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Warm ocean currents melting Antarctic ice

26 April 2012, by Tamera Jones

Warm ocean currents are the main reason Antarctica has lost so much ice in recent years, says a UK-led team of scientists.

Ice shelf

Ice shelf in the Antarctic.

Instead of warm air melting the ice from above, it seems that warmer sea water is attacking the undersides of ice shelves which sit on the ocean surface, weakening them.

This in turn allows glaciers to speed up, transporting more ice to the ocean surrounding the white continent. This ice loss ultimately drives up sea levels.

Understanding exactly why Antarctica's glaciers are accelerating towards the sea brings researchers a step closer to accurately predicting future sea-level rise.

A team of researchers from the UK, the Netherlands and the US mapped the changing thickness of almost all the floating ice shelves around Antarctica using measurements made by a laser instrument mounted on NASA's ICESat satellite.

'It means we can lose an awful lot of ice to the sea without ever having summers warm enough to make the snow on top of the glaciers melt – the ocean can do all the work from below.'

Dr Hamish Pritchard, NERC's British Antarctic Survey

After analysing 4.5 million of these measurements, they discovered a strong pattern. Of the 54 ice shelves they mapped, they found that 20 are being melted by warm ocean currents. Most of these are clustered around the coast of West Antarctica, in the Amundsen Sea.

In every case, the inland glaciers that flow down to the coast and feed into these thinning ice shelves have accelerated, draining more ice into the sea and raising sea levels.

'We've looked all around the Antarctic coast and we see a clear pattern: in all cases where ice shelves are being melted by the ocean, the inland glaciers are speeding up. It's this glacier acceleration that's responsible for most of the increase in ice loss from the continent and this is contributing to sea-level rise,' explains Dr Hamish Pritchard from NERC's British Antarctic Survey, who led the study, published in Nature.

The researchers also found that the greatest losses from the Antarctic ice sheet – also in West Antarctica – coincide with the most ice shelf thinning.

'In most places in Antarctica, we can't explain the ice-shelf thinning through melting of snow at the surface, so it has to be driven by warm ocean currents melting them from below,' says Pritchard.


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Wilkins ice shelf

Distinguishing between ice loss driven by snow melt on the surface of ice shelves, and ice loss from melting at the base of ice shelves – known as basal melt – isn't a trivial task.

'We used computer models, which use climate data and the physics of how snow changes to look at the way the surfaces of the ice shelves were changing,' says Pritchard.

'Once we'd taken away that signal, we were left with the signal of basal melt of the ice shelves. This is the first time that's been done comprehensively for the whole of Antarctica,' he adds.

Pritchard and his colleagues think ice shelves normally act like natural buttresses, slowing down Antarctica's glaciers' march towards the sea.

'It means we can lose an awful lot of ice to the sea without ever having summers warm enough to make the snow on top of the glaciers melt – the ocean can do all the work from below,' he says.

'It also shows that the Antarctic ice sheet is very sensitive to even quite minor changes in climate, including those that have already taken place in the southern hemisphere.'

The researchers say changes in wind patterns could be behind warmer ocean currents. Previous research has shown that Antarctic winds have changed in response to changes in the climate. This has in turn led to changes in the strength and direction of ocean currents, and helped to funnel warm water beneath vulnerable ice sheets.