According to a new study, a vital section of the world's largest ice shelf is losing ice 10 times faster than the overall melt rate for the structure, posing a potential risk to its future stability. The Ross Ice Shelf stretches out over 500,809 km2 (193,363 miles2), and accounts for 32 percent of Antarctica's total ice shelf area.
Antarctica's ice shelves are known to play a vital role in mitigating rising sea levels, however they do not do so directly. Whilst ice shelves are always connected to a landmass, their vast frozen bulk is actually afloat in the ocean. Because of this, even when a significantly-sized chunk of a shelf breaks off, drops into the ocean and melts, it does not contribute to the rise in sea level, as the mass that formed the fragment was already displacing the water.
Their ability to slow the rise in global sea-level stems from their ability to slow the flow rate of the ice streams and glaciers that feed them. Think of an ice shelf as a stopper. If one were to fully break apart and disappear, the material flowing from the glaciers behind it would flow directly into the ocean.
"Previous studies have shown that when ice shelves collapse, the feeding glaciers can speed up by a factor of two or three," warns Dr. Poul Christoffersen of the University of Cambridge's Scott Polar Research Institute, who co-authored a new study detailing an important threat to the world's largest ice shelf.
It is very unlikely that the Ross Ice Shelf is going to collapse any time soon. However, the newly-published research has highlighted a potential threat to the long-term stability of the frozen leviathan.
The goal of the study was to shed light on the amount of ice that was melting at the base of the shelf, in a region surrounding a landmass called Ross Island. Secondly, the scientists behind the research sought to discover the extent to which ocean water warmed by the Sun's radiation drove the melting process.
The shelf environment surrounding Ross Island is of particular importance, as the landmass is responsible for partially pinning the icy structure back against the shore, and slowing the outward flow of the shelf.
Data used in the study was collected over the course of four years using a range of instruments. A custom-made radar system took measurements from 78 sites surrounding Ross Island in order to keep track of the changing thickness of the shelf.
The team was also able to draw on data collected by instruments placed along a wire anchored to the ocean floor below the shelf, known as a mooring. These sensors measured ocean currents, temperature and salinity on an hourly basis. Upwards Looking Sonar (ULS) was also employed to track how much ice was melted.
The instruments revealed that relatively warm water was flowing into a cavity located beneath the shelf, causing it to lose ice at roughly three times the normal rate for the region during summer months. For context, the region is losing ice 10 times faster than the average melt rate expected for the entire shelf.
It was determined that the water invading the cavity was heated by the Sun in a stretch of open ocean near to the shelf known as the Ross Sea Polyna, which is kept largely free of sea ice thanks to powerful offshore winds. Once warmed, the water was driven into the cavity by a combination of strong winds and tidal forcing.
The Ross Ice Shelf is currently relatively stable in terms of mass, as ice lost to melting is largely replaced by material flowing in from tributary glaciers and the long-term accumulation of snow. However, the balance is dependent on Ross Island's ability to pin the shelf back, slowing its outward progress, and further warming in the future could weaken this mechanism.
In the coming decades, the waters surrounding the shelf are predicted to become increasingly free of sea ice. Without these enormous floating ice cubes acting as heatsinks, the surface water temperature is likely to rise significantly.
According to the authors of the study, this temperature increase could intensify the rate of melting, and thus undermine the stabilizing pinning points that allow Ross Island to exert its influence.
A paper detailing the findings has been published in the journal Nature Geoscience.
Source: University of Cambridge
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