Ocean acidification refers to an increase in the level of acidity in the world’s ocean water. Acidity is measured on the 14-point pH scale, with lower numbers indicating a greater level of acidity. As of mid-2020, the level of global ocean acidity was approximately 8.1.
While this may not seem alarming on the surface, it represents an increase of about 30% in acidity over the last two centuries. Based on the current path of future emissions, the pH level by the end of the 21st century may be as low as 7.8. This is a level not seen in approximately 15 million years.
About 30% of the CO2 released into the atmosphere is absorbed by oceans. When combined with sea water, CO2 forms carbonic acid. With the amount of CO2 in the atmosphere increasing due to human activity, the amount being absorbed into the world’s oceans has increased commensurately. This, in turn, has caused the level of acidity to rise as more carbonic acid is created.
The most visible and well-known impact of ocean acidification is its effect on coral reefs. The chemical processes associated with acidification impair the ability of coral to form protective skeletons, leading to the gradual erosion of reefs. Because coral reefs are home to thousands of species of fish and other sea creatures like shellfish, sea urchins and sponges, the destruction of the reefs has detrimental impacts on the habitats and food sources for about 25% of the world’s marine life.
Like coral, shellfish such as oysters, clams and scallops also find it more difficult to build their protective shells in more acidic conditions. The same is true of sea snails. Should acidity continue to increase, the effect will become worse and shells will actually begin to dissolve. In fact, at pH levels associated with the current path of greenhouse gas emissions projected for the year 2100, a shell may completely dissolve in as little as 45 days. Given the number of sea species that include shellfish and snails in their diets, the future effect on ocean food chains could be substantial.
Another way in which acidification may negatively impact ocean food chains is by impairing the ability of some species to detect predators. There is evidence that certain types of fish experience sensory degradation in more acidic water, causing them to stray farther from their normal habitat and leaving them susceptible to predators that they would otherwise likely be able to avoid.
Downstream impacts of ocean acidification extend to human populations who count on the ocean for food and for their economic wellbeing. A significant portion of the world’s population utilizes the ocean as a primary source of protein, and over half a billion people globally rely in some form on coral reef ecosystems.
In the United States, there is significant exposure in areas that leverage the ocean for sustaining employment and tourism, including Alaska, New England, Florida and Puerto Rico. Among the most affected populations globally are those who depend on the ocean for basic subsistence, which are often also the most economically vulnerable populations overall.