AP® Environmental Science · Unit 9 · Topic 9.7
Ocean Acidification
Ocean acidification happens when seawater absorbs CO2, forms carbonic acid, and releases hydrogen ions that bond with carbonate, leaving less of it for shell-building organisms.
A shell does not need acidic water to dissolve. It needs a shortage of carbonate ions, and that shortage is exactly what rising CO2 causes. When CO2 gas dissolves into seawater, it reacts with water to form carbonic acid, H_2CO3. That weak acid splits apart into a hydrogen ion and a bicarbonate ion. The extra hydrogen ions do not just sit there lowering pH. They go hunting for carbonate ions (CO32-) already dissolved in the water and combine with them to make more bicarbonate. That reaction directly consumes the carbonate that clams, corals, and pteropods pull from the water to build calcium carbonate shells and skeletons. Less carbonate available means organisms spend more energy building shell, and in water undersaturated enough, existing shell starts to dissolve faster than it can be repaired.
Because pH is a negative log scale, small-looking drops are large changes in hydrogen ion concentration. A fall from pH 8.2 to 8.1 is only 0.1 units, but hydrogen ion concentration rises by a factor of 100.1, about 1.26 times higher. Average surface ocean pH has already fallen from about 8.2 to 8.1 since preindustrial times, meaning the ocean is already roughly 25 to 30 percent more acidic.
Acidification sits right next to warming in this unit because both trace back to the same rising atmospheric CO2, yet they are separate problems with separate mechanisms. Warming comes from CO2 trapping infrared radiation as heat; acidification comes from CO2 dissolving directly into water and reacting chemically. A pteropod in warmer, more acidic water at year 2100 projected chemistry can face both a metabolic stress from heat and a shell that is thinning or pitting from carbonate loss, two independent attacks on the same animal.
Key terms
- Carbonic acid
- The weak acid formed when CO2 dissolves in water and reacts with it, H_2CO3.
- Carbonate ion
- CO32-, the ion marine organisms combine with calcium to build shells and skeletons.
- Ocean acidification
- The ongoing drop in seawater pH caused by absorption of atmospheric CO2.
- pH scale
- A logarithmic scale measuring hydrogen ion concentration, where each one unit drop means ten times more hydrogen ions.
- Saturation state
- A measure of how much carbonate ion is available in water relative to what shell-building organisms need.
Watch it explained
- How it worksWalk me through CO2 dissolving, making carbonic acid, and stealing carbonate ions.
- Worked exampleFind how much the hydrogen ion concentration rises as ocean pH drops from 8.2 to 8.1.
- Going furtherWhat happens to a pteropod shell over 45 days in water at year 2100 chemistry?
- Common mistakeWhy isn't ocean acidification the same problem as ocean warming?
Check yourself
End of topic.