Ocean acidification (OA) is the long-term decrease in the pH of the Earth’s oceans. This process is fundamentally driven by the absorption of carbon dioxide (\(\text{CO}_2\)) from the atmosphere into seawater. The change in ocean chemistry results from a chain of reactions that increase the concentration of hydrogen ions (\(\text{H}^+\)), which defines increasing acidity. Understanding this process is necessary to grasp the consequences for marine life and the overall health of the planet’s largest ecosystem.
The Role of Atmospheric Carbon Dioxide
The primary engine of ocean acidification is the carbon dioxide released into the atmosphere by human activities, mainly through the burning of fossil fuels. Since the Industrial Revolution, the concentration of atmospheric \(\text{CO}_2\) has risen dramatically, creating an imbalance between the atmosphere and the ocean surface. The ocean naturally acts as a carbon sink, absorbing gases directly from the air through gas exchange across the water surface.
This absorption process is a planetary buffer, mitigating the greenhouse effect by removing a significant portion of human-generated emissions. Scientists estimate that the ocean has absorbed approximately 25 to 30 percent of the total anthropogenic carbon dioxide released over the last few centuries. While this absorption slows the rate of climate warming, it simultaneously changes the fundamental chemistry of the seawater.
The Chemical Reactions That Define Acidification
The process of ocean acidification begins when atmospheric \(\text{CO}_2\) dissolves into the surface waters. The \(\text{CO}_2\) molecule reacts with a water molecule (\(\text{H}_2\text{O}\)) to form carbonic acid (\(\text{H}_2\text{CO}_3\)). This reaction is represented as \(\text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3\), which introduces a weakly acidic compound into the marine environment.
Carbonic acid is unstable in seawater and quickly dissociates. This process releases a hydrogen ion (\(\text{H}^+\)) and forms a bicarbonate ion (\(\text{HCO}_3^-\)). This is the step that defines acidification, as the increase in free hydrogen ions is what lowers the ocean’s pH. The reaction is shown as \(\text{H}_2\text{CO}_3 \rightarrow \text{H}^+ + \text{HCO}_3^-\), directly linking atmospheric carbon to the measurable increase in ocean acidity. Since the pH scale is logarithmic, a small numerical decrease in pH represents a substantial percentage increase in the concentration of these hydrogen ions.
The presence of excess hydrogen ions triggers a secondary chemical reaction within the marine carbon system. These newly available \(\text{H}^+\) ions readily react with carbonate ions (\(\text{CO}_3^{2-}\)), which are naturally present in seawater. This reaction forms an additional bicarbonate ion: \(\text{H}^+ + \text{CO}_3^{2-} \rightarrow \text{HCO}_3^-\).
The net result is a significant decrease in the overall concentration of free carbonate ions in the ocean. This reduction in carbonate ion availability is a critical aspect of ocean acidification, as carbonate ions are a building block for many marine organisms. The chemical balance shifts toward bicarbonate, which does not serve the same biological function as carbonate.
Consequences for Marine Ecosystems
The chemical changes from acidification have biological consequences, particularly for organisms that rely on calcium carbonate (\(\text{CaCO}_3\)) to build their shells and skeletons. These organisms, known as calcifiers, include corals, oysters, clams, mussels, and tiny planktonic snails called pteropods. The reduced concentration of carbonate ions makes it significantly more difficult for them to extract the necessary materials from the water for calcification.
Calcifying organisms must expend more energy to build and maintain their structures in waters with lower carbonate saturation, diverting resources away from growth or reproduction. In some cases, the water chemistry can become corrosive enough to cause the existing calcium carbonate shells and skeletons to dissolve. This dissolution is problematic for organisms like pteropods, which are a foundational component of many marine food webs.
A decline in pteropod populations could have cascading effects throughout the entire marine food chain, impacting fish and whale species that feed on them. Coral reefs are also vulnerable, as their structures are built from calcium carbonate. The reduced calcification rates threaten the integrity of reefs, which provide habitat for a quarter of all marine life. Changes in ocean chemistry can also affect the behavior of non-calcifying species, such as reducing the ability of some fish to detect predators or locate suitable habitats.