Ocean Acidification

The current debate on the connection between CO₂ emissions and climate change has largely overlooked an independent and equally serious problem, the increasing acidity of our oceans. Last December, the respected  journal “Oceanography” published projections (see graphic below) for this rising acidity, measured by falling pH [i], through to the end of the century [ii].

CO₂ in the atmosphere has increased from 278 ppm in pre-industrial times to 390 ppm today. During this time, the amount of CO₂ dissolved in the ocean has   risen by more than 30%, decreasing the pH of the ocean by 0.11 units. As with CO₂ and global warming, there is a lag between cause and effect. That means we are yet to see the worst of the problem. According to the Australian Antarctic Division of our Department of the Environment, “even if all carbon emissions stopped today, we are committed to a further drop of 0.1 to 0.2 pH units” [iii]. However, if CO₂ is allowed to rise along a business-as-usual trajectory, they are concerned that pH will “fall by 0.5 pH units by 2100, a 320% increase in acidity”.

The close relationship between CO₂ in the atmosphere, CO₂ dissolved in the ocean, and the effect of the latter in falling pH, is illustrated by the graph [iv] below.

CO₂  dissolves in water to form carbonic acid. (It is worth noting that carbonic acid is what eats out limestone caves from our mountains.) In the oceans, carbonic acid releases hydrogen ions (H+), reducing pH, and bicarbonate ions (HCO3-). 

CO₂ + H₂O => H⁺ + HCO₃^–     (1)

The additional hydrogen ions released by carbonic acid bind to carbonate ions (CO₃^2-), forming additional HCO₃^–.   

H⁺ + CO₃^2- => HCO₃^–     (2)

This reduces the concentration of CO₃^2-, making it harder for marine creatures to take up CO₃^2- to form the calcium carbonate needed to build their exoskeletons.

Ca²⁺ + CO₃^2- => CaCO₃   (3)

The two main forms of calcium carbonate used by marine creatures are calcite and aragonite. Decreasing the amount of carbonate ions in the water makes conditions more difficult for both calcite users (phytoplankton, foraminifera and coccolithophore algae), and aragonite users (corals, shellfish, pteropods and heteropods).

The photo below left shows healthy specimens of calcifying phytoplankton Gephyrocapsa oceanica. The photo below right shows the damage to the same creature under conditions expected by the end of the century.

Source: Nature, Reduced Calcification of Marine Phytoplankton in Response to Increased Atmospheric CO2, Issue 407 p.364 -367

It is often said that a picture is worth a thousand words.

Research in the Southern Ocean provides evidence that the formation of foraminifera shells is already being affected. Even though these creatures use calcite, which is less soluble than aragonite, there are already clear signs of physical damage. According to Dr. Will Howard of the Antarctic Climate and Ecosystems Cooperative Research Centre in Hobart, shells of one species of foraminifera (Globigerina bulloides) are 30 to 35 percent thinner than shells formed prior to the industrial period. [vi]. The photo below left shows a pre-industrial exoskeleton of this species obtained from sea-floor sediment. The photo below right shows a exoskeleton of a live specimen of the same species obtained from the water column in the same area in 2007. These stunning images were obtained using an electron microscope. (An interview with Dr. Howard was broadcast on the Catalyst television program). [vii] What is staggering is the amount of erosion in the right image compared to the left. The right sample look porous with larger holes and a 10-fold increase in their number. These and creatures like them are at the base of an ocean food chain, and they are already seriously damaged. If they are lost, it is not just biodiversity we are losing, but our food supply as well.

Source: Australian Broadcasting Corporation, Ocean Acidification – The Big Global Warming Story, 13 September 2007

The implications of all of this are disturbing. For corals to absorb aragonite from seawater, the latter needs to be saturated in this mineral.

Now a report from NOAA scientists found large quantities of water undersaturated in aragonite are already upwelling close to the Pacific continental shelf from Vancouver to northern California [v]. Although the study only dealt with the area, the authors suggest that other shelf areas may be experiencing similar effects.

For corals like those in Australia’s Great Barrier Reef, the outlook is grim. They are threatened with destruction on two fronts, both caused by CO₂ emissions. Not only do increased ocean temperatures bleach coral by forcing them to expel the algae which supplies them with energy (see photo at left) [viii], but increased ocean CO₂ reduces the availability of aragonite from which reefs are made.

It is time to wake up. Our planet is dying. I urge you to find the time to view a 20 minute documentary on the problem of ocean acidification produced by the international Natural Resource Defence Council. Simply go to: www.acidtestmovie.com

References and Notes