambos concluded in the study.
It found that, for the first time, the 2000-square-kilometre Wordie ice shelf had completely disappeared, and an area of the Larsen B shelf more than three times the size of the ACT had gone since 1986.
Such changes are testing the capacity of map-makers to keep up. Not only must they redraw shelf fronts; islands once covered in ice are appearing for the first time. But these pale in comparison to changes that could come with the loss of the West Antarctic ice sheet.
In another study prepared for this week's Antarctic Treaty meeting in the US, polar scientists warn that the Amundsen Sea sector of the West sheet, which holds one-third of its ice, is the most rapidly changing region of the continent.
The international Scientific Committee on Antarctic Research report said the rate of mass ice loss from the Amundsen Sea embayment ranged from 50 to 137 billion tonnes, equivalent to the entire mass loss from the Greenland ice sheet.
"Observed recent rapid changes give cause for concern — especially for the stability of parts of West Antarctica," it said.