Around half of all the carbon dioxide (CO2) released by human activities is now in the world’s oceans. The Southern Ocean absorbs about 40% of this.

As CO2 dissolves in the ocean, it increases ocean acidity. This makes it harder for some marine organisms to form shells. The ecological changes, in turn, reduce the capacity of the ocean to absorb CO2.

The Southern Ocean contains more CO2 than other oceans. This is because cooler water absorbs more CO2 than warmer water. This means that the impacts of ocean acidification will appear first in the Southern Ocean.

How does the water change?

When (CO2) dissolves in water (H2O) it forms carbonic acid (H2CO3). This is the same weak acid found in carbonated drinks. Increasing levels of carbonic acid interfere with the formation of calcium carbonate (CaCO3) — a major structural component of the shells of many important planktonic organisms.

What will the effect be?

As it becomes more difficult for calcium carbonate to form, it will become more difficult for some planktonic organisms to form shells. If their shells are thinner and/or deformed, the organisms may be unable to function properly.

Many of these organisms are key components of the food chain. They are important in the diets of krill, fish, squid, penguins, seals and whales. They are also important in the removal of carbon from surface waters to the deep ocean and the release of oxygen into the air.

Important metabolic processes, such as respiration in fish, may also be impaired by the acidity, as lowering the pH reduces the efficiency of oxygen exchange in their gills.

What organisms will be affected?

In the Southern Ocean and other open-ocean ecosystems, calcifying organisms will be affected. These include snail-like molluscs called 'pteropods', single-celled algae called 'coccolithophorids' and protozoan 'foraminifera'.

In tropical coastal ecosystems, coral reefs, which are comprised of colonies of small animals that secrete calcium carbonate skeletons, are also at risk.

Changes in microbial populations are likely to flow on to species throughout the food chain.

How acidic will the oceans get?

The pH of seawater has historically stayed at about 8.2. This is slightly alkaline (pure water is neutral at pH 7). However, CO2 from human activities has caused the pH of ocean surface waters to drop by 0.11 pH units. This might not sound like much, but it is equivalent to a 30% increase in acidity. Unless CO2 emissions are curbed, the pH is expected to fall by 0.5 pH units by the year 2100. This would be a 320% increase in acidity.

How can we stop it?

Even if all carbon emissions stopped today, we would still see a further drop of 0.1 to 0.2 pH units. It will take thousands of years for the oceans to recover.

However, action now can prevent conditions that are corrosive to calcifying organisms from becoming more widespread.

The AAD researches ocean acidification and its effects on marine ecosystems. These research programs will assist governments, scientists, modellers and society to understand the emerging impacts of ocean acidification.

What research is being done?

In 2014–15 AAD scientists created a 'future ocean' under the sea ice off Casey. It uses 4 underwater chambers to measure the impact of ocean acidification on seafloor (or 'benthic') communities.

During the Antarctic 'Free Ocean Carbon Enrichment' (antFOCE) experiment, a team of scientific divers, technicians and engineers increased CO2 concentrations in the water in 2 of the chambers. This decreased the pH of the water by 0.4 pH units, without changing light or nutrient concentrations. The other 2 chambers were used as 'controls' to track natural fluctuations in pH.

Observations from the 'future ocean', allow the team to see how benthic marine habitats respond to more acidic seawater.

Scientists are also studying the effects of ocean acidification on Antarctic marine microbial communities. This includes bacteria, phytoplankton and protozoa. These organisms sit at the base of the food web and directly, or indirectly, support all life in the Southern Ocean.

Six 650 L 'minicosms' or incubation tanks have been set up in a shipping container on the shore at Davis. They are filled with filtered seawater containing different concentrations of CO2. They range from ambient Antarctic concentrations in summer of 84 ppm (parts per million), to the maximum predicted for the year 2300 of 2,423 ppm. For comparison, the current atmospheric CO2 concentration is just over 400 ppm.

Early results indicate that CO2 concentrations at or above those predicted by 2100, negatively affect phytoplankton. It reduces their productivity and growth, and changes the community composition. Such changes at the base of the food chain will have flow-on effects up the food chain, including to krill, seabirds and whales.