Aug 27 2020
According to a recent study, many of the coastal ice shelves in Antarctica would be rapidly destroyed if growing temperatures continue to push the meltwater into various fractures that are presently entering their surfaces.
Image Credit: Adapted from Lai et al., Nature 2020.
The ice shelves help slow the slide of interior glaciers toward the ocean, which means, if these ice shelves were to fail, then global sea levels could surge quickly. The new study was recently published in Nature—a leading journal.
Ice shelves can be described as massive tongues of ice that float on the ocean surrounding the borders of the continent. The large land-bound glaciers behind these ice shelves are steadily pushing seaward.
However, several ice shelves are mostly confined inside expansive gulfs and bays, and hence they are compressed from the sides and tend to slow down the movement of glaciers—somewhat similar to individuals staying in a narrow hallway and bracing their arms against the walls to slow down someone attempting to push past them.
However, ice shelves go through competing stress—that is, they expand as they reach the ocean. As revealed by satellite observations, these ice shelves consequently break apart and most of them are raked with various long fractures that are perpendicular to the direction of stretching.
Fractures that form at the surface can have a depth of tens of meters, while others that form from the bottom can enter the ice that is hundreds of meters upward. A few fractures measure hundreds of meters in width.
At present, a majority of the ice shelves are stable and frozen year-round. However, scientists believe that extensive warming is likely to take place later in the century. Furthermore, present-day studies have demonstrated that even slight temperature changes can cause extensive melting. This may drive increased meltwater toward the surface fractures.
Surges like these can possibly cause hydrofracturing—a procedure where liquid water, which is heavier than ice, would strongly push the fractures to zip open and lead to the rapid disintegration of the ice shelf. The latest study projects that 7%-50% of the regions of the ice shelves supporting the glaciers are susceptible to these processes.
“It’s not just about melting, but where it’s melting,” stated Ching-Yao Lai, the study’s lead author and a postdoctoral researcher at the Lamont-Doherty Earth Observatory in Columbia University.
The ice shelves—that’s the weak spot, where the atmosphere, the ice and ocean interact. If they fill up with melt water, things can happen very quickly after that, and there could be major consequences for sea levels.
Jonathan Kingslake, Study Co-Author and Glaciologist, Lamont-Doherty Earth Observatory, Columbia University
Certain locations have already experienced hydrofracturing. Portions of the Larsen Ice Shelf, which had remained stable for a minimum of 10,000 years, decomposed within a matter of days between 1995 and 2002. The Wilkins Ice Shelf disintegrated partially in 2008 and 2009. Therefore, hydrofracturing was agreed to be the main cause.
The Larsen and Wilkins Ice Shelves contain some of the northernmost ice on the continent, and hence, they have been the first to be affected by seasonal melting and rising temperatures.
The latest study is based on a 2017 research work headed by Kingslake that demonstrated that seasonal streams and ponds on the ice surface are relatively more common over Antarctica than believed earlier; some of them reach within 375 miles of the South Pole. However, a majority of the cataloged features are present in places that are not exposed to hydrofracturing.
To date, the latest study demonstrates that only around 0.6% of East Antarctic ice shelves providing the buttressing effect undergo melt-water ponding, rendering them susceptible. If warming occurs, a relatively larger percentage would possibly be endangered.
Lai teamed up with a Google researcher Cameron Chen and trained a machine-learning model to detect ice-shelf fracture sites over Antarctica. The aim was to generate the first continent-wide map of these features.
At present, a majority of the fractures do not contain any liquid water, but the researchers considered upcoming cases, where melt water may completely fill the surface fractures.
This enabled the team to estimate which portions of the shelves are likely to be susceptible to hydrofracturing, when considering the competing compression forces from the sides, and the expansion of the ice from back to front.
The researchers estimated that the inflow of liquid within the prevalent fractures may possibly apply sufficient force to cause extensive hydrofracturing over 50% to 70% of the ice-shelf regions that offer the buttressing effect.
The major exceptions were smaller, solider regions of ice most proximal to land, which are subjected to less stress from expansion. The end parts of the shelves, which are largely surrounded by the open ocean, are equally susceptible, but since they float freely, they do not help much in restraining the glaciers.
Although the latest work indicates a warning, the team believes that the behavior of the ice shelves cannot be predicted with exact precision.
How fast melt water would form and fill in those cracks is the first question.
Jonathan Kingslake, Study Co-Author and Glaciologist, Lamont-Doherty Earth Observatory, Columbia University
He added that the worst-case situation would be that “lots of places will be covered by lots and lots of water by the end of the century.”
However, projections can differ extensively, based on the type of models used by scientists, and also based on how strongly humans reduce greenhouse gases.
The second question is whether specific sites will experience hydrofracturing, added Kingslake. The third question: whether the procedure would run away, making the shelf to experience explosive disintegration similar to the Larsen Ice Shelf.
Theodore Scambos, a prominent Antarctic glaciologist and the National Snow and Ice Data Center, stated that the study “does a great job of pointing to areas where one can say, ‘If it floods with melt here, it’s likely to break up the shelf.” He further added that it “has huge implications for sea level” if there is an increase in summer temperatures along the coasts.
Eventually, all the ice shelves could be covered by melt water. But we don’t have a time frame, and there are a lot of big questions left.
Ching-Yao Lai, Study Lead Author and Postdoctoral Researcher, Lamont-Doherty Earth Observatory, Columbia University
Martin Wearing from the University of Edinburgh; Pierre Gentine, Harold Li, and Julian Spergel from Columbia University; and J. Melchior van Wessem from Utrecht University have coauthored the study.
Journal Reference:
Lai, C.-Y., et al. (2020) Vulnerability of Antarctica’s ice shelves to meltwater-driven fracture. Nature. doi.org/10.1038/s41586-020-2627-8.