ANTARTICA: The sea ice has no immediate impact on ocean levels, but could ultimately cause catastrophic floods worldwide, writes Dick Ahlstrom, Science Editor
A sheet of ice bigger than counties Dublin and Meath combined has broken off the coast of Antarctica and disintegrated into thousands of icebergs. It is the largest single event of its kind in 30 years according to US specialists.
The slab of ice 220 metres thick and covering 3,250 square kilometres collapsed from the Larsen B ice shelf, which floats on the sea's surface off the Antarctic Peninsula due south of the tip of South America. This staggering loss of ice took place over less than six weeks and occurred because of rising temperatures in the region according to Dr Chris Doake of the British Antarctic Survey.
Large chunks of floating sea ice or "ice shelf" regularly snap off when they get too large to support their size, Dr Doake said yesterday. They are fed and replenished by Antarctica's glaciers, and only yesterday satellite observations detected a newly released piece of ice shelf in the Amundsen Sea measuring 53 miles long and 40 miles wide.
"That is quite different to what has happened to Larsen B," Dr Doake said. The ice loss at Larsen B is important because it is not part of a natural process and is due specifically to a gradual rise in temperatures off Antarctica. "It is not a cyclic process that has happened here. It is a once-off effect," he said. Larsen B "is not going to recover. There is going to be open sea \ for the foreseeable future."
The Larsen B shelf is old, at least 400 years and perhaps up to 12,000 years old according to studies. Average annual temperatures in the Antarctic Peninsula region have risen steadily over the past 50 years however and at a faster rate than average global warming trends.
The area is now 2.5 degrees C warmer on average, which has led directly to the collapse of Larsen B, Dr Doake said. "Larsen B was something that was predicted, it is simply a consequence of a rise in temperature."
The current disintegration has been underway since 1995 and the 2002 collapse released a stupendous 720 billion tonnes of ice. This is enough for 29 trillion five lb bags or enough water to irrigate 1,500 golf courses for a year, according to the University of Colorado's National Snow and Ice Data Centre (NSIDC).
The latest loss released thousands of icebergs that are now drifting into the Weddell Sea. Over the last five years the shelf has lost 5,700 square kilometres and is now less than half its previous minimum stable size.
The break-up process seems to involve melt water on the surface of the ice shelf, which can be as thick as two football pitches end to end according to Dr Ted Scambos of the NSIDC. The water either filters down through cracks enhancing fracturing of the enormous ice slabs or refreezes to prise the gaps further apart.
The loss of sea ice has no impact on global ocean levels because it is already in the water like an ice cube floating in a glass. If sea ice continues to melt as is predicted, however, the losses could have an impact by speeding up the loss of ice from the land, Dr Doake said.
"It is a bit uncertain what effect the floating ice has on the land ice," he said yesterday. "We would expect to see some increase in speed" of glacier ice flow towards the sea, he added.
Continued loss of sea ice could have serious long term consequences according to the NSIDC. "Ice shelves act as a buttress or braking system for glaciers," according to the Centre. "They moderate the amount of melting that occurs on their surfaces."
Sea ice loss could mean faster discharge from Antarctica's massive land-locked ice cover.
The largest ice shelf in Antarctica is the 500,000 square kilometre Ross Shelf south of New Zealand. It is the main outlet for the major glaciers draining the West Antarctic Ice Sheet, land ice which contains enough melt water to raise world sea levels by five metres.
The loss of even part of this land ice would represent a world catastrophe, submerging coastal cities such as Dublin and washing away entire countries including Bangladesh.
The balance between stability and collapse for the sea ice can be finely balanced, according to the Centre. The warmest part of the giant Ross Shelf is currently only a few degrees too cool in summer to undergo the same kind of melt water process that has taken Larsen B apart. This could change as regional temperatures continue to climb.