The carbon cycle describes the continuous movement of carbon atoms as they travel between the Earth’s atmosphere, oceans, land, and living organisms. This biogeochemical cycle is a fundamental planetary process, ensuring that carbon, the element upon which all known life is built, remains available in its various forms. The circulation of carbon sustains life and regulates the planet’s habitable environment.
The Foundation of Life
Carbon’s ability to form four stable bonds allows it to serve as the structural backbone for the complex molecules that comprise all living things. Every cell in every organism is constructed from carbon-based compounds like proteins, fats, carbohydrates, and nucleic acids. The flow of energy and matter through ecosystems depends entirely on the movement of carbon atoms.
The process of photosynthesis is the initial gateway for drawing atmospheric carbon into the biosphere. Plants absorb carbon dioxide from the air and water, using sunlight to convert it into glucose and other simple sugars. This conversion creates the organic matter that forms the base of nearly all terrestrial and aquatic food webs. When organisms consume these plants, the carbon is transferred up the food chain, sustaining life through every trophic level.
Global Climate Regulation
Beyond its biological role, the carbon cycle governs the Earth’s long-term climate by regulating the concentration of carbon dioxide ($\text{CO}_2$) in the atmosphere. $\text{CO}_2$ is a naturally occurring greenhouse gas that absorbs and re-emits infrared radiation, which is the heat radiating from the Earth’s surface. This natural trapping of heat is called the greenhouse effect, and it is necessary for maintaining a temperate planet.
Without greenhouse gases, Earth’s average surface temperature would be approximately $33^\circ \text{C}$ ($59^\circ \text{F}$) colder, rendering the planet frozen and uninhabitable. The natural carbon cycle works to keep the atmospheric $\text{CO}_2$ concentration within a range that ensures the planet remains warm enough for liquid water and life to flourish.
Mechanisms of Carbon Storage
The global carbon cycle operates on different timescales, moving carbon between four major reservoirs, often called “sinks” when they store carbon away from the atmosphere. The largest reservoir is the lithosphere, where carbon is stored in sedimentary rocks, such as limestone. This geological storage component operates over millions of years and holds the vast majority of the planet’s carbon.
The deep ocean represents the second largest sink, holding fifty times more carbon than the atmosphere through physical and biological processes. Cold, dense surface water absorbs $\text{CO}_2$ and sinks, transferring carbon to the deep sea where it can be sequestered for hundreds or thousands of years. Terrestrial ecosystems, including forests and soil, also serve as a significant short-term reservoir by storing carbon in biomass and organic soil matter.
Human Influence and Imbalance
The balance achieved by these natural storage mechanisms is being significantly altered by human activities, accelerating the movement of carbon from the land and geological reservoirs into the atmosphere. The primary source of this disruption is the burning of fossil fuels—coal, oil, and natural gas—which are the compressed remains of ancient organisms that stored carbon millions of years ago. Combusting these fuels rapidly releases this long-sequestered carbon back into the atmospheric pool as $\text{CO}_2$.
Land-use changes, such as large-scale deforestation, further contribute to this imbalance by removing plant life that would otherwise absorb $\text{CO}_2$ through photosynthesis. The increased atmospheric concentration of $\text{CO}_2$ amplifies the natural greenhouse effect, leading to a measurable rise in global temperatures.
The ocean attempts to absorb a portion of this excess atmospheric $\text{CO}_2$, but this absorption creates carbonic acid, which lowers the water’s pH in a process known as ocean acidification. This acidification poses a threat to marine organisms, especially those that rely on calcium carbonate to build their shells and skeletons, such as corals and mollusks.