In Antarctic Summer, Signs of Accelerating Melt
November 29, 2016
The Pine Island Glacier, part of the ice shelf that bounds the West Antarctic Ice Sheet, is one of two glaciers that researchers believe are most likely to undergo rapid retreat, bringing more ice from the interior of the ice sheet to the ocean, where its melting would flood coastlines around the world.
A nearly 225-square-mile iceberg broke off from the glacier in 2015, but it wasn’t until Ohio State University researchers were testing some new image-processing software that they noticed something strange in satellite images taken before the event.
In the images, they saw evidence that a rift formed at the very base of the ice shelf nearly 20 miles inland in 2013. The rift propagated upward over two years, until it broke through the ice surface and set the iceberg adrift over 12 days in late July and early August 2015.
They report their discovery in the journal Geophysical Research Letters.
“It’s generally accepted that it’s no longer a question of whether the West Antarctic Ice Sheet will melt, it’s a question of when,” said study leader Ian Howat, associate professor of earth sciences at Ohio State. “This kind of rifting behavior provides another mechanism for rapid retreat of these glaciers, adding to the probability that we may see significant collapse of West Antarctica in our lifetimes.”
While this is the first time researchers have witnessed a deep subsurface rift opening within Antarctic ice, they have seen similar breakups in the Greenland Ice Sheet—in spots where ocean water has seeped inland along the bedrock and begun to melt the ice from underneath
Howat said the satellite images provide the first strong evidence that these large Antarctic ice shelves respond to changes at their ocean edge in a similar way as observed in Greenland.
“Rifts usually form at the margins of an ice shelf, where the ice is thin and subject to shearing that rips it apart,” he explained. “However, this latest event in the Pine Island Glacier was due to a rift that originated from the center of the ice shelf and propagated out to the margins. This implies that something weakened the center of the ice shelf, with the most likely explanation being a crevasse melted out at the bedrock level by a warming ocean.”
Another clue: The rift opened in the bottom of a “valley” in the ice shelf where the ice had thinned compared to the surrounding ice shelf.
The valley is likely a sign of something researchers have long suspected: Because the bottom of the West Antarctic Ice Sheet lies below sea level, ocean water can intrude far inland and remain unseen. New valleys forming on the surface would be one outward sign that ice was melting away far below.
The origin of the rift in the Pine Island Glacier would have gone unseen, too, except that the Landsat 8 images Howat and his team were analyzing happened to be taken when the sun was low in the sky. Long shadows cast across the ice drew the team’s attention to the valley that had formed there.
“The really troubling thing is that there are many of these valleys further up-glacier,” Howat added. “If they are actually sites of weakness that are prone to rifting, we could potentially see more accelerated ice loss in Antarctica.”
Here, my look at increasing evidence of changes East Antarctica, once thought to be nearly impervious to global warming, at least for the coming century.
While scientists and environmentalists are understandably concerned about ocean warming melting the undersides of sea-fronting West Antarctic glaciers — resulting in risks for rapid sea level rise for the near future, another consequence of global warming is also starting have a more visible impact on the frozen and now thawing continent. Surface melt, which was hitherto unheard of for most of East Antarctica, is now starting to pop up with increasing frequency.
East Antarctica, according to Stewart Jamieson, a glaciologist at Durham University in the U.K., is “the part of the continent where people have for quite a long time assumed that it’s relatively stable, there’s not a huge amount of change, it’s very, very cold, and so, it’s only very recently that the first supraglacial lakes, on top of the ice, were identified.”
But now, even in austral springtime, we find evidence of surface melt in the satellite record.
On November 27, 2016, what looks like an approximate 2 mile long melt pond appeared in a section of ice shelf along the Scott Coast and just North of the Drygalski Ice Tongue in the region of McMurdo Sound. The lake — which is visible as a light blue swatch at center mass in the NASA-MODIS satellite image above — suddenly showed up in November 27 satellite image along a region where only white ice was visible before. And it appears in a region of East Antarctica that, before human-forced warming altered the typically-stable Antarctic climate, had rarely, if ever, seen surface melt.