Australian researchers have discovered that the tipping point for ocean acidification caused by human-induced carbon dioxide emissions is much closer than first thought.

Scientists from the University of New South Wales (UNSW) and the Commonwealth Scientific and Industrial Research Organization (CSIRO) looked at seasonal changes in pH and the concentration of an important chemical compound, carbonate, in the Southern Ocean.

The results, published in Tuesday's Proceedings of the National Academy of Science, show that these seasonal changes will actually amplify the effects of human carbon dioxide emissions on ocean acidity, speeding up the process of ocean acidification by 30 years.

Ben McNeil, senior research fellow at the UNSW's Climate Change Research Centre, says the ocean is an enormous sink for carbon dioxide, but unfortunately this comes at a cost. "The ocean is a fantastic sponge for CO2, but as it dissolves in the ocean it reduces the pH of the ocean, so the ocean becomes more acidic."

This acidification makes life especially hard for marine creatures such as pteropods — an important type of plankton found in the Southern Ocean — whose shells are made up largely of calcium carbonate.

Tipping point

Once the acidity of the Southern Ocean reaches a certain level, the shells of these and other calcareous marine creatures will start to dissolve.

"That's a really bad point to get to," says McNeil. "After that point, we can't go back unless we suck the CO2 out of the atmosphere."

This so-called 'tipping point' of acidification had been predicted to occur when atmospheric CO2 levels hit 550 parts per million, around the year 2060.

However, the new research shows levels of the carbonate that these creatures need to build and maintain their shells drops naturally in winter, due to natural variations in factors such as ocean temperature, currents and mixing, and pH.

This means the tipping point is likely to be reached at far lower atmospheric CO2 levels — around 450 ppm, says McNeil, which also happens to be the target set by the IPCC for stabilisation of CO2 emissions.

"That's the benchmark that a lot of climate scientists have said we want to reach," he says, but this concentration is forecast to be reached around 2030.

McNeil says ocean acidification could lead to large-scale ecosystem changes, affecting not just plankton but other marine life including fish, whales and dolphins.

"They're at the base of the food chain ... so right now we don't really know the ramifications."