Last year, a 580 square km chunk of West Antarctica's Pine Island Glacier broke off and tumbled into the sea. Now, Earth scientists at Ohio State University have pinpointed the root cause of the iceberg calving event: A crack that started deep below ground and 32km inland.
A new rift in West Antarctica's Pine Island Glacier, photographed during a NASA Operation IceBridge flyover on 4 November 2016. Image: NASA/Nathan Kurtz
It's like nothing scientists have witnessed in West Antarctica before, and it doesn't bode well for the ice sheet's future.
A frozen fortress containing enough water to raise global sea levels many metres should it melt, the West Antarctic ice sheet is separated from the ocean by a series of large glaciers. For now, these glaciers act like corks in wine bottles to hold the ice at bay, but that may not be the case for much longer. Recent research has shown that Pine Island, Thwaites and other glaciers along the Amundsen sea are retreating rapidly, as warm ocean waters lap against their margins. At this point, NASA says, collapse of the entire Amundsen sea sector appears to be "unstoppable".
The biggest question on everyone's mind is how quickly all of that ice will go, and to find out, we need to pinpoint the mechanisms responsible for ice sheet collapse. To that end, a study published today in Geophysical Research Letters takes a deep dive into an iceberg calving event in the summer of 2015. It arrives at a startling conclusion.
Bird's eye view of the Amundsen sea embayment, where major glaciers of the West Antarctic ice sheet empty into the ocean. Pope, Smith, and Kohler glaciers were the focus of this study. Image: NASA/GSFC/SVS
"The calving event itself wasn't a big deal," lead study author Ian Howat of Ohio State University told Gizmodo, noting that iceberg break-offs of this size happen about every five to six years at Pine Island. "What made this one different is how it got started."
At Pine Island and elsewhere along the Amundsen sea embayment, calving occurs at the glacier's outer margin, where the ice shelf is detached from bedrock. "It's kinda like a diving board sticking out over a pool," Howat said. Normally, cracks will start to form in areas experiencing extreme shear from ice flowing off the continent. They will propagate laterally across the shelf, eventually causing the entire diving board to break away.
Not so with last year's Pine Island calving event. Analysing several years of images taken by the Sentinel-1A satellite, Howat and his colleagues traced the break-off to a rift that formed at the base of the ice shelf nearly 32km inland, in 2013. Over the course of two years, the rift propagated all the way from bottom to top, until finally, it spat out an iceberg 10 times the size of Manhattan.
What could have caused so much ice to break away in this unusual manner? In all likelihood, melting that started at the contact point between ice and bedrock is to blame. This would explain why the rift overlapped with a topographic valley — a place where the ice appeared to have thinned — in satellite images taken before the calving.
"I think what we're seeing is the surface expression of a much bigger valley at the base of the ice shelf," Howat said. "This tells us the ice shelf has weaknesses that are being exploited by increased ocean temperatures."
Troublingly, as waters around West Antarctica heat up, those weaknesses could be exploited more and more often. "If the ice sheet was going to retreat very slowly on long timescales, we'd just expect to see the usual calving," Howat said. "This event gives us a new mechanism for ice sheets falling apart quickly. It fits into that picture of a rapid retreat."
Howat noted that a second interior ice shelf rift (pictured above) was spotted during a NASA Operation Ice Bridge survey earlier this month. And there are many other topographic valleys — possible sites of future calving events — further up-glacier, but our ability to study them is hampered by a lack of good data.
One can't help but note that NASA's Earth science program, which makes such data available to scientists and the public, faces the possibility of major cuts under a Trump administration. These cuts would come at the precise moment when our planet is changing in rapid and hard-to-predict ways, which is when Earth science research is needed the most. Like cracks in an ice sheet, the irony runs deep.