While ocean water is not carbonated in the same way a fizzy drink is, it naturally contains dissolved carbon dioxide (CO2). The ocean’s capacity to hold carbon dioxide allows for a constant exchange with the atmosphere. This natural process helps regulate atmospheric CO2 levels, influencing global temperatures.
The Science of Dissolved Carbon Dioxide in Seawater
Carbon dioxide from the atmosphere dissolves into seawater, initiating a series of chemical reactions. When CO2 encounters water (H2O), it forms carbonic acid (H2CO3). This weak acid then dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-). Bicarbonate can further dissociate into more hydrogen ions and carbonate ions (CO3^2-).
The acidity or alkalinity of a solution, including seawater, is measured using the pH scale. This scale ranges from 0 to 14, where values below 7 indicate acidity, 7 is neutral, and values above 7 indicate alkalinity. The concentration of hydrogen ions primarily determines a solution’s pH. Ocean water is naturally slightly alkaline, with a pre-industrial average pH of about 8.2. The ocean’s natural buffer system, involving these carbon compounds, helps resist large pH changes.
How Carbon Dioxide Enters and Moves Through the Ocean
Carbon dioxide primarily enters the ocean through atmospheric exchange at the sea surface. This process is driven by the difference in CO2 partial pressure between the atmosphere and the seawater. Colder waters can absorb and hold more CO2 than warmer waters, influencing where significant absorption occurs.
Biological processes also play a role in carbon movement within the ocean. Marine phytoplankton absorb CO2 during photosynthesis, converting it into organic carbon and forming the base of many food webs. Conversely, marine organisms release CO2 through respiration, and decomposition of organic matter also returns CO2 to the water. The ocean acts as a significant natural carbon sink, absorbing approximately 30% of the carbon dioxide released into the atmosphere from human activities. This absorption amounts to around 2.9 billion metric tons of carbon from atmospheric CO2 each year, helping to slow the rise of atmospheric CO2 levels.
Understanding Ocean Acidification
Ocean acidification refers to the ongoing decrease in the pH of the Earth’s oceans, primarily due to the excess absorption of human-caused carbon dioxide from the atmosphere. While the ocean naturally absorbs CO2, the increased levels released by activities such as burning fossil fuels and deforestation overwhelm this natural capacity. This increased absorption leads to a higher concentration of dissolved CO2 in seawater.
When this excess CO2 dissolves, it forms more carbonic acid, which then releases additional hydrogen ions into the water. This increase in hydrogen ions causes the ocean’s pH to drop. Since the Industrial Revolution, the average pH of surface ocean waters has fallen by approximately 0.1 pH units, from about 8.15 to 8.05. This seemingly small change is significant because the pH scale is logarithmic, meaning a 0.1 unit decrease represents about a 26-30% increase in acidity. Forecasts suggest that if current emission trends continue, surface ocean pH could drop to around 7.8 by the end of this century.
This process also reduces the availability of carbonate ions (CO3^2-), as the excess hydrogen ions bond with them to form bicarbonate. Carbonate ions are an important building block for many marine organisms. The reduction in these ions is a defining characteristic of ocean acidification, impacting marine life even though seawater remains alkaline with a pH higher than 7.
Impacts on Marine Ecosystems
The changes in ocean chemistry due to acidification pose major challenges for marine life, particularly organisms that build shells and skeletons. Calcifying organisms, such as corals, shellfish like oysters, clams, and mussels, and various forms of plankton (e.g., pteropods and coccolithophores), rely on carbonate ions and calcium to create their calcium carbonate structures. Reduced availability of carbonate ions makes it harder for these organisms to build and maintain their shells and skeletons, requiring them to expend more energy to do so. In severe cases, existing shells and skeletons can even begin to dissolve.
For corals, ocean acidification can impede the thickening process of their skeletons, making them less dense and more vulnerable to damage. Studies have shown a decrease in calcification, growth, and development across various calcified marine organisms. For example, oyster larvae have shown significant declines in survival and development under increased CO2 conditions.
These direct impacts can ripple through the marine food web, affecting the broader ecosystem. Plankton, including calcifying species, form the base of many marine food chains; their decline or altered nutritional quality can disrupt the food supply for other organisms. This can lead to population declines in fish species and, subsequently, impact larger predators that rely on them for food. While some organisms, like certain crustaceans, may show resilience or even benefit from increased acidity.