Carbon cycle: movement of carbon through Earth's systems

Key reservoirs and flows

• Atmosphere: carbon exists mainly as CO2 and methane and is the fastest-changing reservoir.
• Terrestrial biosphere: forests, grasslands and other vegetation store carbon in living biomass through photosynthesis and release it by respiration.
• Soils and sediments: organic matter and partially decomposed plant material lock carbon in soils or accumulate as sediment.
• Oceans: surface waters absorb CO2 and transport carbon to the deep ocean by biological and physical processes.
• Fossil fuels: deposits such as coal, oil and natural gas formed from ancient organic matter can store carbon for millions of years and release it when burned (fossil fuel combustion).
• Earth’s crust: vast amounts of carbon are bound in sedimentary rock and carbonate minerals like limestone.

Several key processes move carbon between these reservoirs. Plants and some microbes convert CO2 into organic molecules during photosynthesis, while animals and microbes return carbon to the atmosphere through respiration and decomposition. Combustion of biomass and fossil fuels transfers stored carbon rapidly into the air. In the ocean, CO2 dissolves at the surface and participates in chemical equilibria that can form carbonate ions and affect acidity.

Over geological timescales, weathering of rocks plays an important role: rainwater and dissolved CO2 form weak carbonic acid, which reacts with minerals during weathering and carries dissolved carbon to the sea. There it may precipitate as carbonate sediment, eventually becoming part of rock formations. Plate movements and subduction return some of this carbon to the surface; melting and volcanism release CO2 back to the atmosphere along active margins associated with plate tectonics.

Human activity has altered the natural balance of the carbon cycle. During most of Earth’s history volcanic emissions were the dominant long-term source of atmospheric CO2, but the industrial-era combustion of coal (coal), oil and gas has released large quantities of carbon dioxide in a short time. This enhancement of atmospheric CO2 is a principal driver of recent climate change and also increases the amount of dissolved CO2 in the oceans, a process that leads to ocean acidification and affects marine life.

The carbon cycle has practical and scientific importance. It underpins terrestrial productivity, determines soil fertility and influences the global climate system. Understanding which reservoirs absorb or emit carbon helps guide strategies for mitigation—such as protecting forests, improving soil management, or storing carbon in geological formations—and informs models that project future climate change. For further reading on the cycle’s components and dynamics see materials linked to carbon cycle and specialized references on fossil fuel impacts and rock carbon.