Conservative carbon targets will tip oceans sooner


Ocean acidifcation will occur faster if lower CO2 targets are recommened.


Corals like these may dissolve away once the atmospheric carbon dioxide levels rise above 450ppm, new research says.

Credit: Wikimedia / R.Ling

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Southern ocean marine life may start to dissolve away, say Australian scientists, who have discovered that the tipping point for ocean acidification is arriving much sooner than expected.

The news is a death warning to the world's coral species, which cannot survive the change in water conditions.

"Our new results point to irreversible and detrimental impacts to Southern Ocean marine calcifying organisms if atmospheric carbon dioxide exceeds 450 ppm [parts per million]," said Ben McNeil, who led the team from the University of New South Wales and CSIRO.

The study was published today in the journal, Proceedings of the National Academy of Sciences.

Every year the ocean absorbs one-third of the 30 billion tonnes of carbon dioxide emitted into the atmosphere by burning fossil fuels.

Scientists have shown that this absorption leads to ocean acidification, which fundamentally changes the chemistry of the marine environment. Carbon dioxide dissolved in water sources creates by-products, including acid.

This has significant consequences for a many marine plants and animals. Most affected are those that use calcium carbonate as a major 'ingredient' in their skeletons, this includes groups such as shellfish and corals.

As the ocean becomes more acidic, calcium carbonate in shells and corals is literally eaten away, causing marine organisms to die.

"It is likely that marine organisms which use calcium carbonate to form skeletons will be among the first to be affected by ocean acidification, especially in colder waters where calcium carbonate is naturally found in lower concentrations," said Bayden Russell, a marine biologist at the University of Adelaide, who was not involved in the study.

Carbon targets

The more carbon dioxide in the atmosphere, the more the ocean absorbs, and the more acidic the ocean becomes.

Currently levels of carbon dioxide in the atmosphere are estimated at 385 ppm. If this level reaches 450 ppm, the ability of microscopic creatures called zooplankton to retain their calcium carbonate shells will be severely affected, according to McNeil.

Zooplankton are important because they are the basis of the entire ocean food web. If their numbers decline, the effects could ripple up the food chain, to even the large marine mammals. Particularly vulnerable are the small winged snails called pteropods, which make up a quarter of zooplankton biomass in parts of the Southern Ocean such as the Ross Sea, off Antarctica.

Previous estimates of the tipping point ranged up to 550 ppm of atmospheric carbon dioxide. Australia's Garnaut Report on Climate Change considers the 550 ppm target a more feasible goal than the ambitious 450 ppm.

However, the study showed that the new 450 ppm limit may be reached by 2030, providing immediate incentive to sharply reduce carbon dioxide emissions.

The study "highlights the importance of understanding the dynamics of carbonate equilibrium in seawater in our greenhouse world," said Ove Hoegh-Guldberg, the director of the Centre of Marine Studies at the University of Queensland.

"More importantly, however, it confirms the extremely worrying conclusion that marine calcification is in big trouble if atmospheric levels of carbon dioxide exceed 450 ppm carbon dioxide. Rigorous observations such as these should spur our political leaders to make much more decisive steps to curb the rise of carbon dioxide in the atmosphere. Anything less, will be disastrous."