Ocean acidification has become one of the most heavily publicised areas of oceanographic research. It is firmly placed on the agenda for discussion in the federal and international policy arena and frequently appears in the popular press as one of the largest impacts of climate change. As such, it is incredibly important that the science surrounding ocean acidification is communicated correctly to both policy makers and the public alike. This is no easy task as many aspects of carbonate chemistry, and its application to real world scenarios, are complex. The following is by no means an exhaustive discussion of the mechanisms behind ocean acidification, but seeks to highlight some of the features of the system imperative to informed discussion and opinion.
Ocean acidification, or OA, affects the ability of marine organisms, such as corals, to produce calcium carbonate. To understand why, the properties of seawater that are most important are pH, carbonate ion concentration and saturation state. The relationship between pH and the different carbon species (i.e disolved CO2, bicarbonate and carbonate ion concentration) is outlined here. As the pH lowers, or becomes more acidic, carbonate ions are consumed in the reaction that forms bicarbonate ions (see cartoon above). This lowers the ‘saturation state’ or potential for calcium carbonate to form. As seawater becomes less ‘saturated’ with carbonate ions, calcium carbonate becomes vulnerable to dissolution.
Calcium carbonate minerals occur in several forms, or polymorphs, depending on crystal arrangement. The most common forms are calcite and aragonite. Most corals have aragonite skeletons, which is the more soluble form of calcium carbonate, and is therefore more prone to dissolution. While coral reefs get the most media attention they are not the only organism at risk, so are oysters, clams, sea urchins, pteropods, deep-sea corals, and calcareous plankton to name a few. However we need to look at the bigger picture. We know very little about how complex marine ecosystems will continue to function in response to these changes in seawater chemistry, for example some organisms such as algae may benefit from the increased CO2 available for photosynthesis.
One has to be careful when calling ocean acidification the ‘other’ CO2 problem as this implies it is separate to global warming. Warming and acidification are both being caused by increasing anthropogenic CO2 emissions. It might be more correct to say that they are different manifestations of the SAME problem. Thermal stress will effects both biological function, and seawater chemistry. While temperature plays a role in the solubility of gases and therefore the uptake of CO2 by the ocean, it is the buffering capacity of seawater that is of greatest importance in terms of acidification and calcification.
Climate change denialists that seek to discredit the science of ocean acidification point to periods in the geological record where the pH of the ocean was known to be lower, yet organisms continued to calcify. What they neglect to discuss is that this has to do with the RATE of change. There are feedback mechanisms in the earth climate system that can balance change on longer timescales. This includes the dissolution of terrestrial rocks due to weathering, that delivers carbonate ions to the ocean. If atmospheric CO2 increase is slow enough, the pH of the ocean will lower but the input of carbonate ions from land will keep the ocean ‘saturated’, and calcification can continue unabated. In the modern-day, the increase in atmospheric CO2 is occurring at an unprecedented rate that far exceeds the rate of any natural feedback mechanism. The comparison with this previous time in Earth’s history is simply not valid.