Carbon dioxide, photosynthesis, respiration, oceans, soils, rocks, fossil fuels, climate, reservoirs, fluxes, and long-term storage
The carbon cycle
The carbon cycle is the movement and storage of carbon through the atmosphere, oceans, rocks, soils, living organisms, and human systems, linking life, climate, geology, energy, and the chemistry of Earth.
What the carbon cycle is
The carbon cycle describes how carbon moves through Earth. Carbon atoms can be in carbon dioxide gas, plant tissues, animal bodies, soils, ocean water, shells, rocks, fossil fuels, and many other forms. The cycle matters because carbon is a building block of life and because carbon-containing gases help control Earth's climate.
Fast and slow pathways
Some carbon moves quickly through the living world. Plants and phytoplankton take in carbon dioxide, organisms eat and respire, and dead material decomposes. Other carbon moves slowly through rocks, sediments, deep ocean water, volcanoes, weathering, and fossil fuel formation. The fast cycle can operate over days to centuries, while the slow cycle can take thousands to millions of years.
Photosynthesis and respiration
Photosynthesis removes carbon dioxide from air or water and turns it into organic matter using sunlight. Respiration does the reverse: plants, animals, microbes, and other organisms release carbon dioxide as they use energy from food. These two processes create a major exchange between the atmosphere, oceans, and living things.
Oceans and carbon
The ocean stores and moves enormous amounts of carbon. Carbon dioxide dissolves into seawater, reacts chemically, and is used by marine organisms. Some carbon sinks with dead plankton, shells, or organic particles into deeper water and sediments. Ocean circulation can store carbon for long periods, but added carbon dioxide also changes seawater chemistry and contributes to ocean acidification.
Soils, forests, and ecosystems
Forests, grasslands, wetlands, farms, and soils store carbon in plant material, roots, microbes, and organic matter. When ecosystems grow, they can take up carbon. When they burn, dry, erode, are cleared, or are disturbed, they can release carbon. The balance depends on climate, land use, species, fire, water, soil, and management.
Rocks and fossil fuels
Over long timescales, carbon is locked into limestone, sediments, fossil fuels, and other geologic reservoirs. Weathering, sedimentation, burial, plate tectonics, and volcanism move carbon through the slow cycle. Coal, oil, and natural gas formed from ancient organic matter. Burning them releases long-stored carbon back into the atmosphere much faster than natural geologic processes usually would.
Human disruption
Human activities have changed the carbon cycle by burning fossil fuels, making cement, clearing forests, draining wetlands, and altering soils. These activities increase atmospheric carbon dioxide and methane, strengthening the greenhouse effect. Land and oceans absorb part of the added carbon, but not all of it, so greenhouse gases accumulate and climate changes.
Why it matters
The carbon cycle matters because it links the chemistry of life to the stability of climate. It explains why forests, oceans, soils, fossil fuels, agriculture, and industry are connected. Understanding the cycle helps people interpret climate change, carbon storage, emissions, ocean acidification, ecosystem recovery, and the limits of simple fixes.