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Journeys to the bottom of the sea

AUSTRALIAN historian and lyrical Antarctic observer Tom Griffiths once likened Antarctica to a giant, breathing organism clamped to the base of the globe, a ''billowing creature rhythmically expanding and contracting'' from winter to summer.

''When the surface of the sea turns to ice, it releases a dense brine that plunges to the ocean depths, and that thrust of salty water to the sea floor is the piston-stroke that drives the engine of ocean circulation, sending cold Antarctic bottom-water northwards, even infiltrating the northern hemisphere,'' he wrote in his award-winning book documenting a voyage through the southern ice, Slicing the Silence.

Last summer, scientists on another voyage into the high latitudes - one coinciding with the centenary of Sir Douglas Mawson's Australian Antarctic Expedition - continued in Mawson's footsteps, collecting measurements and observations across a route from Hobart due south to the end of the earth, then north to Fremantle.

A particular preoccupation was reaching deep into the Southern Ocean abyss to collect and analyse samples of Antarctic bottom-water, the density of which is determined by coldness and salinity.

Preliminary analysis of the material that the team hauled back on board the icebreaker Aurora Australis over 25 arduous days - working round-the-clock shifts to collect measurements from the ocean floor at 77 sites - indicates that the character of the deep ocean is changing profoundly. The densest waters in the world are gradually disappearing and being replaced by less dense waters.

Yesterday the chief scientist of this latest expedition, Dr Steve Rintoul of CSIRO and the Antarctic Climate and Ecosystems CRC, reported his team's findings back to a meeting of the Australian Academy of Science convened in Canberra to mark 100 years of Antarctic science.


"It's a clear signal to us that the oceans are responding rapidly to variations in climate in polar regions. The sinking of dense water around Antarctica is part of a global pattern of ocean currents that has a strong influence on climate, so evidence that these waters are changing is important," Rintoul says.

After comparing this year's data against an archive of similar measurements collected since 1970 (the material is still being prepared for publication), Rintoul and his team estimate there has been as much as a 60 per cent reduction in the volume of Antarctic bottom water, that cold, dense, salty water that is the pulse of global ocean currents.

''We found that the layer that used to be about 1000 metres thick in 1970 is only about 300 metres thick now. And so what that tells us is that the dense water sinking around Antarctica is becoming less dense over time, and there's less of it.''

The findings are consistent with work published by oceanographers with the US National Ocean and Atmospheric Administration and the University of Washington last month. They used temperature data from 1980 to 2011, tracing Antarctic bottom-water right up into the North Atlantic, and concluded that it was disappearing, and surprisingly rapidly.

It's a story that has emerged consistently in work of the past few years, turning scientific understanding of the deep oceans on its head. ''Until a decade or two ago, we thought the deep ocean didn't change,'' Rintoul says. ''Now we see in places like Antarctica responding very rapidly to changes in climate - man-made or natural - and transferring those signals rapidly into the deep ocean. It's a very dynamic environment.''

Whether this change is part of a long natural cycle, or whether it is a consequence of increased warming of the oceans as a consequence of man-made climate change we don't know, Rintoul says.

''The evidence is growing that the most likely cause of the freshening that we see is increased melt around the edge [of the continent], where ice floats on the ocean.''

This is written in the chemistry of the oxygen isotopes, which are different for melted glacial ice as opposed to sea ice, he says.

What it will mean in the future is an equally compelling, and perhaps more urgent, consideration.

Scientists have long understood that the movement of these waters via the world's deep ocean currents plays a critical role in transporting heat and carbon around the planet and thus regulating climate. There has been increasing interest in how they respond to climate change. ''When we speak of global warming, we really mean ocean warming,'' the chief scientist on the modern voyage says. ''More than 90 per cent of the extra heat energy stored by the earth over the last 50 years has gone into warming up the ocean.

''The Southern Ocean is particularly important because it stores more heat and carbon dioxide released by human activities than any other region, and so helps to slow the rate of climate change.

"A key goal of our work is to determine if the Southern Ocean will continue to play this role in the future. It's possible that changes of pattern of ocean currents might reduce the oceans' ability to store heat and carbon. If that happened, that would tend to accelerate the rate of climate change.

''We don't know that is happening yet, but that is why we are so interested.''

The findings come on the heels of a study by the British Antarctic Survey published in the journal Nature last week, which found that most of the ice being lost from Antarctica was vanishing as a result of warm water eating away at floating ice shelves, eroding the buttresses of the grounded ice sheets. This pulls the plug on glaciers in the continent's interior, hastening their flow.

The team used satellite laser to measure the thinning of ice shelves. The analysis found the shelf shrinkage could not be attributed simply to warmer air, but to warm water eating at the ice from below.

The researchers told BBC News that the work had major implications for more precise projections of future sea-level rise as a result of Antarctic ice loss.

''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,'' lead author Dr Hamish Pritchard says. ''But what we show is 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.''

And in another significant study published last week looking at oceans and salinity, work by Australian scientists collaborating with US colleagues observed clear changes in salinity on the ocean's surface, signalling shifts and an acceleration in the global rainfall and evaporation cycle.

They inferred that the water cycle had accelerated by about 4 per cent between 1950 and 2000, and - controversially - that further projected warming by 3 degrees celsius this century could spin the water cycle as much as 24 per cent quicker.

''These changes suggest that arid regions have become drier and high rainfall regions have become wetter in response to observed global warming,'' lead author Dr Paul Durack, formerly of CSIRO, now relocated to the US, says. ''Changes to the global water cycle and the corresponding redistribution of rainfall will affect food availability, stability, access and utilisation.''

Leading scientists unconnected with the paper were variously intrigued, excited and cautious about the findings when questioned by The New York Times. Dr Kevin Trenberth of the US National Centre for Atmospheric Research said he doubted that the global intensification could be as large as Dr Durack's group had found.

Understanding of the cycle of evaporation and rainfall is one of the most confounding and disputed areas of climate investigation.

Jo Chandler is an Age senior writer and author of Feeling the Heat.