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 centres 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 1.2m of sea-level rise, and the ice contained in West Antarctica as a whole could raise sea levels by 3m.
Several of the region's largest glaciers have inspired some of the greatest concern. Just last week, US 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 per cent 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 stabilise 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 today 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.
The researchers analysed 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 70m per year for a total loss of nearly half a km 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 stabilise. Smith Glacier, on the other hand, continued to retreat. The researchers attribute this behaviour 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.