Climate change and the Antarctic: Ocean acidification

Ocean acidification is a consequence of rising global carbon dioxide levels that is predicted to have serious consequences for cold-water ecosystems, including cold-water corals. Rising CO2 levels upset the balance of carbonate ions in seawater, making it difficult for some organisms, which have shells composed of the mineral calcium carbonate, to form their shells. Many of the organisms that are affected, and are predicted to be affected, are critical for ecosystem health. The Southern Ocean will be impacted early by acidification due to its unique chemistry.

Background

The process of ocean acidification involves several steps. CO2 in the air above the sea surface constantly dissolves in seawater and forms carbonic acid. Carbonic acid can break up into hydrogen ions and bicarbonate ions. The hydrogen ions react with carbonate ions and create more bicarbonate ions, while concentrations of carbonate ions decreases. Decreased carbonate ion concentration means that less carbonate is available for the formation of calcium carbonate, a mineral that forms the shells of many organisms. The decreased concentration also can result in existing shells dissolving. Under normal circumstances the amount of carbon dioxide dissolving in seawater occurs at a level that is not detrimental to shell-forming organisms. However, as CO2 increases, the balance is tipped towards decreased carbonate availability and mineral dissolution. The increased concentration of hydrogen ions under the process described above tends to drive down ocean pH, making the water more acidic – hence the term ocean acidification.


Impact on the Southern Ocean

Colder water is naturally lower in calcium carbonate concentration. Furthermore, scientists believe that ocean acidification will affect the Southern Ocean food webs first because the Southern Ocean is closer to undersaturation levels with respect to calcium carbonate than other oceans. Numerous experiments have reported that calcium carbonate-dependent organisms (also called calcifying organisms or calcifiers) experience significant problems when exposed to lower pH environments. Current atmospheric CO2 concentrations have resulted in a drop of about 0.1 pH units (a 30% increase in acidity), and if current trends continue, ocean pH could drop by an average of 0.5 units to about 7.8 around the year 2100 under the IS92a “business as usual” emissions scenario. The latter represents an ocean that is 320% more acidic than it was in pre-industrial times. Despite that change, the ocean will still be in a slightly alkaline state, the boundary between acid and alkali lying at a pH of 7. Calcifying organisms play critical roles in marine ecosystems, such as the Southern Ocean, and declining populations will have serious consequences for the food web.

The impact on cold-water corals could also be severe. Cold-water corals are particularly vulnerable to ocean acidification, since cold water makes it more difficult for corals to survive. Corals, like calcifying organisms, rely on calcium carbonate to form their skeletons. These cold-water corals found in the Southern Ocean are believed to be several hundred years old and provide valuable information on the history of the ocean and past climate.

ASOC believes it is critical that global leaders act now to minimize sources of carbon dioxide emissions to mitigate the impact of climate change and ocean acidification on the world’s oceans. Scientists are still learning about Southern Ocean ecosystems and species – who knows what we could lose if we upset ocean chemistry.

Read more about ocean acidification in our recent paper.

Antarctic and Southern Ocean Coalition, Web site.