Ocean acidification (OA) should be considered one of the primary results of human-generated climate change. OA is a critical problem that policy makers, marine managers and scientists face together. The flux of carbon into the atmosphere (in the form of carbon dioxide and methane) has major implications for the chemistry of the oceans. In short, anthropogenic carbon changes the pH of seawater, which decreases the availability of carbon for the production of shells, skeletons and reefs.
The seawater carbonate system, as it is known, is a chemical buffering system that responds to aqueous concentrations of carbon. It is through the seawater carbonate system that we can understand how greenhouses gasses (i.e. CO2) impact the acidity of the ocean. The pH of seawater is dependent on the concentration of dissolved inorganic carbon (DIC) in seawater. Seawater pH lowers in response to the addition of CO2 from the atmosphere, making seawater more acidic. This increase in acidity alters one of the most fundamental chemical features of seawater.
The biological process of calcification (i.e. the creation of calcium carbonate shells and bones) is altered by changes in seawater pH. The CaCO3 saturation state of seawater (W), which is a metric of the availability of CaCO3 (calcium carbonate) for calcification uptake, is modified by the seawater carbonate system. Waters may be saturated or undersaturated in regards to CaCO3. In a carbon-rich future, the change in carbon concentrations in surface and in deep water has been predicted to expand the extent of undersaturated waters. This is commonly discussed as the shoaling of aragonite and calcite saturation horizons (aragonite and calcite being two precipitated crystalline forms of CaCO3).
Since the industrial revolution the surface ocean’s pH has dropped by 0.1 pH units and is projected to drop 0.3-0.4 units by the end of the 21st. Current models predict that surface water CaCO3 saturation states are variable dependent on latitude. Polar waters, which are cold and can absorb more gas, have been predicted to become undersaturated in CaCO3 by as early as 2050. For shallow tropical environments, the concern with OA is not that surface waters will become undersaturated. Warm surface waters, in direct exchange with the atmosphere, cannot become undersaturated. They can, with the progressive encroachment of pCO2 (partial pressure of carbon dioxide), become more and more acidic and have a declining saturation state.
When considering the phenomenon of OA in our modern oceans, it is important to consider the future commitment to a carbon-rich world. “Commitment embodies the concept of unstoppable inevitability, according to which the nature and health of future environments will be determined, not by our actions at some future date but by what is happening now." (Veron 2008) Our global commitment to an acidified ocean may be sealed via the amount of modern carbon that has been moved into the atmosphere and the rate at which we continue to mobilize carbon.
Why does ocean acidification threaten marine ecosystems?
Many laboratory experiments have demonstrated that a decrease in seawater (W) induces a 10-30% decrease in coral calcification. Similar studies have been undertaken to quantify larval and planktonic calcification in undersaturated, acidic waters. It is generally assumed that W (CaCO3 saturation state) controls calcification rates at an organismal level. This is because in more acidic environments (i.e. in undersaturated) it is more energetically expensive for an organism to create and maintain skeletons. This is of grave concern, as marine calcifiers are ubiquitous community members that often maintain the basal level of food chains or create the three-dimensional structure of the seafloor.