The question of whether ocean water is “carbonated” often brings to mind the visible fizz of soda, which is carbon dioxide (CO2) gas suspended under high pressure. In a strict sense, the ocean is not carbonated like a beverage because it lacks the intense saturation and pressure required for visible bubbles to form readily. However, the world’s oceans hold a massive amount of dissolved CO2, making them the largest active reservoir of carbon on Earth. The ocean contains about 50 times more carbon than the atmosphere, and this dissolved gas chemically interacts with the seawater, leading to profound consequences for the marine environment.
The Physical Reality of Dissolved Gas
The visible bubbles in soda are CO2 gas molecules forced into the liquid under high pressure and low temperature. When the pressure drops, the gas rapidly escapes, creating the characteristic fizz. Ocean water, in contrast, contains carbon dioxide that is fully dissolved, similar to how salt dissolves, meaning the molecules are individually dispersed within the water.
The deep ocean has high pressure and low temperature, conditions that favor gas solubility. Although CO2 is more soluble in cold water, the immense hydrostatic pressure keeps it in a dissolved, non-gaseous state. The concentration of dissolved inorganic carbon is substantial, but most of it exists as chemically transformed ions rather than simple CO2 gas.
How the Ocean Absorbs Atmospheric Carbon Dioxide
The ocean acts as a “carbon sink,” absorbing approximately 30% of the carbon dioxide released into the atmosphere from human activities. This absorption is governed by the difference in the partial pressure of CO2 between the air and the surface water. When atmospheric CO2 concentration increases, the gas naturally diffuses across the air-sea interface into the seawater until a balance is reached.
The solubility of CO2 is higher in colder water, making the gas exchange process efficient in high-latitude, polar regions. As this cold, CO2-rich water becomes denser, it sinks and transports the dissolved carbon to the deep ocean, a process known as the solubility pump. Separately, the biological pump involves marine organisms like phytoplankton absorbing CO2 for photosynthesis; when they die, the carbon sinks as organic matter, moving it away from the surface.
The Chemical Transformation: From CO2 to Carbonic Acid
Once atmospheric carbon dioxide dissolves into seawater, it undergoes a series of rapid chemical reactions instead of remaining as CO2 gas. The dissolved CO2 first reacts with water to form a weak acid called carbonic acid. This is the same chemical compound that gives soda water its slight acidic tang.
Carbonic acid quickly dissociates, or breaks apart, in the water. The first step releases a hydrogen ion and forms a bicarbonate ion. The bicarbonate ion can then dissociate further to release another hydrogen ion and a carbonate ion.
These three forms—dissolved CO2, bicarbonate, and carbonate—are collectively known as dissolved inorganic carbon. They establish a chemical equilibrium that regulates the ocean’s pH. The vast majority of carbon in seawater exists as bicarbonate ions, followed by carbonate ions, with dissolved CO2 being the least abundant form.
This buffering system allows the ocean to absorb large quantities of atmospheric CO2 without an instantaneous shift in its overall pH. However, the absorption of human-generated CO2 pushes this equilibrium, increasing the concentration of hydrogen ions and slightly lowering the water’s overall pH.
Consequences of Increased Carbonic Acid
The influx of carbon dioxide from the atmosphere has caused the ocean’s average surface pH to drop by about 0.1 units since the Industrial Revolution. This change represents roughly a 30% increase in the concentration of hydrogen ions, a process defined as ocean acidification. While the ocean remains alkaline, the shift toward a lower pH has significant environmental repercussions.
The primary impact stems from the increased hydrogen ions, which chemically bond with the available carbonate ions, effectively “stealing” them from marine life. Carbonate ions are a fundamental building block that many organisms, known as calcifiers, require to construct their shells and skeletons.
Calcifiers use carbonate and calcium ions to form calcium carbonate structures. These organisms include:
- Corals
- Clams
- Oysters
- Pteropods
- Calcareous plankton
The reduction in available carbonate ions forces these calcifying organisms to expend more energy to build and maintain their shells. In more acidic waters, the shells of vulnerable organisms like pteropods, or “sea butterflies,” can even begin to dissolve. Since many of these shelled organisms are foundational to the marine food web, their reduced ability to calcify places the ecosystem at risk.