Have we spoken about the crack(en) yet? If not, it’s probably about time, because it’s reared its ugly head again this field season.
In 2015 the MIDAS team published a short communication detailing the development of a rift in the Larsen C ice shelf. The crack had been a feature in the ice shelf visible in satellite imagery since the 1980s, but it had rapidly grown during the summer of 2014 (that’s December-February for all of us Northern Hemisphere-centric folk).
The crack had been propagating at an alarming rate, as well as moving through ice that scientists had previously thought it would not be able to penetrate. This is because they thought that it would be difficult for a rift to traverse different types of ice.
The ice shelf is made up of different sections, called flow units, which all originate from different sectors of the upstream mountains, and from different parts of the glaciers that feed the ice shelf. This means that they have slightly different properties, including how fast they flow and their temperature. More established shelf ice is typically colder and more brittle, whereas marine ice is warmer and softer. Marine ice is often formed in ‘suture zones’, aka regions where gaps and holes form in the ice, so marine ice sort of acts like polyfilla, plugging the gaps that form naturally.
It was previously thought that ice shelf fractures terminated when they encountered such suture zones, therefore making this marine suture ice critical for maintaining ice shelf stability. However, the crack(en) has defied this previously held knowledge and traversed the region of marine ice, and is poised to calve an iceberg around a quarter of the size of Wales, representing about 12% of Larsen C’s total ice area. Indeed, glaciologist Adrian Luckman, who is head of the MIDAS team, says that “if it doesn’t go in the next few months, I’ll be amazed.”
Now that sounds ominous. However, because Larsen is a floating ice shelf, it already displaces its own weight, meaning that it won’t directly contribute to sea level rise (in the same way that an ice cube melting in your drink won’t cause it to overflow). BUT! It will have an indirect effect – the removal of ice in the shelf can destabilise the ice shelf, because: 1) it can alter the compressive arch which jams everything in – think that crucial last jenga block; and 2) it will remove some of the shelf’s buttressing effect, i.e. its ability to act as a plug on the glaciers that flow into the shelf. It’s been shown in several studies that the removal of shelf ice causes glaciers upstream to accelerate, and THAT is where the contribution to sea level rise starts to kick in.
The scientists working on the MIDAS project believe that the calving off of this iceberg will destabilise the ice shelf, though probably not enough to trigger its total collapse á la Larsen A and B (yet). However, it shows that things are changing rapidly on Larsen C and presages future events that will likely spell the end for the shelf at some point. How soon is anyone’s guess, and the subject of my PhD, so I’ll get back to you on that.