By Kevin Krajick
A global team of scientists has observed the sudden drainage of a large, deep, ice-covered lake within an Antarctic ice shelf—a rare phenomenon that could be interpreted as an ominous sign for the future survival of the ice sheet, and potential global sea-level rise.
The event, chronicled in a study just published in the journal Geophysical Research Letters, occurred during the 2019 Antarctic winter on the Amery Ice Shelf in East Antarctica. The scientists estimate that 600 million to 750 million cubic meters of water, about twice the volume of California’s San Diego Bay, were suddenly lost to the ocean when the lake drained from the bottom of the 1,400-meter (4,590 feet) -thick floating ice shelf.
The study’s authors used radar images from a satellite to pin down the event’s timing to within a week or less in June. After the unnamed lake drained, in its place was a crater-like depression in the ice surface, covering about 11 square kilometers (4.25 square miles). The depression, known as an ice doline, contained the fractured remains of the ice cover.
The event was also captured by a laser instrument on NASA’s ICESat-2 satellite, revealing the vertical scale of the disruption. The ice surface fell as much as 80 meters (260 feet) into the doline cavity. The loss of the water load also made the floating ice shelf lighter, and ocean pressure caused it to flex upwards, with the lake’s immediate surroundings rising as much as 36 meters (118 feet).
“We believe the weight of water accumulated in this deep lake opened a fissure in the ice shelf beneath the lake, [causing] the water to drain away to the ocean below,” said study lead author Roland Warner, a glaciologist at the University of Tasmania. The process, known as hydrofracturing, has been seen on other, smaller ice shelves, but rarely driving through ice as thick as this, especially in the winter. Warner said the drainage probably took about three days. “The flow into the ocean beneath would have been like the flow over Niagara Falls, so it would have been an impressive sight,” he said.
While the Amery Ice Shelf has many melt-lakes and streams in austral summer, the amount of water lost from the lake was many times the annual melt water input to it. “This abrupt event was apparently the culmination of decades of melt-water accumulation and storage beneath that insulating lid of ice,” said study coauthor Jonathan Kingslake, a professor at Columbia University’s Lamont-Doherty Earth Observatory. Kingslake coauthored a study last year warning that many ice shelves around Antarctica could be vulnerable to eventual disintegration caused by hydrofracturing.
Hydrofracturing has already been implicated in the sudden collapses of ice shelves in the warmer, more northerly Antarctic Peninsula, where melt water also forms on the surface during austral summer. In recent decades, with rising air temperatures, some ice shelves have experienced increased surface melting, and the most recent projections suggest that the trend will continue. This would increase the risk of widespread hydrofracturing, leading to more ice-shelf collapses that would in turn permit faster discharge of ice from grounded ice sheets behind the ice shelves, and and resultant increases in sea level.
The uplift of the Amery Ice Shelf lake surroundings created a new lake out of a shallow arm of the original one. During the following melt season, in 2020, more than a million cubic meters of water flowed into the new lake each day for several days, then overflowed into the doline cavity. The team measured a melt-water channel 20 meters (65 feet) wide, freshly cut into the doline. It appears that the fissure below also reopened briefly in the 2020 melt season.
Study coauthor Helen Amanda Fricker of the Scripps Institution of Oceanography said it is too early to conclude that the drainage of the lake was related to broader warming trends in Antarctica. But, she said, with evolving satellite-observation capabilities and more data, researchers should be able to further understand how common these deep lakes are, and how they evolve over time.
The future of the newly formed doline is uncertain: It may accumulate melt water again, or drain to the ocean more frequently. But it is certain that scientists will now be watching.
The study was also coauthored by Susheel Adusumilli and Philipp Arndt of Scripps; and Julian Spergel of Lamont-Doherty.