The ocean operates on a complex chemical balance fundamental to its health. This balance is measured using the pH scale, which indicates the concentration of hydrogen ions in a solution. A lower pH signifies greater acidity (higher hydrogen ion concentration), while a higher pH indicates a more alkaline state (lower acidity). Monitoring the pH value provides a direct measure of the ocean’s stability, as marine life has evolved to thrive within a specific chemical environment.
The Baseline pH
The pH scale ranges from 0 to 14, with 7 being neutral. Substances above 7 are alkaline, and those below 7 are acidic. Historically, the open ocean has maintained a slightly alkaline pH of approximately 8.2.
The ocean’s natural alkalinity results from dissolved minerals carried into the sea, primarily containing buffering agents such as carbonate and bicarbonate ions. This natural buffering system allows seawater to resist drastic changes in hydrogen ion concentration, keeping the pH relatively stable. Since the industrial era began, the average surface pH has dropped by about 0.1 units to a current value near 8.1.
The Driving Force of Change
The primary reason for the shift in seawater pH is the ocean’s absorption of increasing amounts of atmospheric carbon dioxide ($\text{CO}_2$), a process known as ocean acidification. Since the Industrial Revolution, the oceans have absorbed roughly one-third of the $\text{CO}_2$ released from human activities. While this absorption temporarily regulates climate, it fundamentally alters the ocean’s chemistry.
When $\text{CO}_2$ dissolves into seawater, it reacts with water to form carbonic acid. Carbonic acid is a weak acid that quickly dissociates, releasing a hydrogen ion ($\text{H}^+$) and a bicarbonate ion. The release of this excess $\text{H}^+$ directly causes the pH decline. For example, a drop of 0.1 pH units represents a 26% increase in hydrogen ion concentration.
The newly introduced hydrogen ions also react with naturally occurring carbonate ions ($\text{CO}_3^{2-}$), a component of the ocean’s natural buffer. This reaction converts carbonate ions into more bicarbonate ions, consuming the available carbonate in the water. Consequently, the concentration of free carbonate ions, a necessary building block for many marine organisms, is significantly reduced.
Consequences for Marine Life
The reduction in both pH and carbonate ion availability creates challenges for a wide range of marine organisms. The most direct impact is on calcifying organisms, which rely on calcium carbonate to construct their shells and skeletons. Corals, clams, oysters, and sea snails are vulnerable to this chemical change.
For organisms like pteropods, or “sea butterflies,” which form a base of the marine food web, lower carbonate ion availability can lead to shell dissolution and difficulties in forming protective structures. Scientists measure aragonite saturation state, which indicates how easily organisms can build these structures. A value below one means the water is corrosive, causing shells to dissolve. Larval stages of species such as commercial oysters are sensitive, with low pH resulting in smaller size, thinner shells, and higher mortality rates.
Impacts are not limited to shell-builders; lower pH also affects the physiology and behavior of non-calcifying species like fish. Elevated $\text{CO}_2$ levels can impair the sensory functions of some fish, including their ability to detect predators through smell, as observed in species such as clownfish. The overall disruption of predator-prey dynamics and the food web structure poses a risk to entire ecosystems.
Monitoring and Future Outlook
Tracking the changing ocean chemistry requires sophisticated, long-term monitoring efforts across the globe. Researchers use autonomous sensor platforms, such as moored buoys and wave gliders, to continuously measure $\text{CO}_2$ and $\text{pH}$ levels in real-time. These advanced instruments provide stable, high-resolution measurements in challenging ocean conditions.
These long-term observation programs confirm the ongoing decline in ocean $\text{pH}$ and provide data for future projections. Under current high-emission scenarios, models from the Intergovernmental Panel on Climate Change (IPCC) project that the average surface ocean $\text{pH}$ could drop by an additional 0.3 to 0.4 units by the year 2100. This change represents a significant increase in the ocean’s hydrogen ion concentration, highlighting the severity of the global chemical challenge.