For several years, Project MIDAS has been tracking a large rift on the Larsen C ice shelf. Here are some answers to questions we are frequently asked about the rift, and about the iceberg which will be produced as a result.

When will the iceberg break away?

Ice fracturing depends on a large number of factors, many of which are unknown, and the calving is quite unpredictable. Although the event could occur anywhere from days to years, the iceberg is likely to break free within the next few months simply because the leverage of 175km of iceberg on ~20km of what remains connected to the ice shelf is overwhelming. We are watching with bated breath.

How do you measure the crack? How do you know when the crack has extended?

The rift can be seen in satellite images, but seeing the rift tip requires high spatial resolution (~10 m). Larsen C is very cloudy, so we can often go months without being able to see the rift tip visually. We use Synthetic Aperture Radar (SAR) data from the ESA Sentinel-1 satellites which uses microwave energy to ‘see through’ clouds and can produce an image night or day, winter or summer. Even in these data, the tip is not directly visible, so we have to combine sequential pairs of images in a process known as interferometry. This technique is sensitive to small changes and can normally spot the rift tip except when there has been melt at the surface, which can be the case during the Antarctic summer.

Would this iceberg calve away even without man-made climate change?

Yes. We have no evidence to link this event to climate change. Although the general southward progression of ice shelf decay down the Antarctic Peninsula has been linked to a warming climate, this rift appears to have been developing for many decades, and the result is probably natural.

Will the iceberg calving raise sea level?

No. The ice shelf is already floating, so when a part of it detaches, no extra water is displaced. Although there is plenty of debate on this, our research shows that the remaining ice shelf may be less stable, and may eventually follow the example of it northerly neighbour Larsen B. Other scientists expect the ice shelf to regrow. Only time will tell. However, even if it less stable, the point at which collapse may occur would be many years away. Only following this potential far-away collapse would sea level rise follow, and even then the extra rates of sea level rise would be small - measured in mm per year at most.

Are any penguins at risk when it breaks off?

No. Penguins live where they can access fish in the sea. The ice shelf is ~200m thick at its outer edge. Since nine tenths of it is underwater, as in any iceberg, the ice cliff at the seaward edge is at least 20m high. Penguins can jump, but not that high.

Will a wave be created when the iceberg breaks off?

No. The calving event will probably be slow and graceful. The ice is already floating so when the fracture breaks all the way through, it will simply start to drift away.

Tracks of icebergs A47 and A48, produced in the aftermath of the Larsen B collapse. Iceberg positions from QuikSCAT data, as processed by the BYU Center for Remote Sensing. Base map from ADD.

Where will the iceberg go?

Icebergs are freely floating, so they tend to move with the ocean currents. In this part of Antarctica, that usually means that they drift north along the coast, and then north-east, out into the South Atlantic. We believe that the iceberg will take a track similar to the icebergs produced when the neighbouring Larsen B ice shelf collapsed in 2002.

How long will it take for the iceberg to completely melt once it breaks off?

This is even less easy to predict than when the iceberg will calve. It all depends on how quickly it moves to a warmer climate, and how quickly it breaks into smaller pieces. The iceberg (or icebergs if it breaks up) may remain in the region, where the ocean is quite cold, and stick around for years, even decades. Or it (they) may move with ocean currents and winds in a northwards direction where it will be eroded more quickly.

Can the ice shelf ever reform?

Yes it can, because calving is a natural part of the mass balance cycle of ice shelves. Ice flows gradually into the shelf, the shelf expands until stresses become too much, and then icebergs are calved. In this case, whether or not Larsen C will reform is currently debated. We think there is a possibility that the remaining shelf is too fragile to simply grow back to its former size.

Will the iceberg have an official name?

It will be issued a numeric designation by the US National Ice Center. This will be formed of the letter A (indicating which quadrant of Antarctica it originated from) and a two-digit number, probably in the high sixties. Then, as it breaks up, the resulting bergs will get additional letters added to this name.

Why is this important research?

Ice shelves hold back the glaciers which feed them. When they disappear, ice can flow faster from the land to the ocean and contribute more quickly to sea level rise. This was amply demonstrated by the break-up of Larsen B Ice Shelf, after which its feeding glaciers accelerated, and are still flowing faster than before some 15 years later. This is why scientists are interested in ice shelves, and why a large rift which threatens Larsen C is of scientific interest.

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