The team was inspired to look closer at the causes of ice shelf weakening after analyzing the catastrophic breakup of the Larsen B ice shelf. That breakup made headlines in 2002 as 1,250 square miles of ice broke away into the ocean, leaving glaciologists stunned.
Examining data, scientists noticed that in the months leading up to the breakup, the ice shelf was dotted with more than 2,000 meltwater lakes.
“Up until Larsen B, glaciologists thought that we understood ice shelf breakup—they would advance north until they got too thin,” said study co-author Doug MacAyeal, a UChicago professor of geophysical sciences who has been traveling to the Antarctic to study the behavior of ice and snow for decades. “What Larsen B taught us was that once an ice shelf has copious meltwater running across its surface, it will defy the predictions of stability for drier ice shelves and break up.”
During the melting season, lakes may form on the surface of ice shelves, pooling the weight of melting snow and ice into many areas of liquid water. These lakes can weigh 50,000 to 2 million tons each, and that pushes downward on the ice, creating an indent. If the lake drains, this indent pops back up. If the resultant stress is large enough, the ice surrounding the lake basin weakens, and may start to break, the researchers predicted.
To measure just how much these meltwater lakes were distorting the floating Antarctic ice, Banwell, MacAyeal and the team first had to scout where they thought the lakes would develop. They identified four lake basins to outfit with GPS sensors.
In November 2016, before the melt season began, the team drove snowmobiles from the U.S. McMurdo Station over the frozen sea ice to access their field site on the McMurdo Ice Shelf, pulling hundreds of pounds of equipment on sleds. At each of the four lakes, they installed self-contained instruments that measured vertical elevation and lake water depths—each fixed on a metal pole drilled over 6 feet deep into the ice. Three months later, they flew via helicopter to retrieve the instruments (by then, the sea ice was too thin to support the weight of a vehicle).
The team found that at the center of each lake, the ice shelf moved down and then up by around 3 to 4 feet in response to each lake filling and then draining.
Climate models predict that there will be more melting across more ice shelves over the next few decades, leading to an increase in the occurrence of meltwater lakes, the scientists said.
“These observations are important because they help us better understand the triggers of ice shelf breakup, which leads to sea level rise,” said Banwell. “Our results can be used to improve models to better predict which ice shelves are more vulnerable and are most susceptible to collapse.”
Citation: “Direct measurements of ice-shelf flexure caused by surface meltwater ponding and drainage.” Banwell et al, Nature Communications, Feb. 13, 2019. doi: 10.1038/s41467-019-08522-5
Funding: U.S. National Science Foundation, the U.K. Leverhulme Trust, NASA, CIRES- University of Colorado Boulder.
—Story adapted from a CIRES press release