Two years ago, a pair of scientific studies documented that the glaciers of West Antarctica, which hold back over 3 meters (nearly 10 feet) of potential sea level rise, are melting and retreating from below. The cause? It appears that these glaciers, which are perched on the seafloor deep below the ocean surface, are being lapped at by flows of warm ocean currents.
Since then, researchers have been focusing more and more urgent attention on West Antarctica — and new research published Monday in Nature Geoscience uncovers yet another consequence of this warm water intrusion, one that further highlights the region’s vulnerability.
The enormous glaciers of West Antarctica, and indeed, around the Antarctic continent, are held in place by protrusions called “ice shelves.” These are large and thick sheets of ice that float atop the sea and often attach to islands or other features, and thus provide bracing, or stabilization, holding back the flow of the ice behind them. If these floating ice shelves are weakened, or fracture and fall apart, the ice behind them will flow faster into the ocean.
Ice shelves are often hundreds of meters thick, but they float atop water that is deeper still, and so have a deep ocean cavity beneath them. That cavity, in turn, eventually ends in a kind of underwater ice wall, where the glacier touches the seafloor at a place called the “grounding line” — which is where the crucial melt in West Antarctica is believed to be occurring.
But that’s not all that’s going on. In the new study, led by Karen Alley of the National Snow and Ice Data Center at the University of Colorado in Boulder, the researchers document that the warm ocean water that’s undermining West Antarctica from below may also be weakening its ice shelves. It appears to be slowly carving deep channels into their bases, cavities ranging from 50 to 250 meters in vertical extent.
These channels appear to be formed as the warm water that hits the grounding line then bursts upward in a plume, combined with meltwater, and cuts into the ice shelf from below, Alley said. “They’re kind of like upside down rivers, or streams. Instead of the water flowing downhill, it’s flowing uphill, because it’s buoyant,” she says.
The channels are so large that, even though they are occurring on the underside of a very thick sheet of ice, they can be measured from satellites from above, because they cause depressions on the ice surface itself. Here’s an example, in an image from the Landsat 8 satellite sent by Alley:
“When you carve a big channel from underneath an ice shelf, the top sags, and so it leaves a depression that you can see on the ice shelf’s surface. So we were able to use satellite imagery to look for these depressions all around Antarctica,” says Alley.
The channels could be found in many places, but sure enough, the study found the “highest density” of sub-ice shelf channels in West Antarctica. It further implies that warm ocean water is driving their formation, and that is leading to the creation of “polynyas,” or areas of open sea without any ice, as the warm water surges through the channels and ultimately to the surface.
The study also finds that the channels are tied to crevasses on the surface of ice shelves, and could be setting the stage for ice shelf weakening or fracture — a process we’ve already seen play out multiple times in the increasingly warm Antarctic Peninsula region. “Ice-shelf basal channels could lead to large-scale destabilization through the reduction of iceshelf backstress,” the research notes.
If ice shelves fracture, glaciers behind them will flow outward faster and raise seas more quickly. The deepest fear, then, is that researchers have identified a new process that will further accelerate ice loss in West Antarctica – although the study emphasizes that more research will be needed to confirm this.
And it’s not just West Antarctica. While the study found that channels were most prevalent there in general, it also found the “highest channel density” in the ice shelf protecting the enormous Totten Glacier of East Antarctica, which itself holds back as much sea level rise as all of West Antarctica. Totten, too, has become an increasingly important focal area for research lately, given mounting evidence that it, too, is retreating because of the intrusion of warm ocean water at its base.
Research published last year suggested that a subsea valley beneath the Totten ice shelf was allowing warm water to reach the glacier’s grounding line, a worrisome finding, given how much ice this region contains.
Most troubling of all is that recent research has suggested that the ice shelves of West Antarctica, and the Totten ice shelf, cannot afford to lose much ice if they are to continue to play their buttressing role. There is less “passive ice” in these particular areas, meaning ice that is not serving a critical buttressing function.
In sum, not only are warm waters apparently melting the glaciers of West Antarctica from below — they may also, it seems, be weakening the buttressing support that holds these glaciers in place. But as scientists race to learn more about West Antarctica’s vulnerability, more research will be required to figure out just how fundamental the channels are to weakening ice shelves and, ultimately, the glaciers behind them.
“We can tell that they’re important, because they’re everywhere and related to warm water and ice shelf stability,” says Alley of the channels. “But really what we’ve found is that we need to do a lot of work on these features to understand how important they will be in the future.”