As noted earlier, in addition to warming, the ocean is absorbing a significant fraction of the CO₂ emitted by human activities. This is changing the chemistry of the ocean. When the ocean dissolves atmospheric CO₂, carbonic acid is formed. As a result, seawater becomes more acidic. This is called ocean acidification.
As the oceans absorb CO₂, the dissolved CO₂ reacts with water (H₂O) to form carbonic acid (H₂CO₃). Carbonic acid is relatively unstable and breaks down into a bicarbonate ion (HCO₃-) and a hydrogen ion (H+). The conversion of CO₂ to bicarbonate removes a CO₂ molecule from the seawater solution, making room for another atmospheric CO₂ molecule to dissolve; this property of seawater allows it to absorb more CO₂ from the atmosphere than an equivalent volume of freshwater in a lake or a river. Hydrogen ions, the other product of the conversion process, make seawater more acidic; as the concentration of hydrogen ions increases, the pH decreases. Some of the free hydrogen ions react with carbonate ions to form more bicarbonate ions, shifting the balance to favor bicarbonate over carbonate, and reducing the number of carbonate ions in the seawater.
It’s important to clarify that this process will not lead to the ocean changing completely to an acid. The term acidification refers to the relative change in the acidity levels of the ocean. Although the pH is decreasing through this process, the ocean will remain basic, or above 7.0 on the pH scale. However, organisms in the ocean are adapted to very specific conditions, and even small changes in pH can lead to major impacts. Ocean acidification is measurably changing the pH of seawater. The ocean before the start of the Industrial Revolution had a pH of about 8.1 or 8.2, depending on latitude (Caldeira and Wickett 2005). So far, the pH of the ocean has declined by about 0.1 unit (Bindoff et al. 2007). That may not sound like much, but it represents a 26 percent increase in acidity! With continued emissions of CO₂, this situation will continue to worsen. Mid-range projections for 21st century emissions suggest that ocean pH could decline by another 0.3 or 0.4 unit by 2100, and that figure could be as high as 0.7 unit by 2300 for higher emission trajectories (Bindoff et al. 2007; Caldeira and Wickett 2003).
L. Jeremy Richardson. Chapter 6: Changes in ocean temperature and chemistry, pp. 110-112. One Ocean Teacher Guide. National Geographic. Teacher guide.