Ocean acidification threatens marine ecosystems

By Karen Barlow

ABC News Online, 3 March 2011

As politicians debate the merits of a carbon tax scheme, antarctic waters are being used to create an early warning system for oceans affected by climate change.

Carbon dioxide does not just affect global warming. Ocean acidification is picking up pace, threatening entire marine ecosystems.

Famed coral, like the Great Barrier Reef, is at risk from more acidic seawater, but the planet's polar waters are especially vulnerable.

Aboard the temporary floating science laboratory Aurora Australis, marine geologist Dr Will Howard and a team of scientists are investigating the marine repercussions of climate change by sampling the curious life of the southern ocean.

Rarely seen, tiny marine snails called pteropods are vulnerable to subtle changes in ocean chemistry.

The concern is that if seawater becomes too acidic they and other organisms like coral will not be able to form.

Dr Howard says entire ecosystems are at risk.

"The raw ingredient that a lot of these shell making organisms require is being reduced," he said.

"Now some of the evidence we are getting from the field would suggest that they are sensitive enough to already be feeling the impact of that reduction in this raw material they need for their shells."

Dr Howard says seawater is slightly alkaline, but increasing carbon dioxide is making the ocean more acidic.

"It is unprecedented for millions of years the last time the carbon dioxide of the atmosphere was as high as we have now," he said.

Dr Howard says the acidity of the oceans has increased by about 30 per cent.

"That acid in this case has been added by additional carbon dioxide because unlike other gases, when it dissolves in water it forms a weak carbonic acid and that is the source of acidification, and that is the source of the concern," he said.

Major ecosystem shifts

Scientists want to set a benchmark for rarely seen creatures now to detect changes in the future.

Chemical measurements are done on Aurora Australis to calculate the current state of play, while the thickness and chemistry of the snail shells will be studied later in mainland labs.

CSIRO chemical oceanographer Dr Bronte Tilbrook says no one knows how severe ocean acidification will be on these organisms, but they expect to see major shifts in ecosystems.

"It seems like some of the biggest changes are on the polar regions, but this ocean acidification is occurring all through the earth's oceans and tropical reefs. Coral reefs are also considered at great risk," she said.

Oceans absorb carbon dioxide as part of the planet's natural carbon cycle, but global carbon emissions, including Australia's, are increasing.

"We think actually that threshold is very close, within decades, and that is true for both the Antarctic and the Arctic oceans as well so those two sets of polar ecosystems that we think are facing quite a significant chemical challenge now," Dr Howard said.

"I would say it is the most persistent and pervasive impact on the oceans that we have ever seen, because it is happening everywhere that the ocean is in contact with the atmosphere, which is to say almost everywhere in the ocean."

Australian Government figures show carbon emissions are on track to rise 24 per cent above 2000 levels by 2020.

The CSIRO's Dr Tilbrook says the acidification problem will continue to grow.

"And it is not going to go away," she said.

Dr Howard agrees.

"The ocean will ultimately buffer this carbon dioxide but will take centuries possibly thousands of years for it to happen," he said.

"So I think the real source of concern [is] that an impact will not be easy to turn around. No matter if we cut emissions drastically, we will still have to live with this impact for quite some time."

While the effects of ocean acidification are just being felt, Dr Howard says the tide is turning.

"I think it will start to actually affect ecosystems that we can see and that we do experience quite regularly things like the Great Barrier Reef or the reef at Ningaloo," he said.

"These are all systems we are used to actually seeing and experiencing and to some extent depending on for tourism and fisheries and other activities so it may be invisible now but I don't think that it will stay invisible for long."