Dissolved carbon dioxide refers to carbon dioxide gas that has been absorbed into a liquid. This phenomenon is commonly observed in everyday life, such as the familiar fizz in carbonated beverages like soda, where CO2 is intentionally dissolved under pressure. Beyond manufactured drinks, dissolved carbon dioxide naturally occurs in large bodies of water, including oceans, lakes, and rivers. Its presence in these natural environments plays a significant role in various biological and chemical processes.
The Process of CO2 Dissolving in Liquids
The dissolution of carbon dioxide in a liquid, most commonly water, involves a dynamic exchange between the gas in the atmosphere and the liquid’s surface. Carbon dioxide molecules move across this interface until a state of equilibrium is reached, where the rate of CO2 entering the liquid equals the rate of CO2 escaping back into the air. This physical process is influenced by several external factors.
Pressure is a significant determinant of how much CO2 can dissolve; higher pressure forces more gas into the solution, as exemplified by a sealed soda can where CO2 remains dissolved until the pressure is released. Temperature also plays a role, with colder liquids capable of holding more dissolved gas compared to warmer ones, which is why sodas go flat faster at room temperature. The salinity of the water impacts solubility as well, with saltier water generally dissolving less carbon dioxide. Once dissolved, a small portion of the CO2 molecules then react with water to begin forming carbonic acid.
Chemical Changes Caused by Dissolved CO2
Once carbon dioxide dissolves in water, it undergoes chemical transformations. A fraction of dissolved CO2 reacts with water (H2O) to form carbonic acid (H2CO3). This reaction is reversible, meaning carbonic acid can also convert back into dissolved CO2 and water.
Carbonic acid is a weak acid, meaning it does not fully break apart in water. It dissociates, releasing hydrogen ions (H+) into the solution. This increases hydrogen ions, lowering the water’s pH and making it more acidic. Carbonic acid can further dissociate into bicarbonate ions (HCO3-) and carbonate ions (CO3^2-), releasing additional hydrogen ions.
Ocean Acidification and the Global Carbon Cycle
The increasing amount of carbon dioxide in the atmosphere, primarily from human activities such as burning fossil fuels and deforestation, has a significant impact on the world’s oceans. The ocean acts as a vast carbon sink, absorbing approximately one-third of the excess atmospheric CO2. This uptake helps regulate atmospheric CO2 levels, but it comes at a cost to ocean chemistry.
Ocean acidification refers to the ongoing decrease in the pH of the Earth’s oceans caused by this absorption of anthropogenic CO2. While the ocean remains alkaline, with a pH generally above 8, the average ocean surface pH has already dropped from about 8.15 to 8.05 between 1950 and 2020, representing a 26% increase in acidity. This shift is significant because the pH scale is logarithmic, meaning even small changes represent substantial increases in hydrogen ion concentration.
This process is a component of the global carbon cycle, which describes the movement of carbon between the atmosphere, oceans, land, and living organisms. Historically, the ocean’s carbonate system has maintained a relatively stable pH, but the rapid rate of CO2 absorption from human emissions is overwhelming this natural buffering capacity.
Consequences for Aquatic Life
The chemical changes associated with ocean acidification impact marine ecosystems. A primary concern is the impact on calcifying organisms, which rely on calcium carbonate to build and maintain their shells and skeletons. These include creatures such as corals, shellfish (like oysters, clams, mussels), and pteropods (tiny sea snails).
Lower pH levels and reduced availability of carbonate ions make it increasingly difficult and energetically costly for these organisms to form their calcium carbonate structures. In severe conditions, existing shells and skeletons can even begin to dissolve. For example, studies have shown that mussels, sea urchins, and crabs may dissolve their protective shells to counteract elevated acidity in their body fluids, potentially impairing their overall health.
Beyond calcifiers, other marine life also faces challenges. Fish larvae may experience impaired development, and some species like clownfish and damselfish larvae have shown a reduced sense of smell, leading to riskier swimming behaviors in acidified conditions. The disruption of calcifying organisms, which often form the base of marine food webs, can lead to cascading effects throughout the ecosystem, impacting larger fish, marine mammals, and ultimately human food supplies.