The oceans absorb large amounts of carbon dioxide from the atmosphere. They currently remove about 25 per cent of the emissions of carbon dioxide (CO2) produced by human activities, which is more than 2 billion tonnes of carbon a year. Around 40 per cent of this enters through the Southern Ocean.
This ocean carbon ‘sink’ occurs because CO2 is absorbed by ocean waters when atmospheric CO2 concentrations are higher than those at the ocean’s surface. The carbon dioxide absorbed by surface waters is eventually mixed down into the deep ocean in high latitude polar waters where the waters of the upper ocean are cold and salty enough to sink to the deep sea. The sinking waters carry the absorbed carbon dioxide and are replaced by upwelling of deeper water that was last at the surface when atmospheric CO2 levels were lower. These waters can absorb more carbon dioxide. This connection between the surface waters and deep sea at the higher latitudes is the main pathway for the oceans to take up our CO2 emissions.
Microscopic marine plants known as phytoplankton also remove carbon dioxide from the surface layer of the ocean when they photosynthesise. Just like land plants, phytoplankton convert carbon dioxide and water to carbohydrates and oxygen through photosynthesis. These phytoplankton are the base of the ocean food chain and fuel other forms of ocean life. When phytoplankton, and other organisms that have consumed them, die they sink to the ocean floor, carrying the carbon with them and away from the surface ocean. While biological pathways play an important role in controlling atmospheric CO2 levels over centuries to millions of years, the physical absorption and transport of carbon into the deep sea is the main control on the ocean sink for the emissions in the present day. It is unclear whether how they will contribute to the control of atmospheric CO2 from human activities in the future as changes in climate may drive shifts in productivity of the oceans and alter the biological transport of carbon to the deep sea.
The service provided by the oceans through the removal of carbon dioxide from the atmosphere comes at a price. The carbon dioxide absorbed in the surface waters cause a decrease in both the pH and the dissolved carbonate ion concentration of seawater, and these changes are referred to as ocean acidification.
When carbon dioxide is absorbed in seawater it forms carbonic acid, which separates to form hydrogen ions and bicarbonate ions. It is the increase in hydrogen ions that lowers the pH of the water. Over the past 200 years, the concentration of hydrogen ions in the seawater has increased by about 25 per cent. This has increased the level of ocean acidity.
The acidity of a substance is a function of its concentration of hydrogen ions, and is measured using the logarithmic pH scale. A pH of 0 represents a very acidic solution (high concentration of hydrogen ions), while a pH of 14 represents a very alkaline solution (low concentration of hydrogen ions). A neutral substance has a pH of 7. The pH of seawater is around 8. Because the scale is logarithmic, a one-unit decrease in pH represents a ten-fold increase in the concentration of hydrogen ions.
The absorption of carbon dioxide by the oceans also reduces the concentration of carbonate ions in ocean waters. Under elevated carbon dioxide levels, carbonate ions react with the excess hydrogen ions in the ocean, making more bicarbonate and reducing the amount of carbonate ion available to marine organisms that build shells or skeletal material of calcium carbonate.
Ocean absorption of carbon dioxide in the last 250 years has decreased near-surface ocean pH by about 0.1. It is expected to decrease it by a further 0.2–0.4 by 2100, depending on the rate of increase of carbon dioxide in the atmosphere. The current rates of ocean acidification are 10 to 100 times greater than those experienced at the end of the last ice age and over many millions of years in the earth’s history.
Laboratory and field experiments show that the formation of shells or skeletons of calcium carbonate by key organisms such as shelled-plankton, corals, and molluscs, will decrease due to ocean acidification. Not all species behave in the same, but there are multiple lines of evidence that corals will take longer to build reefs and these structures are likely to be more fragile and vulnerable to erosion. Increased acidity levels have also been shown to impair the ability of some reef fish to avoid predators while other species like the non-calcifying algae may benefit from increased CO2 in the water. The end result is the ecosystems and biodiversity are likely to change; the ability of coral reefs to provide ecosystem habitats, protect coastal regions from storms and support tourism and fisheries are predicted to decline.
Profound effects on corals and other marine organisms have already been demonstrated, and there is increasing evidence that impacts will occur on a wide range of organisms across the entire marine food web. For example, foraminifera (tiny, single-celled, shelled organisms) have more than 30 per cent less shell weight than their counterparts in the fossil record. Juvenile pteropods (lentil-sized snails) have been observed to have partly dissolved shells as a result of exposure to aragonite-undersaturated waters in the Southern Ocean. Oyster production in the Pacific Northwest of the USA has also been impacted by ocean acidification leading to extensive research to identify ways to maintain an economically viable industry into the future.
Marine ecosystems provide food and employment to people across the globe, worth many billions of dollars annually. In Australia, the combined value of marine industries and ecosystem services is estimated to be in excess of $100 billion per year. A disruption to these services will have significant and far-reaching economic and social impacts.
Australian ocean acidification research mostly focuses on the Southern Ocean and the tropical reef systems of the Great Barrier Reef and Ningaloo. These are critical areas where ocean acidification is expected to have early and lasting impacts on marine ecosystems that are of economic and environmental importance to Australia.
Threats associated with ocean acidification have only been recognised in the past decade. CSIRO scientists have been able to take advantage of data from long-term ocean monitoring programs to establish trends in ocean pH and changing carbonate chemistry. This information is also helping to unravel the detail of the processes involved in ocean acidification. The research uses a variety of moorings and commercial and research vessels operating from the Tropics to Antarctic coastal waters, including research carried out as part of Australia’s Integrated Marine Observing System.
A number of other research centres and universities are investigating ocean acidification impacts on marine species in the Australian marine realm. Researchers at the Antarctic Climate and Ecosystems Cooperative Research Centre and the Australian Antarctic Division are expanding their work from single-species effects to whole-of-community responses in the natural polar setting. The Australian Institute of Marine Science recently found that increased carbon dioxide will reduce the structural diversity of coral reefs. Many universities including James Cook University, University of Queensland and University of Western Australia, University of Sydney, and University of Tasmania have active research programs on ocean acidification impacts.
In May 2016, Australia will host the 4th International Oceans in a High CO2 World Symposium. The symposium, held every four years, is the most significant international meeting on ocean acidification. It brings together the world leading experts in this rapidly emerging frontier of marine research, and will be held for the first time in the Southern Hemisphere. The success in securing the meeting is recognition of the excellent research being carried out in the Australian region on ocean acidification, and the consequences for marine ecosystem and the economic and environmental benefits they provide.