Ice shelves are disappearing
The polar landscape is changing. Belinda Smith reports on shrinking ancient ice shelves.
Better book a cruise soon if you want to experience the awe-inspiring sight of Antarctic Peninsula ice shelves towering above you. They’re 10,000 years old and crumbling, one by one.
Over the past few months, measurements from ice-penetrating radar suggest a chunk half the size of Tasmania could disappear within the next century. “We’re watching a large-scale natural experiment unfold before our eyes,” says Ala Khazendar, climate scientist at NASA’s Jet Propulsion Laboratory in California.
And it’s not only bad for tourism.
Ice shelves form long coastal sections of Antarctica and are also found in northern Canada and Greenland. They create an ever-changing coastline as they calve huge icebergs into the sea. Like the congealed plug on your toothpaste, ice shelves can also hold back seaward glaciers. Were Antarctica’s ice shelves to disappear completely, sea level rises of tens of metres would obliterate the low-lying regions of the world.
Luckily, for the moment, Antarctica’s collapsing ice shelves are confined to the continent’s warmest, most northerly region, the Antarctic Peninsula. The peninsula sticks out from the Antarctic continent like a thumb from a fist, extending 1,300 kilometres towards South America. Its rocky islands are mostly covered by an ice sheet up to 500 metres thick, the middle of which is drained by glaciers running east and west. The Larsen ice shelves, named for the British-Norwegian explorer and sea captain Carl Larsen, plug the east-running glaciers.
Just 20 years ago, seven ice shelves dotted the bayside edge of the peninsula. But in 1995, Larsen A – a 2,000-square-kilometre shelf at the northernmost tip of the thumb – collapsed completely in a single storm. Moving south, two thirds of Larsen B followed in 2002, and its remaining third looks set to crumble within a few years. Then there’s Larsen C. At nearly 50,000 square kilometres – or half the area of Tasmania – it’s the biggest of the peninsula’s ice shelves. It’s still standing – but for how long?
“It’s basically haemorrhaging ice”
Like living organisms, ice shelves shed and grow. They calve off massive chunks of ice that bob around the ocean as icebergs and are usually replenished by snowfall and tributary glaciers each year. But in a Science paper in April, climate scientist Fernando Paolo from the University of California, San Diego reported the balance has been lost: ice shelves are losing ice faster than they can regrow.
From 1994 to 2003, ice shelves lost an average of around 25 cubic kilometres of ice per year. Then from 2003 to 2012, the ice loss ramped up more than tenfold to 310 cubic kilometres per year. At some point, these ice shelves could reach a tipping point that sends them into sudden, dramatic collapse.
So why are ice shelves retreating? No surprise: global warming. But there’s nothing simple about Antarctic weather patterns.
Out on Antarctica’s thumb, the ice shelves are melting from warmer ocean currents flowing below, and warmer temperatures above. The peninsula’s annual average temperature has increased by around 3 °C in the past 50 years – several times faster than the global average. Scientists are yet to crack the exact processes that have led to this localised warming, because across most of the continent’s fist, winds are strengthening and increasing the spread of sea ice [see "Two types of ice and why they matter", below].
Stronger westerly winds around Antarctica can be linked to the ozone hole and, perhaps, to global warming, Paolo says. While the ozone hole appears to be filling, he says “climate models suggest in the near future global warming will play a more important role in changing these winds”, which will increase ice shelf melt.
Using radar satellite imagery, Khazendar traces vast fractures in ice shelves. In a paper in Earth and Planetary Science Letters he and colleagues described one such crack creeping across what’s left of Larsen B, 12 kilometres from its grounding line –where the ice shelf sits on rock. In addition, the flow of surrounding glaciers is also picking up pace. “It’s basically haemorrhaging ice,” Khazendar says.
Paul Holland, an oceanographer with the British Antarctic Survey, analysed 14 years’ of aircraft radar, which he says
is a way to “see through the ice”. Holland saw that Larsen C has been thinning too. He and his colleagues reported their findings in the journal The Cryosphere.
“Larsen B collapsed because it retreated past a critical point,” Holland says. “It’s probably what will happen to Larsen C – we just don’t know where that point is.”
While Larsen B held back 10 or so small glaciers before its collapse, Larsen C stems around 30. When it collapses, we can expect to see a few centimetres of sea level rise over a few decades. And while that may not sound much, the rapid onset will be tough to handle, Holland says: “We could deal with a rise of a metre over a 1,000 years, but over 100 years it would be really hard.” The Intergovernmental Panel for Climate Change’s fifth assessment report has already forecast a 25-85 centimetre sea level rise by 2100.
The stakes are even higher in other parts of Antarctica, Khazendar says. Modelling suggests the far larger West Antarctic ice sheet, which lies south of the peninsula, is tipped to cause up to four metres of sea level rise over the next 1,000 years. West Antarctica’s Pine Island glacier, where temperatures are also rising, is the fastest melting glacier on the continent and already accounts for a quarter of the continent’s ice loss.
So what can we do? Not much – collapse of the remaining part of Larsen B is inevitable, Khazendar says: “All we can do is diagnose and forecast.”
“I’ve been studying Larsen C for 10 years now and I’ve always been extremely sceptical about its collapse within a century,” Holland says. “But now I’ve accepted the possibility that it’ll go within my children’s lifetime.”
Two types of ice and why they matter
Last summer, the extent of Antarctica’s sea ice reached a record high, denying supply ships access to Mawson station, and requiring all supplies to be flown in by helicopter. How can ice shelves be melting while sea ice is growing?
It’s important not to confuse the two, says NASA climate scientist Ala Khazendar: “They both float – and that’s about it.”
Sea ice is only a metre or two thick. Its extent depends mostly on wind strength, but also on ocean temperature, waves, currents and tides. Paradoxically, increased sea surface temperatures boost the extent of sea ice. Thinner ice layers break up and are spread more easily by winds.
The winds swirling around Antarctica have been blowing more strongly over the past few decades due to the steepening temperature gradient between the warming equator and the atmosphere over Antarctic, which has been cooled by the hole in the ozone layer. This has concentrated the winds into a tighter circle.
Down near the surface, strong southerlies, fresh off the West Antarctic ice sheet blow sea ice away from the coast to extend far into the Southern Ocean. But warmer winds from the north, such as those blowing onshore to the western edge of the Antarctic Peninsula, produce far less sea ice.