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.