About one third of the 30 billion tonnes of carbon dioxide emitted into the atmosphere via fossil fuel burning is absorbed in the world's oceans, says Ben McNeil of the University of New South Wales. Continually increasing atmospheric carbon dioxide concentrations are therefore fundamentally altering ocean chemistry by lowering pH.

The tipping point for this acidification is reached when ocean chemistry is altered so much that the calcium carbonate-containing shells of marine organisms such as crustaceans and molluscs start to dissolve. These creatures produce calcium carbonate to help protect themselves, and must produce more as they grow.

Previous estimates indicated that the "dissolution point" for marine shells in the Southern Ocean would occur after atmospheric carbon dioxide concentrations reached 550 ppm. This was projected to occur in the second half of this century.

But by using a new technique that better quantifies natural variations of carbon dioxide in the Southern Ocean, McNeil and colleagues have found that natural processes – such as cool winter temperatures – bring the onset of this dissolution point forward so that it occurs when atmospheric carbon dioxide concentrations reach just 450 ppm. "This could occur as early as 2030 – much earlier than previously thought," McNeil told environmentalresearchweb.

The team, which includes researchers from the Centre for Australian Weather and Climate Research in Hobart, obtained its results by analysing 15-years' worth of carbon dioxide measurements for the surface of the Southern Ocean. The scientists used an empirical technique to estimate seasonal variations in carbon dioxide and pH levels and say that 450 ppm is the point at which the Southern Ocean will become permanently damaging to some calcifying marine organisms. In other words, the effects would be irreversible, states McNeil.

Early ocean acidification might also have a direct or indirect impact on higher organisms like fish or whales that feed on crustaceans or molluscs, he adds.

The researchers say that the only way to prevent this from happening is to stabilise atmospheric levels of carbon dioxide below 450 ppm. To do that, emissions would have to peak by 2015 and be cut in half by 2050.

"Our work provides additional and direct scientific evidence for the world to do everything in its power to limit carbon dioxide concentrations to 450 ppm to avoid the irrevocable consequences associated with ocean acidification," says McNeil.

The results were published in PNAS.