By Michael Byrnes

SYDNEY, Feb 22 (Reuters) - The pristine Southern Ocean, which swirls around the Antarctic and absorbs vast amounts of carbon dioxide from the atmosphere, is slowly losing a fight against industrial gases responsible for global warming, scientists say.

The Southern Ocean's unique wind and storm conditions make it the world's greatest carbon "sink"; the earth's oceans absorb a third of the carbon dioxide in the atmosphere, and the Southern Ocean absorbs a third of that.

But the waters that surround Antarctica are becoming more acidic as they absorb increasing amounts of carbon dioxide produced by nations burning fossil fuels such as oil, coal and natural gas.

Deforestation and slash-and-burn farming also releases vast amounts of carbon dioxide stored in timber or peat bogs.

The more acidic an ocean gets, the less carbon dioxide it can soak up.

"It is becoming more difficult for the Southern Ocean to absorb the excess carbon dioxide," said Dr Will Howard of Australia's Antarctic Climate and Ecosystems Cooperative Research Centre.

Howard has just returned to the Australian Antarctic and Southern Ocean Research Programme's base in southern Tasmania state after leading a team of 60 international scientists on a five-week expedition to gather evidence on how ocean systems are struggling to cope with the build-up of greenhouse gases.

"I would not say it's being killed," Howard said in a telephone interview. But it is being changed. "And once the system is altered ... it's going to be a different ecosystem," he said.

Rising acidification of the Southern Ocean has already begun to affect the ability of plankton -- microscopic marine plants, animals and bacteria -- to absorb carbon dioxide, scientists have found.

In the sea as on land, plants absorb carbon dioxide from the atmosphere through photosynthesis. Oceans soak up carbon dioxide from the air and sink it to the depths.

CHEMISTRY

Microscopic marine organisms also form tiny shells of calcium carbonate, which sink when they die to also move carbon to the bottom of the sea.

Projections by the Antarctic Climate & Ecosystems Cooperative Research Centre indicate that some organisms will not be able to make shells within the next 100 years, Howard said.

"We're talking about timescales of decades to perhaps a century before at least some of these shell-making organisms are facing an ocean chemistry that they cannot make shells in."

Scientists from Australia, France, Belgium, the United States and New Zealand on board the research ship Aurora Australis have just returned from gathering extensive seawater samples from east of Tasmania, where the warm, east Australian current mixes with colder Southern Ocean waters.

This is also an area that carries iron-bearing dust blown off the vast, arid Australian continent into the sea. And iron is seen as part of a possible solution.

Scientists have discovered that phytoplankton in the Southern Ocean are deficient in iron, and that some parts of the Southern Ocean are persistently more fertile than others, probably because they receive extra iron.

So should Australia, the world's largest exporter of iron ore for the steel mills of Asia, throw its iron ore into the sea to help plankton absorb excess carbon dioxide?

"It's not so easy to manipulate," Howard said.

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