Both anthropogenic climate change and anthropogenic ocean acidification are caused by increasing carbon dioxide concentrations in the atmosphere. Rising levels of carbon dioxide (CO₂), along with other greenhouse gases, indirectly alter the climate system by trapping heat as it is reflected back from the Earth’s surface. Anthropogenic ocean acidification is a direct consequence of rising CO₂ concentrations as seawater currently absorbs about 30% of the anthropogenic CO₂ from the atmosphere.
Ocean acidification refers to a reduction in pH over an extended period, typically decades or longer, caused primarily by the uptake of CO₂ from the atmosphere. pH is a dimensionless measure of acidity. Ocean acidification describes the direction of pH change rather than the end point; that is, ocean pH is decreasing but is not expected to become acidic (pH < 7). Ocean acidification can also be caused by other chemical additions or subtractions from the oceans that are natural (e.g., increased volcanic activity, methane hydrate releases, long-term changes in net respiration) or human-induced (e.g., release of nitrogen and sulphur compounds into the atmosphere). Anthropogenic ocean acidification refers to the component of pH reduction that is caused by human activity.
Since about 1750, the release of CO₂ from industrial and agricultural activities has resulted in global average atmospheric CO₂ concentrations that have increased from 278 to 390.5 ppm in 2011. The atmospheric concentration of CO₂ is now higher than experienced on the Earth for at least the last 800,000 years and is expected to continue to rise because of our dependence on fossil fuels for energy. To date, the oceans have absorbed approximately 155 ± 30 PgC from the atmosphere, which corresponds to roughly one-fourth of the total amount of CO₂ emitted (555 ± 85 PgC) by human activities since preindustrial times. This natural process of absorption has significantly reduced the greenhouse gas levels in the atmosphere and minimized some of the impacts of global warming. However, the ocean’s uptake of CO₂ is having a significant impact on the chemistry of seawater. The average pH of ocean surface waters has already fallen by about 0.1 units, from about 8.2 to 8.1 since the beginning of the Industrial Revolution. Estimates of projected future atmospheric and oceanic CO₂ concentrations indicate that, by the end of this century, the average surface ocean pH could be 0.2 to 0.4 lower than it is today. The pH scale is logarithmic, so a change of 1 unit corresponds to a 10-fold change in hydrogen ion concentration.
When atmospheric CO₂ exchanges across the air–sea interface it reacts with seawater through a series of four chemical reactions that increase the concentrations of the carbon species: dissolved carbon dioxide (CO₂(aq)), carbonic acid (H2CO3) and bicarbonate (HCO3 –):
CO₂(atmos) = CO₂(aq) (1) CO₂(aq) + H₂O = H₂CO3 (2) H₂CO3 = H+ + HCO3 – (3) HCO3 – = H+ + CO3 2– (4)
Hydrogen ions (H+) are produced by these reactions. This increase in the ocean’s hydrogen ion concentration corresponds to a reduction in pH, or an increase in acidity. Under normal seawater conditions, more than 99.99% of the hydrogen ions that are produced will combine with carbonate ion (CO3 2–) to produce additional HCO3 –. Thus, the addition of anthropogenic CO₂ into the oceans lowers the pH and consumes carbonate ion. These reactions are fully reversible and the basic thermodynamics of these reactions in seawater are well known, such that at a pH of approximately 8.1 approximately 90% the carbon is in the form of bicarbonate ion, 9% in the form of carbonate ion, and only about 1% of the carbon is in the form of dissolved CO₂. Results from laboratory, field, and modeling studies, as well as evidence from the geological record, clearly indicate that marine ecosystems are highly susceptible to the increases in oceanic CO₂ and the corresponding decreases in pH and carbonate ion.
Climate change and anthropogenic ocean acidification do not act independently. Although the CO₂ that is taken up by the ocean does not contribute to greenhouse warming, ocean warming reduces the solubility of carbon dioxide in seawater; and thus reduces the amount of CO₂ the oceans can absorb from the atmosphere. For example, under doubled preindustrial CO₂ concentrations and a 2 °C temperature increase, seawater absorbs about 10% less CO₂ (10% less total carbon, CT) than it would with no temperature increase (compare columns 4 and 6 in Table 1), but the pH remains almost unchanged. Thus, a warmer ocean has less capacity to remove CO₂ from the atmosphere, yet still experiences ocean acidification. The reason for this is that bicarbonate is converted to carbonate in a warmer ocean, releasing a hydrogen ion thus stabilizing the pH.