Ocean Acidification

ocean acidification, the worldwide reduction in the pH of seawater as a consequence of the absorption of large amounts of carbon dioxide (CO₂) by the oceans. Ocean acidification is largely the result of loading Earth’s atmosphere with large quantities of CO₂, produced by vehicles and industrial and agricultural processes. Since the beginning of the Industrial Revolution about 1750, roughly one-third to one-half of the CO₂ released into Earth’s atmosphere by human activities has been absorbed by the oceans. During that time period, scientists have estimated, the average pH of seawater declined from 8.19 to 8.05, which corresponds to a 30 percent increase in acidity.

Some scientists estimate that the pace of ocean acidification since the beginning of the Industrial Revolution has been approximately 100 times more rapid than at any other time during the most recent 650,000 years. They note that concentrations of atmospheric CO₂ between 1000 and 1900 ce ranged between 275 and 290 parts per million by volume (ppmv). In 2010 the average concentration was 390 ppmv, and climatologists expect the concentration to rise to between 413 and 750 ppmv by 2100, depending on the level of greenhouse gas emissions. With additional CO₂ transferred to the oceans, pH would decline further; under worst-case scenarios, seawater pH would drop to between 7.8 and 7.9 by 2100.

Marine scientists are concerned that the process of ocean acidification constitutes a threat to sea life and to the cultures that depend on the ocean for their food and livelihood. Increases in ocean acidity reduce the concentration of carbonate ions and the availability of aragonite (a significant source of calcium carbonate) in seawater. Marine scientists expect that coral, shellfish, and other marine calcifiers (that is, organisms that use carbonates) will be less able to obtain the raw materials that they use to build and maintain their skeletons and shells. These scientists also note that rising ocean acidity presents a number of other physiological problems to different groups of marine organisms and that such problems could threaten the stability of marine food chains.