̽��ֱ�� of Cambridge - Alison Banwell

Surface lakes cause Antarctic ice shelves to ‘flex’

sc604 — Wed, 13 Feb 2019 10:00:00 +0000

A team of British and American researchers, co-led by the ̽��ֱ�� of Cambridge, has measured how much the McMurdo ice shelf in Antarctica flexes in response to the filling and draining of meltwater lakes on its surface. This type of flexing had been hypothesised before and simulated by computer models, but this is the first time the phenomenon has been measured in the field. ̽��ֱ��results are reported in the journal Nature Communications.

̽��ֱ��results demonstrate a link between surface melting and the weakening of Antarctic ice shelves and support the idea that recent ice shelf breakup around the Antarctic Peninsula may have been triggered, at least in part, by large amounts of surface meltwater produced in response to atmospheric warming.

As the climate continues to warm, more and more ice shelves may become susceptible to flex, fracture and break up over the coming century.

Most of the Antarctic continent is covered by the Antarctic Ice Sheet, which is up to four kilometres thick and contains enough ice to raise global sea levels by about 58 metres. Over most of the continent and for most of the year, air temperatures are well below zero and the ice surface remains frozen. But around 75% of the ice sheet is fringed by floating ice shelves, which are up to a kilometre thick, mostly below sea level, but with tens of metres of their total height protruding above the water. In the summer months, when air temperatures rise above freezing, the surfaces of these ice shelves are susceptible to melting.

“Surface water on ice shelves has been known about for a long time,” said co-author Dr Ian Willis from Cambridge’s Scott Polar Research Institute. “Over 100 years ago, members of both Shackleton’s Nimrod team and the Northern Party team of Scott’s British Antarctic Expedition mapped and recorded water on the Nansen Ice Shelf, around 300 kilometres from where we did our study on the McMurdo Ice Shelf. For the last few decades, it has also been possible to see widespread surface meltwater forming on many ice shelves each summer from satellite imagery.”

What is not fully known is the extent to which surface water might destabilise an ice shelf, especially in warmer summers when more meltwater is produced. If the slope of the ice shelf is sufficiently steep, the water may flow off the ice shelf to the ocean in large surface rivers, mitigating against any potential instability.

̽��ֱ��danger comes if water pools up in surface depressions on the ice shelf to form large lakes. ̽��ֱ��extra weight of the water will push down on the floating ice, causing it to sink a bit further into the sea. Around the edge of the lake, the ice will flex upwards to compensate. “If the lake then drains, the ice shelf will now flex back, rising up where the lake used to be, sinking down around the edge,” said lead author Dr Alison Banwell, also from SPRI. “It is this filling and draining of lakes that causes the ice shelf to flex, and if the stresses are large enough, fractures might also develop.”

Banwell and co-author Professor Doug MacAyeal from the ̽��ֱ�� of Chicago had previously suggested that the filling and draining of hundreds of lakes might have led to the catastrophic breakup of the Larsen B Ice Shelf 2002 when 3,250 square kilometres of ice was lost in just a few days.

“We had been able to model the rapid disintegration of that ice shelf via our meltwater loading-induced fracture mechanism,” said Banwell. “However, the problem was that no one had actually measured ice shelf flex and fracture in the field, and so we were unable to fully constrain the parameters in our model. That’s partly why we set out to try to measure the process on the McMurdo ice shelf.”

Using helicopters, snow machines and their own two feet, the researchers set up a series of pressure sensors to monitor the rise and fall of water levels in depressions which filled to become lakes, and GPS receivers to measure small vertical movements of the ice shelf.

“It was a lot of work to obtain the data, but they reveal a fascinating story,” said MacAyeal. “Most of the GPS signal is due to the ocean tides, which move the floating ice shelf up and down by several metres twice a day. But when we removed this tidal signal we found some GPS receivers moved down, then up by around one metre over a few weeks whereas others, just a few hundred metres away, hardly moved at all. ̽��ֱ��ones that moved down then up the most were situated where lakes were filling and draining, and there was relatively little movement away from the lakes. It is this differential vertical motion that shows the ice shelf is flexing. We’d anticipated this result, but it was very nice when we found it.”

̽��ֱ��team hope that their work will inspire others to look for evidence of flex and fracture on other ice shelves around Antarctica. Their work will also help in developing ice sheet scale models that could be used to predict the stability of ice shelves in the future and to understand the controls on ice shelf size since they act as buffers against fast-moving ice from the continent. As ice shelves shrink, glaciers and ice streams behind them flow more rapidly to the ocean, contributing to global sea level rise.

̽��ֱ��work was funded by the US National Science Foundation, the Leverhulme Trust, NASA, and CIRES, ̽��ֱ�� of Colorado, Boulder.

Reference:
Alison F. Banwell et al. ‘Direct Measurements of Ice-Shelf Flexure caused by Surface Meltwater Ponding and Drainage.’ Nature Communications (2019). DOI: 10.1038/s41467-019-08522-5

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̽��ֱ��filling and draining of meltwater lakes has been found to cause a floating Antarctic ice shelf to flex, potentially threatening its stability.

Filling and draining of lakes causes the ice shelf to flex, and if the stresses are large enough, fractures might also develop

Alison Banwell

Surface lakes on the McMurdo Ice Shelf

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'Extreme Sleepover #11’ - moulins and meltwater on the Greenland ice sheet

bjb42 — Sun, 01 Jan 2012 09:00:57 +0000

“What? No coffee?” We had been dropped off by helicopter onto the Greenland ice sheet where we were to live and work for three weeks. ̽��ֱ��sky was clear and the sun was out, but the air temperature hovered around 0^oC and a cutting katabatic wind blew from the top of the ice sheet in the east, through our ‘windproof’ jackets, and down the coast to our west. We had speedily put up our personal tents and three mess tents (the ‘kitchen’, the ‘workshop’ and the ‘office’). By late afternoon, we had set up the stove; dug through the snow to get water; the kettle was boiling; and now all we needed was the coffee. But someone had forgotten to pack it. “Three weeks without coffee?” Our heads throbbed, our hands shook and our moods sank into our felt-lined boots.

That early-June morning, we had flown to the ice sheet from the small coastal town of Ilulissat, Disko Bay, west central Greenland. Ilulissat, meaning ‘icebergs’ in the native language, has around 4,000 people and is remarkable for its brightly coloured wooden buildings. To the south is Ilulissat Fjord, which has at its head one of Greenland’s biggest and fastest moving outlet glaciers, Jakobshavn Isbræ. As it shudders forwards, thousands of icebergs calve off the front each year, some the size of aircraft carriers. For the last decade, local people and visiting scientists have witnessed one of the world’s most noticeable effects of climate change, as the floating front of the glacier has retreated by around 10 km. ̽��ֱ��glacier has also sped up; ten years ago it flowed at 7 km a year, but now that figure is closer to 15 km.

Our goal was to measure three key things on the ice sheet surface: melting, water flow through snow and along channels etched into the ice, and the filling (and hopefully drainage) of lakes. We radiated out from our camp each day, sometimes for 2–3 km, trussed up in harnesses jangling with ice screws and carabiners, roped together like beads on a string in case anyone disappeared into a crevasse or a hole under the snow. We advanced slowly as we probed for these hidden dangers, but also in an attempt to avoid large patches of slush that developed within the melting snow. We had 24 hours of daylight, of course, and quickly learnt to tell the time from the direction of the sun gyrating around us, higher to the south around midday and lower towards the north, reminding us to crawl into our sleeping bags at the end of each day.

For several days we had watched a nearby lake grow into what was now a thick sky blue ribbon, tapering at the edges, spread across the bright white of the ice sheet – about 800m across. But, as we watched, it suddenly began to shrink: the thick ribbon became a thin band, then a tiny thread, and then it was gone. During the final dying moments of the lake, ice blocks the size of truck containers swirled on the water like pieces of soap around an emptying plug hole. A volume equivalent to 600 Olympic-sized swimming pools (or 15 Royal Albert Halls) had drained in just 2.5 hours. On the floor of the former lake was a new fracture 600 m long that had been produced by the weight of the water. Six large holes or ‘moulins’ had formed along the fracture, the largest of which was around 10 m wide and still had water thundering into it when we reached it.

After nearly three weeks on the ice sheet we were reluctantly ready to leave. It took a few days to bring our instruments back to camp, collapse our tents and pack our things into the boxes they had arrived in. ̽��ֱ��helicopter retrieved us and whisked us back to Ilulissat which felt warm, peculiarly dry underfoot, and was fused with colours and smells that we had been deprived of for three weeks. And at last there was coffee.

Dr Ian Willis and Alison Banwell

Ian is a Senior Lecturer and Alison is a PhD student at the Scott Polar Research Institute, and both are members of St Catharine’s College. Ian has over 20 years of extreme sleepover experience on top and in front of several of the World’s glaciers. In addition to his Greenland work, he currently has projects in Svalbard, Iceland and New Zealand. Alison has recently been awarded a Dow Sustainability Innovation Student Challenge Prize that will allow her to extend her Greenland work to the study of Himalayan glaciers. Ian and Alison’s work was supported by the Natural Environment Research Council, ̽��ֱ�� of Cambridge Travel Fund, BB Roberts Fund, Scandinavian Studies Fund and St Catharine's College.

A longer version of this article was originally published in the St Catharine’s College Society Magazine 2011.

In the eleventh of a series of reports contributed by Cambridge researchers, glaciologists Dr Ian Willis and Alison Banwell watch as a lake disappears before their eyes.

During the final dying moments of the lake, ice blocks the size of truck containers swirled on the water like pieces of soap around an emptying plug hole.

Dr Ian Willis

Ian Willis

Peering into a moulin

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