Animals are already dissolving in Southern Ocean acid

In a small patch of the Southern Ocean, the shells of sea snails are dissolving. The finding is the first evidence that marine life is already suffering as a result of man-made ocean acidification.

"This is actually happening now," says Geraint Tarling of the British Antarctic Survey in Cambridge, UK. He and colleagues captured free-swimming sea snails called pteropods from the Southern Ocean in early 2008 and found under an electron microscope that the outer layers of their hard shells bore signs of unusual corrosion.

As well as warming the planet, the carbon dioxide we emit is changing the chemistry of the ocean. CO₂ dissolves in water to form carbonic acid, making the water less alkaline. The pH is currently dropping at about 0.1 per century, faster than any time in the last 300 million years.

Lab experiments have shown that organisms with hard shells, such as corals and molluscs, will suffer as a result. To build their shells, corals and molluscs need to take up calcium carbonate from the water, but more carbonic acid means more hydrogen ions in the water. These react with carbonate ions, making them unavailable to form calcium carbonate.

Aragonite shortage

The most vulnerable animals are those, like pteropods, that build their shells entirely from aragonite, a form of calcium carbonate that is very sensitive to extra acidity. By 2050, there will be a severe shortage of aragonite in much of the ocean.

Aragonite is still relatively plentiful in most of the ocean, but Tarling suspected that some regions might already be affected by shortages.

He visited the Southern Ocean near South Georgia where deep water wells up to the surface. This water is naturally low in aragonite, meaning the surface waters it supplies are naturally somewhat low in the mineral ? although not so much so that it would normally be a problem. Add in the effect of ocean acidification, however, and Tarling found that the mineral was dangerously sparse at the surface.

"It's of concern that they can see it today," says Toby Tyrrell of the National Oceanography Centre in Southampton, UK.

Aragonite-depleted regions are still rare, but they will become widespread by 2050, says Tarling. The polar oceans will change fastest, with the tropics following a few decades after. "These pockets will start to get larger and larger until they meet," he says.

Tyrrell says the Arctic will become undersaturated with respect to aragonite before the Antarctic. Patches of undersaturation have already been seen, for instance off the north coast of Canada in 2008.

The only way to stop ocean acidification is to reduce our CO₂ emissions, Tyrrell says. It has been suggested that we could add megatonnes of lime to the ocean to balance the extra acidity. However, Tyrrell says this is "probably not practical" because the amounts involved ? and thus the costs ? are enormous.

Journal reference: Nature Geoscience, DOI: 10.1038/ngeo1635

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