The carbon cycle describes the continuous movement of carbon atoms from the atmosphere into the Earth and then back into the atmosphere. This natural process is fundamental for all life, as carbon is a building block for organic matter. It also plays a significant role in regulating Earth’s climate by influencing the concentration of carbon-containing gases. Carbon is exchanged between different reservoirs on Earth, including the atmosphere, oceans, land, and living organisms.
Carbon Uptake by Plants
Plants, algae, and certain bacteria absorb carbon dioxide from the atmosphere or water. This process, known as photosynthesis, uses sunlight as energy to convert inorganic carbon dioxide into organic compounds, such as sugars and starches. These organic compounds store carbon within plant tissues, forming the base of most food webs. Through tiny pores on their leaves called stomata, plants draw in atmospheric carbon dioxide, acting as carbon sinks.
Carbon Release by Living Organisms
Living organisms, including plants, animals, and microbes, release carbon dioxide back into the atmosphere or water through cellular respiration. This process involves breaking down organic compounds to release energy. Carbon dioxide is produced as a byproduct of this energy conversion. Respiration serves as a counter-balance to photosynthesis.
Carbon Release from Decomposition
Decomposition is another process that releases carbon back into the atmosphere and soil. When plants and animals die, or organisms excrete waste, bacteria, fungi, and other microorganisms break down this dead organic matter. As decomposers consume the material, they release carbon dioxide through their respiration. In environments with limited oxygen, decomposers may also produce methane, another carbon-containing gas. This breakdown of organic material aids nutrient recycling.
Carbon Release from Burning
Burning, also known as combustion, releases stored carbon into the atmosphere. This occurs through natural phenomena like wildfires and human activities. Wildfires convert carbon stored in plant biomass into carbon dioxide. Combustion of fossil fuels—coal, oil, and natural gas—formed over millions of years from ancient organic matter—releases large amounts of carbon dioxide. Burning these fuels for energy increases atmospheric carbon dioxide levels.
Carbon Exchange with Oceans
Oceans represent a reservoir of carbon and exchange carbon dioxide with the atmosphere. Carbon dioxide from the atmosphere dissolves into ocean waters. This dissolution is a two-way process, with carbon dioxide also being released from the oceans back into the atmosphere depending on factors like temperature and atmospheric concentration. Marine organisms, such as phytoplankton, absorb dissolved carbon dioxide for photosynthesis, similar to land plants. Marine life also contributes to carbon storage in sediments over long geological timescales.