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Larsen C ice shelf cracks - Expert reaction


Larsen C ice shelf cracks - Expert reaction
13 July 2017

After weeks of teasing, a trillion-tonne iceberg has calved off the Larsen C ice shelf into the sea.


A UK scientist who has been monitoring the ice shelf cautioned that it was too early to blame the calving of the ice shelf to human-generated climate change, and the event "may simply be a rare but natural occurrence".

The SMC asked Antarctic researchers to comment on the event. Feel free to use these comments in your reporting.

Dr Natalie Robinson, marine physicist, Niwa, comments:

"This is a ‘normal’, if relatively large, calving event, and is very different from the collapse of its neighbouring ice shelves. There appears to be no evidence that the Larsen C has been subject to the surface melt that led to the rapid and very dramatic collapse of Larsen B. The fact that the Larsen C is able to calve such an enormous, contiguous piece of ice, is more indicative of it being in pretty good health, rather than the opposite.

"This large iceberg is the result of the relentless seawards slide of the land-based glaciers feeding the ice shelf. In this case, there were a couple of ‘pinning points’ (i.e. islands) that meant pressure built up within the ice shelf – and very likely contributed to determining the exact location of the fracture that ultimately created the iceberg.

"The significance of this particular event will come in the next weeks to months as scientists (not me, or anyone in NZ) monitor its response to the calving. Will it flow faster? Will it find new locations to pin or ground? Will there be a response from sea ice in the local/regional vicinity?

"Of itself, this event will make no impact on sea level, as it was already floating – the potential for an effect on sea level depends on whether the glaciers feeding the ice shelf speed up as a result of this release of pressure. And even then, the glaciers directly feeding the Larsen C Ice Shelf only have the potential to contribute 1 cm to global sea level.

"What is potentially a much greater immediate impact is the shipping hazard the iceberg presents. While it is large and intact, it can be observed and tracked from space. However, once it begins to break up it could disintegrate into hundreds (maybe thousands?) of smaller icebergs that are too small to be individually tracked, but still represent an enormous hazard from the perspective of a ship navigating these waters.

"For comparison, the B-15 iceberg that broke from the Ross in 2000 could’ve created 10,000 icebergs of 1x1 km in size. Some of these came close to NZ eventually and were labelled as ‘massive’ by the media, but in comparison to the original icebergs, they were merely fragments.

"The event represents a very rare opportunity to study these large-scale processes and responses – I think only four icebergs larger have calved in the entire satellite era (since 1979). So each one presents a new opportunity to use the satellite-based tools to better understand the system as a whole. In this case, Larsen C has lost 12 per cent of its entire mass in a single event – so it is certainly significant from the ice shelf’s perspective. And we can expect to see some significant (if not ‘dramatic’) response from it in the next months to years.

"Incidentally, some people are keeping an eye on a crack that will create the next iceberg from the front of the Ross Ice Shelf. But this may take 10 years or more."

Additional facts about Larsen C:

"The iceberg is about 30 m above sea level, with a further ~200 m hidden beneath the ocean. It covers an estimated 6,000 square km – so about ¾ of the entire Wellington region."

Associate Professor Nancy Bertler, Antarctic Research Centre, Victoria University of Wellington, comments:

"The Antarctic Peninsula is one of the fastest warming regions on Earth. Part of this warming comes from direct temperature increases in the atmosphere due to higher greenhouse gas concentrations and partly this is an indirect effect of ozone-destroying CFCs.

"The increase in greenhouse gas concentration and the development of the ozone hole have caused the westerly winds to shift southwards towards Antarctica and in the process, these winds also became faster. The Antarctic Peninsula sticks out into this shifted westerly wind belt and similar to föhn winds in Christchurch (the warm winds that come over the South Island Alps) these winds rise over the Antarctic Peninsula and when then descend they are much warmer and drier than before, raising the temperature.

"This has led to a strong warming of the Antarctic Peninsula which in turn causes the catastrophic collapse of numerous ice shelves, some of which have been shown to have existed for 10,000 years or more. As these ice shelves collapse, they don’t add to sea level rise (ice shelves are the floating tongue of an ice sheet). But with the ice shelves removed, the grounded ice sheet behind them – accelerate into the ocean – and that causes sea level to rise (now ice is moved from the continent into the ocean).

"The ice sitting behind Larsen B Ice Shelf, which collapsed in 2002, have sped up 8-fold. I read a nice comparison from a colleague at Jet Propulsion Laboratory in California – this is the same difference having a familiar car drive first at 50 km/h and being replaced with a formula one race car, now going 400km/h. Most amazingly, those glaciers are still galloping towards the ocean – some 15 years after the first collapse of Larsen B.

"However, the Antarctic Peninsula doesn’t hold that much ice – if all of it were to slip into the ocean – it would raise sea level by less than 50cm (still a lot of course). But we look at the big ice shelves – the Ronne/Filchner Ice Shelf next to South America and the Ross Ice Shelf – the largest ice shelf on the planet – in the NZ region of Antarctica. If those same processes destabilise the Ross Ice Shelf, this could lead to a catastrophic collapse of West Antarctica, adding about 3.3m of sea level rise from West Antarctica alone.

"Currently, we know little about the how healthy or not the Ross Ice Shelf is but scientists are hurrying to learn more. I lead as chief scientist the RICE project which tries to constrain how quickly the Ross Ice Shelf could collapse and how much it would contribute to sea level rise. We find that the Ross Ice Shelf was very sensitive in the past and capable of rapid change. Tim Naish led as chief scientist the ANDRILL programme, an ambiguous marine sediment drilling project which showed that last time we had 400 ppm CO2 in the atmosphere (which is what we have now again), West Antarctica, Greenland, and some parts of East Antarctica collapsed, raising sea level by 10-20m."

Professor Christina Hulbe, Antarctic researcher and Dean of the School of Surveying at the University of Otago, comments:

"A new iceberg just broke off from the Larsen C Ice Shelf on the eastern side of the Antarctic Peninsula. Here are some ideas about why it’s such an exciting event to glaciologists (people who study glacier ice) and why it matters to the rest of us.

"A lot of the Antarctic continent is fringed by floating ice—places where the big ice sheets and smaller glaciers have gone afloat at the coast and pushed out into the ocean. Icebergs breaking from the floating 'ice shelves' are responsible for about half of the ice lost from Antarctica every year. Given the importance of iceberg 'calving', glaciologists don’t have very good mathematical models to describe it. That’s because these events are hard to observe.

"One of the really exciting aspects of the new iceberg calving event on Larsen C is how well it’s been observed. That's thanks to the number of Earth-observing satellites that now look toward the high latitudes (not just where most of the people live) and to the MIDAS project in the UK (a group of scientists who have been watching and measuring for a while).

"The Larsen C iceberg is huge—twice the area of Stewart Island and 1155 times the volume of Wellington Harbour. Big icebergs are rare and that means we don't have very many high-resolution observations of the processes leading to their production. This makes it hard to either develop or test good theories.

"The most recent really big iceberg calved from the front of the Ross Ice Shelf in March 2000. It bumped its way along the front of the shelf and got stuck for a while around Ross Island. That was trouble for penguins and for shipping to the bases there.

"We can't say exactly what influence climate change had on the Larsen C event. Big icebergs calve when rifts that have been growing for decades finally connect together and the iceberg separates from the rest of the ice shelf. The longer a fracture becomes, the easier it is for the tip to keep propagating. This event won’t change sea level because the ice was already floating, but it could lead to other events that do.

"Ice shelf collapse events, like Larsen A and Larsen B, were triggered by many seasons of surface melting. The meltwater filtered down into the snow on the ice shelf surface and refroze there, so that new meltwater was more likely to stay at the surface, where it collected in ponds. The water at the surface then filled existing cracks (and probably made new ones as well) and drove them to break from the surface all the way through the ice shelves, splitting them into numerous tiny bits. This is very different to what we just saw happen.

"What will happen now on Larsen C is a lot of watching. Glaciologists want to know how the flow of the ice shelf changes; how the stresses in the ice, which cause cracks and rifts to form and propagate, change; and how the glaciers flowing into the ice shelf change. Those observations will help us to build better theoretical understanding and better computer models. Those things, in turn, enable us to better predict the future.

"The Antarctic Peninsula used to be the fastest warming place on the planet but right now it appears to be cooling. Scientists who study processes in the atmosphere and climate have determined that this is due a change in storms over the Weddell Sea (which is itself due to changes in the atmosphere farther north. Put another way, the recent trend is part of the natural variation around the Peninsula."

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