The carbon cycle is the Earth’s natural system that regulates the movement of carbon throughout the planet’s major spheres: the atmosphere, oceans, land, and rocks. Carbon is fundamental to life, forming the structural backbone of all organic molecules. This continuous recycling between its various reservoirs keeps the global system in a dynamic balance.
Carbon’s Starting Point The Atmosphere
The atmosphere serves as the most immediate reservoir for carbon, existing primarily as carbon dioxide ($\text{CO}_2$). This gaseous form is the starting point for the fastest component of the cycle, often referred to as the biological cycle. Photosynthesis is the first major step of carbon movement away from the atmosphere.
Green plants, algae, and some bacteria—collectively known as autotrophs—pull $\text{CO}_2$ directly from the air. Using sunlight, they combine carbon dioxide with water to create glucose and other organic compounds. This converts inorganic atmospheric carbon into organic matter. Once incorporated into plant biomass, the carbon is transferred into the terrestrial biosphere, stored for years to decades.
Movement Through Living Systems
Once carbon is fixed into organic compounds by plants, it begins its journey through the food web. When herbivores consume plants, the carbon stored in the plant’s tissues transfers to the animal’s body for energy and growth. This carbon moves further up the chain when carnivores consume herbivores, distributing the element throughout the living system.
Carbon returns to the atmosphere through respiration, performed by all living organisms. Respiration breaks down organic molecules like glucose to release energy, releasing carbon dioxide as a byproduct. Carbon is also rapidly returned to the atmosphere and soil when organisms die.
Microbes, such as bacteria and fungi, act as decomposers, breaking down dead organic matter and waste products. This decomposition releases stored carbon back into the atmosphere as $\text{CO}_2$ through their respiration. This biological loop is a relatively fast exchange, operating on timescales of hours to years.
Exchange with the Oceans
The ocean represents the largest active reservoir of carbon near the Earth’s surface, holding approximately 50 times more carbon than the atmosphere. Carbon is exchanged between the atmosphere and the ocean surface through two distinct mechanisms: the physical and biological pumps. The physical solubility pump involves $\text{CO}_2$ dissolving directly into seawater.
The solubility of $\text{CO}_2$ is greater in colder water. As surface water cools at high latitudes, it absorbs more atmospheric $\text{CO}_2$ and becomes denser. This denser water sinks into the deep ocean, carrying dissolved inorganic carbon away from the surface and isolating it from the atmosphere for centuries. The biological pump is driven by marine life, primarily phytoplankton.
Phytoplankton use dissolved $\text{CO}_2$ for photosynthesis, converting it into organic carbon. When these organisms die, their remains sink to the deep ocean floor, known as marine snow. This downward transport of carbon, both as organic matter and as calcium carbonate shells, sequesters carbon in deep ocean sediments, removing it from the atmosphere-ocean system for long periods.
Carbon’s Deep Journey
The slow, or geological, cycle involves carbon movement into the lithosphere, the largest carbon reservoir overall, storing carbon for millions of years. Burial and compaction of marine organisms, which form shells of calcium carbonate ($\text{CaCO}_3$), eventually create massive deposits of sedimentary rock, most notably limestone. This process locks carbon away in the Earth’s crust for immense periods.
Ancient organic matter from plants and plankton is buried under layers of sediment. Subjected to intense heat and pressure over geological timescales, this organic carbon transforms into concentrated stores of coal, oil, and natural gas (fossil fuels). Natural processes slowly release this stored carbon, such as through the chemical weathering of rocks or volcanic activity.
Volcanic eruptions are a natural mechanism for returning deep-earth carbon back to the atmosphere as $\text{CO}_2$. This process helps maintain the long-term balance of the cycle. This geological cycling through rock formation and subsequent release operates on a timescale of hundreds of millions of years, sharply contrasting with the rapid exchanges in the atmosphere and biosphere.
The Imbalance Human Influence
Human activities have accelerated specific fluxes within the carbon cycle, overriding the natural speed of the system. The primary source is the combustion of fossil fuels, which releases carbon stored over millions of years almost instantaneously. Burning coal, oil, and natural gas adds significant $\text{CO}_2$ to the atmosphere at a rate that far exceeds the cycle’s ability to reabsorb it.
Land-use changes, particularly deforestation, also contribute to the imbalance. When forests are cleared, the carbon stored in the trees’ biomass is released into the atmosphere through burning or decomposition. Removing large areas of forest eliminates a natural carbon sink, reducing the amount of $\text{CO}_2$ captured through photosynthesis. This rapid injection of stored carbon explains why the concentration of $\text{CO}_2$ has been rising rapidly since the industrial era.