Ocean acidification is a significant impact of a changing climate on the Great Barrier Reef ecosystem.

Acidification occurs because the ocean acts as a carbon sink, absorbing carbon dioxide from the atmosphere. This is changing the ocean’s chemistry by reducing the ocean’s pH — which measures acidity or alkalinity — over an extended period time.

When seawater absorbs carbon dioxide, chemical reactions occur, resulting in a greater concentration of hydrogen ions. This causes the seawater to become more acidic and for carbonate ions to be relatively less abundant.

Carbonate ions are the building blocks for many marine animals such as corals, oysters, clams, sea urchins, molluscs, crustacesans and echinoderms, helping them to produce shells and skeletons.

Trends

The pH of seawater has remained steady for millions of years, and marine life has evolved based on the ocean’s delicate chemical balance. However, the oceans are estimated to have absorbed about 30 per cent of the emitted carbon dioxide  from human activities since pre-industrial times.

The carbon dioxide is contained in the upper 10 per cent of oceans (less than 1000 metres depth) because of slow ocean mixing processes.

A decline of 0.1 from pre-industrial times has already been recorded in the pH of the ocean’s surface, taking it to 8.1.  This corresponds to a 26 per cent increase in acidity.  Reef development is thought to cease at pH 7.8.

The Intergovernmental Panel on Climate Change expects this decline to continue, with average reductions of between 0.06 and 0.32 units over the 21st century.

The effects of global warming and ocean acidification may magnify each other, but may not occur uniformly from place to place and over time.

Effects

Even relatively small increases in ocean acidity reduce the capacity of corals to build skeletons, which in turn reduces their capacity to create protective habitat for the Reef's marine life.

The rate of skeleton formation, known as calcification, is already likely to have been affected, resulting in slower growth rates and weaker coral structures.

However, the impact of acidification is likely to vary between coral species and between organisms. The predicted warming of the oceans speeds up the calcification process, potentially counteracting to some extent the negative effects of decreasing ocean pH at some reefs.

Ocean acidification is also expected to make it more difficult for many plankton — which form the basis of the entire marine food chain — to build calcium carbonate (limestone) shells, plates and skeletons.

It will also likely affect fish reproduction, as fish eggs are more sensitive to pH changes than fish adults, thus potentially reducing populations. Ocean acidification has also been shown to reduce the ability of fish larvae to find suitable habitat and find their way home.