Recent Ocean Acidification

The diagram depicts the ocean’s carbonate buffer, which converts atmospheric CO₂ into nongaseous forms such as bicarbonate (HCO₃^-) and carbonate ions (CO₃^2-). Marine calcifiers create the calcium carbonate (CaCO₃) and organic carbon(C) shown in the middle of the diagram, a process that will become more difficult with decreasing carbonate ion concentrations resulting from ocean acidification. The descending wiggly arrows represent the ocean’s ‘biological pump,’ which transfers carbon into the deep ocean and sediments over long time scales.

The oceans are more acidic as a result of carbon dioxide (CO₂) emissions associated with human activity.

Surface ocean pH has become more acidic by approximately 0.1 units since pre-industrial times.

Ocean acidification affects calcium carbonate saturation in ocean waters, thereby making this building block of shells and skeletons less available which affects the health of corals and other marine organisms (e.g., crabs and clams).

The atmosphere and the oceans play key roles in regulating climate by continually exchanging carbon. As a result of anthropogenic CO₂ emissions, the atmospheric concentration of CO₂ has increased about 38 percent from pre-industrial times to 2009.  The amount of carbon contained in the ocean has increased in tandem with increasing atmospheric concentrations (IPCC, 2007b), due to the increasing atmospheric pressure of CO₂. Over the past 200 years, oceans have absorbed approximately one-half of the CO₂ produced by the burning of fossil fuels and other industrial processes (Raven et al, 2005).  This absorptive capacity of the oceans has resulted in atmospheric CO₂ concentrations that are much lower than they otherwise would be (IPCC, 2007a).

The increase in the amount of CO₂ dissolved in the oceans has increased the concentration of hydrogen ions (see Figure 1) in the oceans (IPCC, 2007b). As a result, the pH (a measure of acidity) of the oceans has decreased, making the oceans more acidic. It is estimated that the mean surface pH of the oceans has decreased by 0.1 units since pre-industrial times due to increased uptake of anthropogenic CO₂ emissions (IPCC, 2007b). Since pH is measured on a logarithmic scale, a decrease of 0.1 in ocean pH equates to a 30 percent increase in the hydrogen ion concentration of the ocean (Raven et al, 2005).

The ocean’s natural carbonate buffer system (see Figure 1) allows seawater to accommodate the addition of an acid or base without appreciable pH change.  Therefore, this system buffers the increased concentration of hydrogen ions that results from elevated levels of dissolved CO₂ in surface waters.  This keeps the oceans much less acidic than they otherwise would be, but it also reduces the carbonate ion concentration of the seawater, making calcification harder for corals and other marine calcifiers.

While the full impact of existing acidification on marine organisms is not well understood, experiments show that the calcification rates of marine organisms are strongly dependent on the saturation state of carbonate ions in seawater, which is affected by acidification (IPCC, 2007a). Future acidification could significantly affect many kinds of marine organisms and is very likely to interfere with the formation of shells and skeletons by corals and other marine calcifiers, such as crabs, marine snails, and clams.