Warm Autumn Winds Could Strain Antarctica's Larsen C Ice Shelf

April 11, 2019

 

melting antarctic ice

Warm winds sweeping down from the Antarctic Peninsula’s central mountains are melting the ice below, especially at the bases of glacial valleys, as seen here. (NASA Earth Observatory/Lauren Dauphin)

The Antarctic Peninsula is the northernmost part of the earth’s coldest continent, making it particularly vulnerable to a changing global climate. Surface melting of snow and ice initiated the breakup of the northernmost Larsen A ice shelf in 1995, followed in 2002 by a large chunk of the Larsen B ice shelf, to the south.

New research shows that the Larsen C ice shelf—the fourth largest ice shelf in Antarctica, located just south of the former Larsen B—experienced an unusual spike in late summer and early autumn of similar surface melting in the years 2015 to 2017. The study shows that much of the additional melting can be ascribed to warm, dry air currents called foehn winds that originate high in the peninsula’s central mountain range–events that can raise air temperatures by as much as 30 degrees Fahrenheit. The study further shows that the foehn-induced late-season melting has begun to restructure the snowpack on the Larsen C. If this pattern continues, it could significantly alter the density and stability of the ice shelf, potentially putting it at risk of suffering the same fate as the others. The study appears online today in an early view of the journal Geophysical Research Letters.

“Three years doesn’t make a trend. But it’s definitely unusual,” said lead author Rajashree Tri Datta, a faculty assistant at the University of Maryland who did the research while working at Columbia University’s Lamont-Doherty Earth Observatory.  “It’s unusual that we’re seeing increased foehn-induced melt in consecutive years—especially so late in the melt season, when winds are stronger but temperatures are usually cooling down. This is when we expect melting to end and the surface to be replenished with snow.”

map of antarctica

Study site near the end of the Antarctic Peninsula, the northernmost part of the continent. (Courtesy NASA)

The researchers quantified patterns of foehn-induced melt from climate models corresponding to satellite observations and weather station data. The data spanned 35 years, 1982 to 2017.

Surface melting causes water to trickle into the underlying layers of uncompacted snow, known as firn, that comprise the upper parts of the ice sheet. This water then refreezes, causing the normally porous firn to become denser. Eventually, firn layers can become too dense for water to enter, leading to a buildup of liquid water atop the ice shelf. The system then enters the next warm season with a very different structure. With less open space for water to filter into, surface runoff increases year after year, dissecting and weakening the ice. The dominant theory suggests that it was this kind of densification that led to the fractures of the Larsen A and B shelves.

“The effect of water sitting on the ice shelves can play a big role in the disintegration of those shelves,” said Marco Tedesco, a glaciologist at Lamont-Doherty and coauthor of the study. “The results point to the importance of understanding the processes that can lead to melting, and their role on the energy and mass balance, especially in a place where topography is a huge problem, like the Antarctic Peninsula. As foehn winds race down the colder eastern slopes of the Antarctic Peninsula’s central mountain range, he says, they exert their greatest effects at the bases of glacial valleys. Here, where the feet of the glaciers adjoin the Larsen C ice shelf, the winds stand to destabilize some of the most fragile and critical structures in the system.

Like other ice shelves, the Larsen C is floating, so its breakup and eventual melting would not directly lead to a rise in global sea level, just as the melting of ice cubes in a glass of water does not change the level there. However, the ice shelf braces against the flow of the glaciers that feed it. So if the ice shelf goes, some of these glaciers will be free to accelerate their rate of flow and melt; that would result in a direct rise in global sea level.

The paper was coauthored by researchers from the University of Liege, Belgium; University of Grenoble, France; Delft University of Technology, Netherlands; and University of Colorado, Boulder.

Adapted from a press release by the University of Maryland.