Sea levels could rise by half a metre by the end of the century due to melting of the Antarctic ice shelf. A new international study shows that the ice shelf is thinning from above and below. Barbara Barkhausen reports from Sydney.
How thick is the ice in Antarctica, where is it thickening, where is it thinning and most important: why? A scientific debate has been held on these and similar questions for more than a decade, but an international study, published last week in the journal The Cryosphere, now seeks to settle the issue once and for all.
When will Larsen C collapse?
Scientists, including those from the British Antarctic Survey (BAS), compared satellite data and radar surveys of the Larsen C Ice Shelf during a 15-year period from 1998 to 2012. The neighbouring Larsen A and B ice shelves already collapsed – in 1995 and 2002 respectively – leading many scientists to assume that the third largest ice shelf on Earth could itself thin and become less stable.
The data confirmed the scientists’ worst fears: the ice shelf is thinning both from above and from below. Over the 15-year period, Larsen C lost an average of 4 metres of ice and had lowered by an average of one metre at the surface. The reason is equally troublesome: the Antarctic Peninsula experienced a temperature rise of 2.5 degrees Celsius over the last 50 years. The warmer air temperature is melting the top layer of snow and ice while warmer ocean currents are likely doing the same from below.
Sea-level rise threatens coastal cities
“If this vast ice shelf – which is over two and a half times the size of Wales and 10 times bigger than Larsen B – was to collapse, it would allow the tributary glaciers behind it to flow faster into the sea,” warns Paul Holland from BAS and one of the lead authors of the study. “This would then contribute to sea-level rise.”
The scientists predict that a collapse could occur within a century, perhaps even sooner and with little warning. Already today a crack is forming in the ice, and the ice shelf appears to be detaching from a small island called Bawden Ice Rise.
If the Larsen C Ice Shelf, which at 50,000 square kilometres is bigger than Larsen A and B combined, should collapse, then it would add to the projections of sea-level rise by 2100, confirmed David Vaughan, a glaciologist and director of science at BAS. “We expect that sea-level rise around the world will be something in excess of 50 cm higher by 2100 than it is at present and that will cause problems for coastal and low-lying cities.”
Freshwater ice melts, sea ice freezes
While the ice shelf – composed of freshwater ice that originally fell as snow – itself is melting, the salty seawater around Antarctica is freezing more quickly these days and creating more sea ice cover. Workshops are even being held right now in the Australian city of Hobart on increased sea ice coverage in the Antarctic because it is making work on the icy continent increasingly more difficult for scientists as the thicker ice is affecting the delivery of supplies to Antarctic research facilities. The research vessel Akademik Shokalskiy made headlines around the world back in 2013 when it was trapped in ice for days with 74 people on board.
Climate scientist Chris Turney of the University of New South Wales in Australia even suggests that the expansion in sea ice coverage is connected to the melting freshwater ice shelf. Freshwater is “less dense than salt water, so it floats to the surface relatively and then it’s more vulnerable to freezing,” explains the scientist. The wind pushes cold air onto the sea water, which encourages the ice to rapidly freeze over and can quickly – and unexpectedly – land a research vessel in distress.