Southern Ocean to acidify much sooner than thought

11 November 2008

The Southern Ocean will acidify much earlier than previously thought from increasing atmospheric carbon dioxide levels, taking it past the point where the shells of sea creatures will start to dissolve, according to a new study.

That point will be reached when atmospheric carbon dioxide (CO₂) levels pass 450 parts per million (ppm), which is projected to occur within 30 years at most, according to University of New South Wales climate scientist Dr Ben McNeil.

Previous estimates found that this "dissolution point" for shells in the Southern Ocean would occur after atmospheric carbon dioxide concentrations reached 550ppm, which is projected to occur in the latter part of the century, says Dr McNeil, whose research with Dr Richard Matear is published in the US journal Proceedings of National Academy of Sciences.

By using a new technique that better quantifies natural variations of carbon dioxide in the Southern Ocean, they found that natural processes amplify the onset of the dissolution point.

"Oceanic acidification is a direct consequence of increasing atmospheric carbon dioxide concentrations," says Dr McNeil. "Our new results point to irreversible and detrimental impacts to Southern Ocean marine calcifying organisms if atmospheric CO₂ exceeds 450ppm.

"It provides additional and direct scientific evidence for the world to do everything in its power to limit CO₂ concentrations to 450ppm in order to avoid the irreversible consequences associated with ocean acidification."

Many marine organisms produce calcium carbonate for protection and growth, particularly in the Southern Ocean. Harming the calcifying process could have significant consequences for the marine food chain, as earlier studies have shown, but these flow-on effects remain unclear.

The problem arises because rising atmospheric carbon dioxide concentrations will fundamentally alter the ocean's chemistry by lowering ocean pH, making it more acidic.

Of the 30 billion tonnes of carbon dioxide emitted into the atmosphere through fossil-fuel burning, about one-third is absorbed by the ocean. There, it not only reduces seawater pH but also reduces carbonate mineral saturation, which plays an important role in calcification for many marine organisms.

Media contact:
Bob Beale: mobile 0411 705 435, bbeale@unsw.edu.au

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