These Antarctic Glaciers Have Experienced Staggering Ice Loss — And Scientists Think They Know Why

Washington Post - Chelsea Harvey

Part of the eastern flank of Crosson Ice Shelf (center left) and Mount Murphy (foreground) as viewed during a NASA IceBridge flight in 2012. Thwaites Ice Shelf lies beyond the highly fractured expanse of ice (center). (Image credit: John Sonntag)

Few regions of the world are as unstable in the face of advancing climate change as frozen West Antarctica, where rapidly melting glaciers have scientists on edge about the potential for huge amounts of future sea-level rise.
Now, a new study has pinpointed some of the most rapid ice losses observed in the region in the past 15 years — and it supports a growing scientific belief that warm ocean water is behind the melting.
“[The study] seems to provide a strong piece of evidence to support a general hypothesis about what’s happening in the Amundsen Sea,” said Ala Khazendar, a polar scientist at NASA’s Jet Propulsion Laboratory and the new paper’s lead author.
Much of the focus on West Antarctica centers around the Amundsen Sea region, whose glaciers may already be experiencing irreversible ice loss. The glaciers backing up to this sea have the potential to cause about 4 feet of sea-level rise, and the ice contained in West Antarctica as a whole could raise sea levels by 10 feet.
Several of the region’s largest glaciers have inspired some of the greatest concern. Just last week, U.S. and British science agencies announced a joint multimillion-dollar research mission to study the massive Thwaites Glacier, which scientists believe may already be contributing about 10 percent of all global sea-level rise. And a recent study on the nearby (and slightly smaller) Pine Island Glacier has documented recent rapid retreat.
Now, research increasingly suggests it’s not just atmospheric warming that’s causing all the problems in West Antarctica, but the influence of the ocean as well. Many glaciers in this region back right up to the edge of the sea, terminating in what’s known as an ice shelf — a ledge of floating ice that’s disconnected from the bedrock and juts out into the water, helping to stabilize the glacier and hold back the flow of ice behind it.
Scientists now believe that rising water temperatures may be helping to weaken ice shelves by seeping into the cavities beneath them and lapping up against the exposed ice. If an ice shelf thins or breaks, the glacier behind it begins to pour ice into the ocean and retreat inland. The point where the bottom of the glacier actually joins to the bedrock is known as the grounding line, and scientists often use it as a point of reference to measure how far a glacier has retreated over time.
Scientists believe this is what’s driving the retreat of the Pine Island and Thwaites glaciers. But while these glaciers hold some of the greatest potential to raise sea-levels, smaller glaciers in the area can also offer some important insights into the processes driving ice loss in the region.
The new study, just out on Tuesday in the journal Nature Communications, focuses on the Smith, Pope and Kohler glaciers, which are buttressed by the Dotson and Crosson ice shelves, not far from Thwaites Glacier. Previous research had already suggested that these glaciers had experienced unusually rapid retreat in the mid-2000s, Khazendar said. For the new study, he and his colleagues were interested in taking a closer look at what was happening to the glaciers below the surface of the water in the hopes of getting a better grasp on the physical processes causing the ice loss.
“I was really curious about what would we see in a place where the grounding line was retreating so fast,” Khazendar said.

Image of flow speeds (Credit: NASA Earth Observatory)

The researchers analyzed radar survey data collected by NASA research aircraft at various points between 2002 and 2014, which provided direct measurements of ice loss below the surface of the ocean. They found that, between 2002 and 2009, the glaciers experienced some of the fastest ice loss observed in decades. This was especially true for Smith Glacier, whose ice shelf thinned below the surface by 40 to 70 meters per year for a total loss of nearly half a kilometer during the study period.
The researchers attribute this extreme melting to an influx of warm water in the Amundsen Sea during that time period, the reasons for which are still not completely understood. Some scientists have suggested that changes in wind patterns and atmospheric circulation carried in more warm water from other parts of the ocean, and that this effect slacked off near the end of the decade.
As a result, between 2009 and 2014 the ice loss slowed significantly, and both Pope and Kohler glaciers seemed to stabilize. Smith Glacier, on the other hand, continued to retreat. The researchers attribute this behavior to differences in the topography at the three study sites. As Smith Glacier retreated, it moved into deeper terrain where the encroaching ocean water had greater access to the exposed ice beneath the surface. Pope and Kohler glaciers, on the other hand, retreated into shallower areas.
The study is the latest installment in a “long line of papers that show how important ocean melting can be to the evolution of these glaciers and ice sheets, both in Greenland and Antarctica,” said Martin Truffer, a physics professor and glacier expert at the University of Alaska Fairbanks, who was not involved with the new study. “And this is a particularly drastic or impressive example of how much ice melting from the ocean can impact the glacier directly.”
He added that the study also demonstrates that the worrisome patterns observed at Pine Island and Thwaites glaciers are not just local effects — they’re happening throughout West Antarctica.

“We benefited from the fact that we were not looking at one ice shelf and one glacier — we were looking at two ice shelves and three glaciers, which allowed us to compare and contrast and reach conclusions about what might be influencing the behaviors of all of them,” Khazendar said.

The dramatic effect observed in this case “lets us examine a greater envelope of glacier response behavior than we have yet observed at [Pine Island] and Thwaites,” said Knut Christianson, a glaciologist at the University of Washington in Seattle who was not involved with the new study, in an emailed comment to The Washington Post.
He added that “this study also highlights the difficulty of assessing future ice sheet behavior, as grounding line position is dependent on environmental forcing (ocean heat content), bed geometry, and glaciological setting…which must all be considered to understand a system’s current behavior and future evolution.”
In the near future, scientists’ primary focus will likely remain on the region’s largest glaciers, especially as the joint U.S and British research mission at Thwaites Glacier progresses. But continued research on other ice shelves in the surrounding area may provide important insight into the processes contributing to their destabilization.
“The changes are so big in these ice shelves and the glaciers that feed them that we definitely can learn a lot about the underlying processes by looking there,” Khazendar said.

Links

• Rapid submarine ice melting in the grounding zones of ice shelves in West Antarctica
• This Antarctic glacier is the biggest threat for rising sea levels. The race is on to understand it
• Scientists report ‘devastating’ coral death at Great Barrier Reef
• We’re adding record amounts of wind and solar — and we’re still not moving fast enough