The Venable Ice Shelf, in West Antarctica, in 2008. (NASA/JPL-Caltech-UC-Irvine/AFP)

The forces affecting the ice in West Antarctica are an area of urgent focus for climate scientists who are all too aware of the ice sheet’s huge potential contributions to global sea-level rise...

A great deal of this attention has centered on a specific region bordering the Amundsen Sea, south of the Antarctic Peninsula, where research has suggested that a set of rapidly retreating glaciers — including the famous Thwaites and Pine Island glaciers — may be increasingly vulnerable to collapse.  

But research is increasingly suggesting that the region is not the only area deserving of concern. Just last month, a new study suggested that the Totten Glacier in East Antarctica, which has typically been considered much less of a threat than West Antarctica, is also thinning quickly and has retreated inland by close to two miles in some areas. Overall, the glacier has the potential to raise sea levels by about 13 feet should it collapse.

And now, a new study just published in the journal Geophysical Research Letters has identified a new area of concern. The new research focuses on the Bellingshausen Sea region, an area just above the Amundsen Sea on the west side of the Antarctic Peninsula. Using four decades’ worth of satellite data, researchers have found that ice in this region has also experienced significant retreat, particularly since 1990, and could be a bigger threat than expected.  

Researcher Robert Bingham, an expert in geophysics at the University of Edinburgh and one of the authors of the new study, said he became interested in the Bellingshausen region during previous expeditions to West Antarctica that were meant to focus on the Amundsen Sea. “In that process, I noticed that the neighboring regions also exhibited some signs of change as well,” he said. “Not quite the same sort of magnitude but nevertheless quite significant.”  

He and a group of colleagues from the University of Edinburgh and Temple University in Philadelphia decided to examine the historical satellite record, using data from NASA’s Landsat program (which has been in existence since the 1970s) to see what kinds of changes had taken place in the Bellingshausen region over the past few decades.

“What we’ve found is there’s a signal of change there all the way back to when records began,” Bingham said. “And it’s actually very consistent and persistent along the Bellingshausen coastline of West Antarctica.”  

Their research focused on a glacial feature known as the “grounding line.” Glaciers along the shoreline, terminating in the ocean, tend to be grounded — or anchored to the bedrock — up to a certain point. Past this point, the ice may become detached from the ground and actually float on the water, while still remaining connected to the glacier itself; this is what’s known as an ice shelf.

The point where the ice goes from grounded to floating is the grounding line. Floating ice shelves tend to act as a kind of stabilizing force for glaciers, holding back the flow of all the ice behind them. If these ice shelves thin, weaken and break off, however, ice may flow more quickly from the glacier into the ocean and the grounding line may start to move farther and farther inland. So looking at grounding lines is a good way to get a sense of how much ice a region has lost over the years.

By looking at the satellite record, the researchers concluded that more than 65 percent of the grounding line for glaciers along the Bellingshausen Sea had experienced a net retreat — in other words, they had moved inland — between 1990 and 2015. On the other hand, just over 7 percent of the grounding line advanced forward.

There are many factors that may affect the retreat process, and scientists are still getting a grasp on how they work and interact with one another. In the past, rising air temperatures — which can cause an increase in melting on the surface of the ice sheet — have been the primary focus for glacier experts. But one factor that’s been gaining attention over the past few years is the influence of warm ocean water, which can melt the ice from underneath, thinning or even breaking the ice shelves and causing grounding lines to retreat backward.  

The considerable research that’s been done in the Amundsen Sea sector suggests that this process is a major influence in that region, and Bingham and his colleagues suspected that it may play a large role in ice losses in the Bellingshausen sector as well. And their results seem to support this idea.  

First, the researchers found that the greatest retreat seemed to be associated with areas where the ice is grounded to the seafloor at the greatest depths beneath the surface of the water. This kind of setup means there’s a deeper column of water lapping at the ice front — in other words, a greater area of ice that the warm ocean water can melt away.

Additionally, the researchers observed that the rate of retreat tended to vary over time in most locations along the Bellingshausen. Over a number of years, there might be a short period in which rapid ice loss took place, followed by a much longer period in which little or no retreat happened.

Despite these variances over time, Bingham pointed out that the changes in rate tended to happen all together along the coastline. “When one [glacier] responded, most of the rest of them responded at the same kind of rate,” he said. “So that starts to tell you that there’s a fairly single influence that’s affecting the whole coastline.”

Logically, one might expect that a factor with the ability to affect the entire coastline at once would probably be related to air temperature or water temperature, Bingham said. But he added that measurements in the area have already suggested that the air temperature typically isn’t warm enough to cause the kinds of changes he and his team have observed.

“So then, really, the only viable influence has to be what’s coming from the ocean,” he said.

Still, there was one place along the Bellingshausen that seemed to confound the researchers’ expectations. An area called the Venable Ice Shelf is notable for the dramatic ice thinning it’s experienced since the 1990s — in fact, it’s considered the most rapidly thinning ice shelf in Antarctica, said Frazer Christie, a PhD candidate at the University of Edinburgh and the lead author of the new study. But despite all this thinning, the researchers observed that the grounding line has retreated only by a small amount over the years.

“What we’ve found is the grounding lines in 2015 are situated upon very rough, shallow topography,” Christie said. “And so we think the grounding line is essentially pinned in place at the moment and unable to retreat because of the topography.”

However, he added that about 40 miles inland of the current grounding line, there’s a deep trough beneath the glacier. So if the ice continues to thin and ocean water eventually gains access to that trough — which would probably allow it to cause significantly more melting from underneath — the ice shelf’s overall stability could be threatened in a major way.

He likened the situation to what’s been observed at the Pine Island Glacier on the Amundsen Sea — one of the fastest melting glaciers on the continent and the subject of great preoccupation for climate scientists.

“Previously, our studies asserted that Pine Island Glacier was also pinned in place at one point,” Christie said. But scientists believe that rapid thinning of the ice over the past 20 years has allowed ocean water to seep into some of the deep troughs underneath and cause the glacier to become increasingly unstable. Now, the authors of the new study are concerned that the same process could repeat itself with the Venable Ice Shelf.

“There’s every reason to believe that at some point it will become unpinned from that ridge and the same sort of process of retreat . . . will happen at the Venable Ice Shelf as well,” Bingham said.

Overall, the significant retreat observed in many places along the Bellingshausen Sea — and the potential for even more ice to be lost in the future in places such as the Venable Ice Shelf — suggests that the region may be worthy of greater attention as scientists continue to deepen their understanding of how the world’s ice sheets might contribute to future sea-level rise and other climate-driven oceanic changes.

According to Bingham, there’s a great deal that we still don’t know about the region that scientists should be looking into in order to better understand how the ice might change in the coming years. In particular, he noted, there’s a lot of missing data about the region’s underwater topography — and that kind of information is crucial for understanding how ocean water might be affecting the ice sheet from below.

And, in a broad sense, the study also underscores the idea that the rapidly melting parts of the world that scientists have been focusing on may not be the only areas of concern.

“I think the study is an example of how we shouldn’t be placing all our observational eggs into one basket or one area,” Bingham said. “It’s a very important message that we need to be —as much as we possibly can, given the resources available — looking at all parts of the Antarctic coastline to investigate potential signals of change.” 

By Chelsea Harvey June 1 at 7:00 AM

source: https://www.washingtonpost.com/

original post HERE

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