Australasian Science: Australia's authority on science since 1938

Ocean Acidification Worse than Thought

By Stephen Luntz

When it comes to carbon dioxide’s effects on the acidity of the oceans we need to fear extreme events rather than the average, a study published in Global Change Biology has warned.

Carbon dioxide was once known as carbonic acid for its acidity when dissolved in water. Higher concentrations in the atmosphere increase the concentration in the upper oceans. This is causing what is known as ocean acidification, although technically the slightly alkaline oceans are becoming closer to neutral (AS, May 2008, pp.31–33).

Emily Shaw, a PhD student at the University of NSW Climate Change Research Centre, examined the natural variability in the pH of seawater around shallow reefs. “At the moment we have a lot of natural processes that cause variability,” says Shaw. “However, the ocean can buffer them. As we increase carbon dioxide levels in the atmosphere we reduce the ability of the ocean to absorb these changes.” As a consequence, not only will the oceans be more acidic on average, but extreme events will fall further from the mean.

Marine plants absorb carbon dioxide during the day but release some at night, changing the local acidity. Low tides produce greater swings in acidity as the effects of biological processes are distributed over a smaller volume of water than when the tides are high.

“In recognising that extreme changes in pH are likely in the future, it is important that further research is done to examine the biological consequences of short-term exposure to extreme carbon dioxide conditions,” says Shaw.

The concern is that while average ocean conditions may take until the end of the century to reach the point where corals dissolve, the temporary conditions may reach this point much sooner, leading to a rapid demise for coral reefs.

“If you’re a manager of a reef you can’t stop carbon dioxide emissions,” says Shaw. “However, you can enhance ecosystem health and resilience. We know that if there is one condition you can’t get rid of, the reef will do better if every other aspect is better. Acidification becomes worse if combined with other stressors. On a tiny scale it might be possible to add alkaline to a local resort to save a small chunk of coral, but on the scale of the Great Barrier Reef, or even a whole reef at a resort, this would not be viable.”