A study in the Proceedings of the National Academy of Sciences (PNAS)indicates that 450 parts per million (ppm) atmospheric carbon dioxide could well be a tipping point beyond which marine organisms will be severely affected by the increasing acidity of the ocean.

Scientists have known for some time that increasing CO2 concentrations in the atmosphere are causing the oceans to become more acidic. This new research by Australian scientists Ben McNeil (UNSW) and Richard Matear (CSIRO and Burea of Meteorology), shows that the widely accepted limit of 450ppm CO2 is a tipping point for some of the oceans most important organisms, the microscopic zooplankton with calcium carbonate shells. The shells of these organisms start to dissolve when sea water becomes too acidic. The atmospheric concentration of CO2 is currently 385ppm. Here experts respond.

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Dr Ben McNeil is Senior Research Fellow at the Climate Change Research Centre, University of New South Wales and the lead author of the PNAS study.

” Oceanic acidification is a direct consequence of increasing atmospheric CO2 concentrations. Our new results point to irreversible and detrimental impacts to Southern Ocean marine calcifying organisms if atmospheric CO2 exceeds 450ppm. It provides additional and direct scientific evidence for the world to do everything in its power to limit CO2 concentrations to 450ppm in order to avoid the irreversible consequences associated with ocean acidification.”

Some quick facts:
- Of the 30 billion tonnes of carbon dioxide emitted into the atmosphere via fossil fuel burning, about one-third of that is absorbed by the ocean. Continually increasing atmospheric CO2 concentrations is therefore fundamentally altering the ocean’s chemistry by lowering ocean pH, leading to an ‘oceanic acidification’.

- A tipping point for oceanic acidification is reached when the oceanic chemistry is altered so much that shells of calcium carbonate start to dissolve. Many marine organisms produce calcium carbonate for protection and growth, particularly in the Southern ocean.

- Previous estimates found that the point where this ‘dissolution point’ for shells in the Southern Ocean would occur after atmospheric CO2 concentration reached 550 parts per million (ppm), which would occur in the latter part of the 21st century.

- By using a new technique that better quantifies natural variations of CO2 in the Southern Ocean, we find that natural processes amplify the onset of this dissolution point to occur when atmospheric CO2 reaches 450ppm or as early as the year 2030 - much earlier than previously thought.

- Earlier Southern ocean acidification has direct consequences on the calcifying organisms, and yet unclear flow-on impacts to the higher trophic organisms like fish or whales which feed on them.”

Professor Barry Brook is Sir Hubert Wilkins Chair of Climate Change and Director of the Research Institute for Climate Change and Sustainability at Adelaide University.

“Ocean acidification has been called the ‘silent climate change issue’ because it gets far less press than global warming, sea level rise and rainfall changes. Yet for marine systems, it could end up being the most important consequence of industrial pollution of all because it risks dissolving coral reefs and undermining the food chain upon which the world’s fisheries and marine mammals depend. The McNeil and Matear paper is important for a number of reasons: (1) they show that the impacts of acidification will not be globally uniform (southern polar regions – the most biologically productive waters - will be hit hardest first); (2) the impacts will act to compromise the ability of organisms to form shells and skeletons within just the next few decades, and (3) because of natural variability, there will be damaging regional and year-to-year impacts well before the average conditions become generally unsuitable.”

Dr Will Howard is an oceanographer at the Antarctic Climate & Ecosystems Cooperative Research Centre in Tasmania.

“This paper is one of the first to highlight the importance of accounting for seasonal cycles of the ocean and of marine organisms in predicting the impacts of ocean acidification. McNeil and Matear’s projections for the Southern Ocean would say key organisms in Antarctic marine ecosystems will experience conditions corrosive to their shells decades earlier than previously anticipated in model simulations. Coupled with ongoing observations of the distribution and seasonal cycles of marine biota and chemistry [being carried out by the Antarctic Climate & Ecosystems CRC, CSIRO Marine & Atmospheric Research, and the Australian Antarctic Division], this kind of simulation will help develop a more complete picture of impacts we can expect in coming decades. This type of study needs to be carried out in other areas of the world ocean, such as the Arctic, where there are large seasonal cycles and where the impacts of climate change and ocean acidification will be felt earliest.”

Professor Ove Hoegh-Guldberg is Director of the Centre for Marine Studies at the University of Queensland and Deputy Director of the ARC Centre for Excellence in Coral Reef Studies.

“This is a key paper which highlights the importance of understanding the dynamics of carbonate equilibrium in seawater in our greenhouse world. 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 CO2. We came to a similar conclusion for coral reefs in a recent article in Science magazine (Hoegh-Guldberg et al 2007) – finding also that net calcification on coral reef ecosystems dwindles to zero at about 450-500 ppm. The implications of failing ecosystems as important as those in the Southern Ocean and coral reefs are extremely serious. 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.”