Dr Bronte Tilbrook checks some of the Southern Surveyor's ocean sampling equipment.

Carbon dioxide, the ocean and climate change

As the Southern Ocean absorbs increasing amounts of carbon dioxide, its chemistry is changing with potentially significant impacts on marine life.

• 19 August 2008 | Updated 14 October 2011

• The Southern Ocean carbon sink
• Consequences of ocean acidification
• Wider ranging consequences

Just like land plants, phytoplankton convert carbon dioxide and water to carbohydrates and oxygen through photosynthesis. These carbohydrates are then used as fuel by all forms of ocean life.

As the carbon-containing carbohydrates make their way through the food chain, the carbon is effectively ‘locked up’ and kept out of the atmosphere.

The carbon can be kept out of circulation for periods ranging from decades to centuries when dead organisms and other organic matter are transferred to the deep ocean by a process known as the biological pump.

In order to predict future carbon dioxide concentrations in the atmosphere, it is necessary to understand the way that the biological pump varies both in space and time, as well as the effects on the pump of changes in temperature, ocean circulation and ocean chemistry, such as acidification due to increased carbon dioxide.

The Southern Ocean carbon sink

The world’s oceans absorb more than 25 per cent of carbon dioxide generated by human activity. 

Nearly half of this total is absorbed in the Southern Ocean. Because the Southern Ocean is a ‘sink’ for carbon dioxide – a region in which more carbon dioxide is absorbed than emitted – total carbon dioxide levels in the Southern Ocean are rising.

The rate of this rise is exceeding the normal capacity of currents and eddies to transfer and store carbon dioxide in the ocean interior.

The resulting build up of carbon dioxide at the ocean surface is causing an increase in ocean acidity in sub-Antarctic zones of the Southern Ocean, south of Australia.

Consequences of ocean acidification

Calcium carbonate is the main component of coral reefs and the shells and external skeletons of many marine animals.

As ocean acidity increases there is a decrease in concentrations of calcium carbonate. This decrease is expected to affect marine species in polar and sub-Antarctic marine ecosystems, in particular marine organisms with external skeletons and shells built from calcium carbonate.

The most threatened species are cold-water calcifying organisms which include:

• sea urchins
• cold-water corals
• coralline algae
• plankton known as pteropods.

Pteropods are winged snails that drift through surface waters, and are abundant in high-latitude regions and in the Ross Sea. They are considered an indicator of ecosystem health.

Biological studies have reported the pitting and partial dissolution of pteropod shells in under-saturated (low calcium carbonate) waters, raising concerns that they may not be able to adapt to under-saturated conditions in the high-latitude surface ocean during the next 100 years.

Pteropods are eaten by many small marine creatures which are in turn eaten by larger animals. If pteropods are adversely affected by ocean acidification there will be significant consequences for the rest of the food chain.

Wider ranging consequences

Higher concentrations of carbon dioxide also may make it harder for some larger marine animals to obtain oxygen from seawater. For example, squid are particularly sensitive because they move by jet propulsion, which is energy-demanding and requires a good supply of oxygen.

In tropical regions, the combined effects of climate change and ocean acidification mean that corals could be rare on tropical and subtropical reefs, such as the Great Barrier Reef, by 2050.

This will have major ramifications for hundreds of thousands of other species reliant on reef systems, as well as for the people that depend on them for food, tourism, and to help to protect coastal areas from, for example, tsunamis.

The Wealth from Oceans Flagship is working closely with other marine research organisations to determine the rate, nature and consequences of the ocean’s changing chemistry.

Find out more about Understanding our Changing Climate.