Carbon dioxide (CO2) is a naturally occurring gas, composed of one carbon atom bonded to two oxygen atoms, present in Earth’s atmosphere. It is an integral part of the planet’s natural carbon cycle, moving between the atmosphere, oceans, land, and living organisms. While essential for processes like plant photosynthesis, CO2’s rising atmospheric concentrations can lead to increased acidity when it dissolves in water, impacting various natural systems.
How Carbon Dioxide Becomes Acidic
When carbon dioxide gas dissolves in water, a chemical reaction occurs, forming carbonic acid (H2CO3). This process is reversible, meaning carbonic acid can also break down into CO2 and water. Carbonic acid is a weak acid that partially breaks apart into hydrogen ions (H+) and bicarbonate ions (HCO3-). An increase in hydrogen ions directly lowers the pH of the water, making it more acidic. This chemical transformation is fundamental to understanding how elevated CO2 levels can influence environmental pH.
Ocean Acidity Explained
Oceans act as a significant natural reservoir for atmospheric carbon dioxide, absorbing a substantial portion of it. Since the Industrial Revolution, oceans have taken up approximately one-third of the CO2 released by human activities, a process that helps mitigate atmospheric warming. This absorption, however, comes at a cost to ocean chemistry.
As CO2 dissolves into seawater, it forms carbonic acid, which then releases hydrogen ions, increasing the water’s acidity. This ongoing process is known as ocean acidification. The average pH of surface ocean waters has decreased by about 0.1 units since around 1850, a change equivalent to a 26% increase in acidity. This rate of change is roughly ten times faster than any observed over the last 55 million years.
Acid Rain Formation
Acid rain is a broad term for precipitation that is more acidic than normal, primarily due to atmospheric pollutants. While carbon dioxide does contribute to the natural acidity of rain by forming carbonic acid, making unpolluted rain slightly acidic (around pH 5.6), it is not the main driver of severe acid rain events.
The primary culprits behind highly acidic rain are sulfur dioxide (SO2) and nitrogen oxides (NOx), largely from the burning of fossil fuels. These gases react with water, oxygen, and other chemicals in the atmosphere to form stronger acids like sulfuric acid and nitric acid. These stronger acids are responsible for the more damaging effects commonly associated with acid rain, often lowering pH levels below 4.5.
Consequences of Increased Acidity
Increased acidity in marine environments poses significant threats to various organisms, particularly those that build shells or skeletons from calcium carbonate. Corals, shellfish, and certain plankton find it more difficult to form and maintain their protective structures in more acidic waters. This can lead to weakened shells, reduced growth, and impaired development, impacting the survival and reproduction of these species. Disruptions at the base of the food web, such as the decline of calcifying plankton, can have cascading effects throughout marine ecosystems, affecting fish and other larger organisms.
Acid rain also has widespread consequences on terrestrial and aquatic ecosystems, as well as human infrastructure. In natural environments, it can acidify lakes and streams, making them inhospitable for fish and other aquatic life, and can leach essential nutrients from soils while mobilizing toxic metals. Forests can suffer from direct damage to foliage and reduced nutrient absorption, leading to slower growth and increased vulnerability to disease. Acid rain also accelerates the deterioration of buildings, statues, and other structures made of stone or metal, and the pollutants that cause it can contribute to respiratory problems in humans.