The carbon cycle is a fundamental natural process involving the continuous movement of carbon atoms between the atmosphere, living organisms, oceans, and land. This ongoing exchange ensures carbon is recycled and reused across Earth’s biosphere, geosphere, hydrosphere, and atmosphere.
Carbon as Life’s Building Block
Carbon forms the foundation of all known life on Earth. It is the primary element in organic molecules like proteins, carbohydrates, fats, and DNA, which are the building blocks for living cells. Carbon’s ability to form stable bonds with up to four other atoms allows for the creation of diverse, complex molecules needed for biological functions. Approximately 18.5% of the human body, by mass, consists of carbon.
Plants, algae, and certain bacteria initiate the biological carbon cycle by absorbing carbon dioxide from the atmosphere or dissolved in water through photosynthesis. They convert carbon dioxide and sunlight into sugars and other organic compounds, forming the base of nearly all food chains. Carbon then moves through food webs as organisms consume these molecules. Cellular respiration, performed by plants, animals, and microbes, releases carbon back into the atmosphere as carbon dioxide when organic molecules are broken down for energy. This exchange between living organisms and the atmosphere is a rapid component of the carbon cycle, operating over timescales from hours to years.
Regulating Earth’s Climate
The carbon cycle moderates Earth’s temperature and climate. Carbon dioxide (CO2) acts as a greenhouse gas, absorbing and re-emitting heat radiated from the Earth’s surface. This natural greenhouse effect keeps the planet warm enough to support life; without it, Earth’s average temperature would be significantly colder, around -18 degrees Celsius (0 degrees Fahrenheit). The natural carbon cycle maintains a balance of atmospheric CO2, ensuring a stable global temperature over long geological periods.
Human activities have altered this natural balance since the Industrial Revolution. Burning fossil fuels like coal, oil, and natural gas releases stored carbon into the atmosphere as CO2, carbon that took millions of years to accumulate underground. This rapid release has increased atmospheric CO2 concentrations by approximately 50% compared to pre-industrial levels, reaching a record high of 422.7 parts per million in 2024. Deforestation also contributes to this imbalance by reducing trees that absorb CO2 through photosynthesis. The amplified greenhouse effect from elevated CO2 levels leads to a rise in global temperatures, impacting Earth’s climate patterns.
Impact on Ocean Chemistry
The ocean serves as a carbon sink, absorbing a significant portion of atmospheric carbon dioxide. CO2 dissolves into surface waters and exchanges with the atmosphere. The ocean currently absorbs about 31% of human-caused CO2 emissions. Cold ocean waters, particularly near the poles, absorb more CO2, which then sinks to deeper layers, storing carbon for centuries.
Increased CO2 absorption due to human emissions is altering ocean chemistry. When CO2 dissolves in seawater, it forms carbonic acid, decreasing the ocean’s pH, a process known as ocean acidification. The ocean’s average pH has decreased by about 0.1 units since the industrial era, making it approximately 30% more acidic. This change reduces carbonate ion concentration, which marine organisms like corals, oysters, and clams need to build their calcium carbonate shells and skeletons. Ocean acidification thus threatens marine ecosystems and the diverse life they support.
Formation of Essential Resources
The carbon cycle’s long-term geological processes form natural resources over millions of years. When ancient organic matter from plants and marine organisms is buried under sediment, it is subjected to immense pressure and heat. Over geological timescales, this transforms the buried carbon into fossil fuels, including coal, oil, and natural gas. These resources represent carbon stored away from the active carbon cycle for millions to hundreds of millions of years.
Beyond fossil fuels, the carbon cycle also contributes to the formation of carbonate rocks like limestone. Marine organisms, such as corals and shell-building plankton, extract dissolved calcium and bicarbonate ions from seawater to form calcium carbonate for their shells and skeletons. Upon death, these remains accumulate on the seafloor. Over vast periods, these deposits are compressed and cemented, forming sedimentary rocks like limestone, which comprise the largest carbon reservoir on Earth. This geological storage highlights the cycle’s role in shaping Earth’s crust.