Antarctic ice shelves fracture under pressure

The Antarctic ice shelves are one of the main victims of climate change. A report published in Nature Communications by a team of researchers co-led by the University of Cambridge finds that the stability of a floating Antarctic ice shelf is threatened by meltwater lakes on its surface. The flexing of ice shelves has been hypothesized and simulated by computer models in the past, but this study represents the first time it has been observed in the real world.

Most of Antarctica is blanketed by the four-kilometer-thick Antarctic ice sheet, which contains enough ice to raise the global sea level by around 58 meters. For most of the year, temperatures above the ice are below zero, keeping the continent solidly frozen. However, the fringes of the ice sheet are made up of floating ice shelves that are no more than a kilometer thick and partially exposed above the ocean. In summer months, these floating icebergs are susceptible to melting, especially as the climate continues to get warmer.

If melted ice water runs off the steep slopes of ice sheets into the ocean, there is little risk to frozen ice shelves. Unfortunately, surface depressions on the ice shelf allow water to pool into large lakes — the weight of these meltwater lakes exerts tremendous pressure on the floating ice, forcing it to sink deeper. Around the edges of the lakes, the ice flexes upwards to keep itself afloat and compensate as the water drains.

“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,” said Dr. Alison Banwell of the University of Chicago, lead author of the paper, in a University of Cambridge press release.

The researchers used helicopters to install pressure sensors and GPS receivers near the surface lakes to monitor the rise and fall of water levels in the ice’s depressions.

Professor Doug MacAyeal of the University of Chicago said in a Cambridge press release, “It was a lot of work to obtain the data, but they reveal a fascinating story. Most of the GPS signal is due to the ocean tides, which move the floating ice shelf up and down by several meters twice a day. But when we removed this tidal signal, we found some GPS receivers moved down, then up by around one meter over a few weeks whereas others, just a few hundred meters away, hardly moved at all. The 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.”

Due to this newfound link between surface melting and weakening of ice shelves, many believe that the recent ice break-up around the Antarctic Peninsula may have been triggered by surface meltwater lakes that created fractures in the ice, a process set into motion by atmospheric warming. Banwell and MacAyeal previously suggested that the disastrous breakup of the Larsen B Ice Shelf in 2002, where 3,250 square kilometers of ice was lost, could have been caused by this very phenomenon.

The team hopes that their report will inspire studies into flex and fracture in Antarctic ice shelves, as well as the development of ice sheet scale models that could predict the stability of the ice sheet in the future.