Glacier's 'irreversible' melt could result in 100bn-tonne annual ice loss
Glaciologists have announced that a huge ice stream in western Antarctica, recognised as the largest single contributor of ice to the sea, has begun an accelerated and irreversible melt rate that could see it shedding 100 billion tonnes a year, equating to a global sea level rise of up to 10mm in 20 years.
According to Gael Durand, a glaciologist with France's Grenoble Alps University and coauthor on a paper describing the find published in Nature Climate Change, the Pine Island Glacier "has started a phase of self-sustained retreat and will irreversibly continue its decline".
The massive ice stream has been the focus of much research after its significant contribution to rising sea levels was discovered. Back in 2000 the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument aboard Nasa's Terra spacecraft identified a huge crack in the glacier. It showed that the crack expanded by 15m each day resulting in the separation of an iceberg known as B-21 that measured 42km by 17km. Then in 2011, another huge calving event was noted when the instrument picked up a 20km by 80m crack that plunged 60m deep.
Understanding why the Pine Island Glacier is melting so fast is key to the forecasts of future sea level rises, and this latest study has found that the receding grounding line -- the line that separates the point where the glacier touches land from the floating ice shelf -- is responsible, causing increased instability the more it recedes and rapid flow.
The team of glaciologists used three ice flow models to come to the conclusion that the glacier's grounding line "is probably engaged in an unstable 40 km retreat". That line had already receded by 10km in the past ten years, and the glacier was shown to have lost elevation, which explains the acceleration of ice loss the team argues.
According to statistics gathered by the team from prior studies, the glacier has been losing 20 billion tonnes of ice each year between 1992 and 2011. Because of the accelerated rate, that figure will increase to 100 billion tonnes a year, they predict.
A series of other factors have led to this irreversible point, however. As the glacier loses ice due to rising temperatures, it has flown more rapidly leading to the development of the kind of cracks that generate huge ice loss events like those instigated in 2000 and 2011.
In December 2013 the European Space Agency revealed West Antarctica ice sheet loss had accelerated and led to a 15 percent increase in contribution to sea level rises, with 150km3 of ice dumped into the ocean every year. Pine Island, along with the Thwaites and Smith Glaciers, are responsible for this increase. These measurements were, however, taken by the Cryosat spacecraft launched in 2010, which has given us the sharpest images of the sheet and glaciers yet by using two antennas to capture the height and shape of slopes and ridges. "Although some of the changes are due to increased ice thinning, others are related to Cryosat's capacity to observe previously unseen terrain and, of course, three years is a very short period for detecting trends," Andy Shepherd of Leeds University, coauthor on a paper revealing the find, told the BBC at the time.
Most recently, another paper published in the journal Sciencethis month by a team from the British Antarctic Survery blamed depletion of the Pine Island Glacier on its vulnerability to climactic and ocean variability. Much of the surrounding ice loss is down to the Circumpolar Deep Water flow on the nearby continental shelf, bringing warm water flow. However, the team found that melting of the ice shelf the glacier flows into dropped by 50 percent between 2010 and 2012, it believes due to ocean cooling caused by increased easterly winds caused by a La Ninã event.
"We found ocean melting of the glacier was the lowest ever recorded, and less than half of that observed in 2010," commented Pierre Dutrieux from British Antarctic Survey. "This enormous, and unexpected, variability contradicts the widespread view that a simple and steady ocean warming in the region is eroding the West Antarctic Ice Sheet. These results demonstrate that the sea-level contribution of the ice sheet is influenced by climatic variability over a wide range of time scales."
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