The other CO2 problem: Tiny planktonic molluscs called pteropods are likely to be impacted by increasing acidity in the Southern Ocean. These organisms are an important component of the food chain, but may be find it difficult to form shells by 2030.
Credit: SPL
SYDNEY: Southern Ocean marine life may start to dissolve away, say Australian scientists, who have discovered that a dangerous 'tipping point' for ocean acidification could arrive as soon as 2030.
This is worrying news, say the experts, because organisms with calcium carbonate skeletons and shells – such as corals, pteropods, krill and other shellfish – may not be able to survive the changing 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 government research agency the CSIRO.
Tipping point
Previous work had suggested that the tipping point wouldn't occur until atmospheric carbon dioxide (CO2) passes a concentration of 550ppm, which some climate models predict will be happen by 2060. The revised estimate is published this week in the U.S. journal, the Proceedings of the National Academy of Sciences.
Every year the ocean provides a useful service by absorbing one-third of the 30 billion tonnes of CO2 emitted into the atmosphere by burning fossil fuels.
However, researchers have shown that this dissolved CO2 creates a weak solution of carbonic acid, which is fundamentally changing the chemistry of the marine environment, with significant consequences for many marine plants and animals.
Most affected are animals that use calcium carbonate to build their skeletons. As the ocean becomes mildly more acidic (only a very slightly lower pH, still less acidic than the human body), these animals will find it more difficult to grow. Eventually, at the predicted 'tipping point', research suggests that they may start to find their skeletons and shells dissolve away.
First signs
"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," commented Bayden Russell, a marine biologist at the University of Adelaide, South Australia, who was not involved in the study.
Study authors McNeil and Richard Matear, a climate modeller with the CSIRO in Hobart, Tasmania, made their prediction of a revised date for the tipping point by comparing changes to carbonate chemistry in the Southern Ocean over time and across seasons.
Currently levels of CO2 in the atmosphere are estimated at 385 ppm, and levels there are in equilibrium with the amount that dissolves into the seas. If the atmospheric level reaches 450 ppm, though, the ability of microscopic plankton to retain their calcium carbonate shells will be severely affected, said McNeil.
Plankton 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.
"Big trouble"
Previous estimates of the tipping point in the Southern Ocean ranged up to 550 ppm of atmospheric CO2. Australia's Garnaut Report on Climate Change considers the 550 ppm target for capping our emissions 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 incentive to more rapidly reduce our carbon dioxide emissions.
The study "highlights the importance of understanding the dynamics of carbonate equilibrium in seawater in our greenhouse world," commented Ove Hoegh-Guldberg, director of the University of Queensland's Centre of Marine Studies in Brisbane, Australia.
"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," added Hoegh-Guldberg.
"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," he said.
