Warm ocean driving Antarctic ice loss

Brunt Ice Shelf Ice shelves float on the ocean and slow the flow of land glaciers behind them

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Most of the ice being lost from Antarctica is going as a result of warm water eating the fringes of the continent, scientists say.

The researchers used a satellite laser to measure the thinning occurring on ice shelves - the floating tongues of ice that jut out from the land.

The team's analysis found the shelves' shrinkage could not be attributed simply to warmer air.

Rather, it is warm water getting under the floating ice to melt it from below.

This is leading to a weakening of the shelves, permitting more and more ice to drain from the continent's interior through tributary glaciers.

Previous studies have already indicated that warmer waters are being driven towards the continent by stronger westerly winds in the Southern Ocean.

The researchers say the new understanding has major implications for their ability to reliably project future sea-level rises as a result of Antarctic ice loss.

"What we realise now is that we're looking at a very sensitive system," Dr Hamish Pritchard, from the British Antarctic Survey (BAS), told BBC News.

"Previously, you would have thought that we needed a lot of warming in the atmosphere to get a substantial loss of ice from Antarctica - because it's such a cold place. But what we show is that that's not necessary; you don't need radical change.

"All you need are quite subtle changes - such as a change in the winds - and that can produce effects at the edges of Antarctica that then lead to a loss of a lot of ice."

The research is published in the journal Nature but has also been presented here at the European Geosciences Union (EGU) meeting in Vienna, Austria.

Weaker shelves

Pritchard's team used the laser altimeter on Nasa's Icesat spacecraft to map the changing thickness in 54 ice shelves around Antarctica.

The survey incorporated some 4.5 million data points between 2003 and 2008.

The researchers draw on modelling work and information from a range of other studies to explain the thinning observed by Icesat.

Twenty of the shelves were assessed to be being melted from below by warm ocean currents. Most of the 20 are in West Antarctica, and show thinning up to seven metres per year.

In every single case, the glaciers on land that feed into these shelves have recorded accelerated movement over the same period.

Icesat Icesat stopped working in 2009 but has returned some very valuable data on polar ice thickness

This will have drained many billions more tonnes of ice into the ocean, contributing to sea level rise.

The explanation, simply put, is that the shelves no longer have the strength to impede glacier flow in the way they once did.

"When ice shelves completely collapse - and we've seen that before - the grounded glaciers behind them will speed up; we know that," explained co-author Helen Amanda Fricker of Scripps Institution of Oceanography at UC San Diego, US.

"But what this study is showing, which is very new, is that you don't need to lose the shelf entirely for this to happen; just a reduction in the thickness of the ice shelf is enough to allow more of the grounded ice behind it to flow off the continent."

Future scenarios

One key tell-tale that warm water is at the root of the thinning is shape of the sea-floor.

Some of the greatest melting has been seen where deep troughs cut across the continental shelf, allowing the water easier access to the shelves' undersides.

Larsen B In contrast, the melting of some shelves on the peninsula can be attributed to warmer air

The picture is not uniform all around Antarctica. Indeed, on the peninsula - the long stretch of land pointing towards South America - the shelves show a different set of thinning symptoms, which very probably can be tied to a warming atmosphere.

But again, the originator is probably the same changing wind patterns.

"Strong westerlies go up over the chain of the peninsula mountains and these winds descend, they warm up, melting the surface of the shelves on the eastern side. So, although we have two different melting mechanisms, the ultimate cause is the same - it's the wind," said Dr Pritchard.

Colleague and co-author Prof David Vaughan said the research provided new understanding to help scientists gauge the likely impact of future ice loss on ocean height.

"This is one study within a programme called ice2sea, which is a European Union-funded programme that's aimed at improving our projections of sea-level rise.

"The idea is that we go and study some of the processes that are causing ice sheets in Greenland and Antarctica to change, and that allows us to improve the models for projection of future sea level rise."

The ice2sea project will be releasing its projections into the 21st and 22nd centuries later this year.

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter

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