How does the carbon cycle work? The short answer
The carbon cycle is the constant movement of carbon atoms between the air, living things, the ocean, soil and rocks.15 Earth doesn't make new carbon or lose it to space in any meaningful amount, so the same atoms keep getting passed around.6
In one sentence: plants pull carbon dioxide out of the air and build it into sugar and wood (photosynthesis); animals eat the plants; plants, animals and microbes breathe carbon dioxide back out (respiration); decomposers return the carbon of dead things to the soil and air; and, much more slowly, carbon sinks into the ocean, gets locked into rocks and fossil fuels, and comes back out through volcanoes.18
Take a breath and let it out. You just released carbon dioxide, and a carbon atom in that breath could end up in a leaf, then a rabbit, then a seashell, and one day in a rock.75
Where is Earth's carbon stored?
Scientists call the places that hold carbon reservoirs (or stores). Think of four big jars:13
The carbon in the air is the smallest store, but it matters a lot, because carbon dioxide traps heat.6 Anything that moves carbon out of one jar puts it into another, and a change in the air jar changes Earth's temperature.1
The carbon cycle steps
Different textbooks list five, six or seven steps. Here are the main processes and which way each one moves carbon.
The fast carbon cycle: carbon moving through life
The fast cycle runs through living things, on time scales of a lifespan. NASA estimates it moves 1,000 to 100,000 million metric tons of carbon every year.1
It starts with a leaf. The chemistry is simple to write down: carbon dioxide + water + sunlight's energy → sugar + oxygen.1 The plant uses the sugar for energy and to build wood, leaves and roots, which is why the carbon in a growing tree (roughly half the weight of dry wood) comes from the air, not the soil.16 Then the reverse reaction, sugar + oxygen → carbon dioxide + water + energy, happens in four ways: the plant uses its own sugar, an animal eats the plant, the plant dies and decays, or it burns.1
Some steps are over in seconds: a breath takes less than a second.7 Others last as long as a tree lives.
The fast cycle is so big you can see Earth "breathe". In the Northern Hemisphere winter, few land plants are growing and many are decaying, so carbon dioxide in the air climbs. In spring, plants start growing again and it drops.1
The slow carbon cycle: carbon moving through rocks
In the slow cycle, carbon takes 100 to 200 million years to travel between rocks, soil, ocean and atmosphere. Only about 10 to 100 million metric tons of carbon move this way each year, far less than in the fast cycle.1
- Into the sea. Carbon dioxide dissolves into the ocean surface. Cold water absorbs more of it than warm water, and when cold water sinks it can carry carbon to the deep ocean for centuries.93
- Into shells and limestone. Rain mixed with carbon dioxide forms a weak acid that slowly dissolves rock, and rivers carry the dissolved minerals to the sea. There, corals and tiny plankton build shells of calcium carbonate. When they die, the shells sink, pile up and over time are cemented into limestone.1 That's how many sedimentary rocks are formed, and why coral reefs are giant carbon stores. About 80% of carbon-containing rock forms this way.1
- Into fossil fuels. When dead plant matter builds up faster than it can decay, buried layers can become coal, oil or natural gas over millions of years.1 Coal comes mostly from land plants (see how coal is formed), while oil and gas come mostly from ocean plankton.8
- Back into the air. Where tectonic plates collide, rock is dragged down and melts, releasing carbon dioxide that volcanoes vent to the air. Volcanoes emit about 130 to 380 million metric tons of carbon dioxide a year.1
Over hundreds of thousands of years, this slow cycle works like a thermostat: if volcanoes add more carbon dioxide, the planet warms, more rain falls, more rock dissolves, and more carbon ends up locked on the sea floor.1
Why is the carbon cycle important?
It recycles the stuff life is made of. Carbon can form up to four bonds per atom, which lets it build the long chains and rings that make up sugars, proteins and DNA.1 Every living thing is built from carbon that was once in the air.
It helps set Earth's temperature. Carbon dioxide is a greenhouse gas: it absorbs heat and sends some of it back toward the surface. With too little, Earth would be frozen; with too much, it warms.62 For most of Earth's recent history, the cycle kept this balanced. During the ice-age cycles of the past million years or so, carbon dioxide never rose above about 300 parts per million (ppm).2
It shapes ocean life. The ocean's chemistry, and the shells of corals, oysters and plankton, depend on how much carbon dioxide the sea absorbs.4
How do humans affect the carbon cycle?
People change the carbon cycle in two main ways: burning fossil fuels and clearing land.1
Burning coal, oil and natural gas. Fossil fuels hold carbon from millions of years of photosynthesis. We're returning it to the air in a few hundred years.2 NASA describes this as moving carbon from the slow cycle into the fast cycle.1 People release tens of billions of tons of carbon dioxide a year this way (NASA put it at about 30 billion tons), which NASA says is 100 to 300 times more than all the world's volcanoes.1 Making concrete from limestone releases carbon too.3
Where the extra carbon goes. Natural sinks, meaning plant growth and the ocean, remove about half of the carbon dioxide people emit from fossil fuels. The rest stays in the air.2 The ocean has absorbed about 30% of human carbon dioxide,4 and land plants about 25%.6
The rise is speeding up: about 0.8 ppm a year in the 1960s, and about 2.6 ppm a year from 2015 to 2024.2 For the latest monthly figure, see NOAA's Global Monitoring Laboratory.14
The explainer's "your turn" toy uses a simple, made-up model, not real measurements: each burn adds one unit of carbon, plants and ocean take about half of it (less once the forest is gone), and cutting a forest releases its stored carbon. It shows the idea of a balance, not a forecast.
How does deforestation affect the carbon cycle?
Deforestation hits the carbon cycle twice.1
- It releases stored carbon. Trees store carbon in their wood, stems and leaves. When a forest is cleared, often with fire, much of that carbon goes back into the air, and the bare soil gives off carbon from decaying plant matter.1
- It removes a sink. Without the forest, there are fewer plants taking carbon dioxide out of the air as they grow, and the crops or pasture that replace them usually store less.1
NASA estimated that deforestation made up about 12% of human carbon dioxide emissions as of 2008.1 The reverse works too: where old farmland grows back into forest, it stores much more carbon in wood and soil than crops would.1
What happens when the ocean absorbs more carbon?
The ocean is a huge help: by soaking up carbon dioxide it slows the build-up in the air.9 But carbon dioxide reacts with seawater to form carbonic acid. Since the Industrial Revolution, the pH of surface seawater has fallen from 8.21 to 8.10, which is about 30% more acidic.24 Seawater is still slightly alkaline, but the change leaves fewer carbonate ions for corals, oysters and tiny sea snails to build their shells, so they have to work harder and their shells can be thinner.41
What helps rebalance the carbon cycle?
This is the hopeful part: we know which dials to turn, and many are already turning.
- Forests. Protecting forests keeps their carbon locked up, and regrowing them pulls carbon back out of the air.113 Many countries have already reduced their rate of deforestation.11
- Soil care. The USGS calls soil the most stable long-term surface store of carbon. Changes in farming and forestry can help soils hold more.12
- Clean energy. Energy that doesn't burn fossil fuels leaves their carbon in the ground. From 2010 to 2019, the cost of solar power fell by 85% and wind power by 55%.11
For how the extra carbon changes the weather and temperature, see what causes climate change.
Common misconceptions
"Plants are made from soil." Dry wood is roughly half carbon,
16 and that carbon comes from carbon dioxide taken out of the air during photosynthesis, not from the soil.
18
"Carbon gets used up." Earth's carbon isn't created or destroyed. It just moves between stores.
6
"The carbon cycle is only about living things." Most carbon is in rocks, and the slow cycle through oceans, rocks and volcanoes matters just as much.
1
"Volcanoes put out more carbon dioxide than people." People emit roughly 100 to 300 times more.
1
"Nature soaks up all our carbon." Plants and the ocean take up about half. The other half builds up in the air.
2
FAQ
How does the carbon cycle work, step by step?
Plants on land take carbon dioxide from the air, and ocean plankton take it from the carbon dioxide dissolved in seawater, and use sunlight to turn it into sugar (photosynthesis). Animals eat the plants and take in that carbon. Plants, animals and microbes give carbon dioxide back out as their cells use food for energy (respiration). When living things die, decomposers such as fungi and bacteria return their carbon to the soil and air. Slowly, some carbon sinks into the ocean, becomes shells and limestone, or is buried as coal (mostly from ancient swamp plants) and oil and gas (mostly from ancient ocean plankton), and volcanoes eventually release carbon from rocks.
What are the 7 processes of the carbon cycle?
A common list is photosynthesis, respiration, decomposition, combustion (burning), ocean exchange (carbon dioxide dissolving into and out of seawater), sedimentation (shells and buried remains settling in layers that slowly harden into rock) and volcanic release. Textbooks group them differently, so you may see five, six or seven steps, but they all describe carbon moving between the air, living things, the ocean and rocks.
Does the carbon cycle ever end?
No. Earth doesn't gain or lose any meaningful amount of carbon, so the same atoms keep moving between stores. A carbon atom might pass through a plant in a single season or stay locked in limestone for millions of years, but eventually weathering or volcanoes put it back into circulation.
Why is the carbon cycle important?
Carbon is the building block of every living thing, and the carbon cycle is how it gets recycled from one living thing to the next. Carbon dioxide in the air also traps heat, so the balance of the cycle helps set Earth's temperature. Too little and Earth would freeze; too much and the planet warms.
How does deforestation affect the carbon cycle?
Trees store carbon in their wood, leaves and roots. When a forest is cleared, much of that carbon returns to the air as the wood burns or rots, and the exposed soil releases carbon too. Fewer trees are left to take carbon dioxide out of the air, and crops or pasture that replace them usually store less. NASA estimated that deforestation made up about 12% of human carbon dioxide emissions as of 2008.
How do humans affect the carbon cycle?
Mainly by burning coal, oil and natural gas, which releases carbon that was buried over millions of years, and by clearing forests. Plants and the ocean take up about half of the extra carbon; the rest stays in the air. NOAA measured a global average of 422.8 parts per million of carbon dioxide in 2024, up from about 280 before the industrial age.
How can you memorize the carbon cycle?
Remember four stores (air, life, ocean, rocks) and two speeds. The fast lane runs through life: plants take carbon in, animals eat it, everything gives it back out through respiration and decay. The slow lane runs through rocks: shells become limestone, buried plants and plankton become fossil fuels, and volcanoes breathe carbon back out.
Sources
- NASA Earth Observatory, The Carbon Cycle
- NOAA Climate.gov, Climate change: atmospheric carbon dioxide (2024 data)
- NOAA Education, Carbon cycle
- NOAA Education, Ocean acidification
- UCAR Center for Science Education, Biogeochemical Cycles
- U.S. Department of Energy, DOE Explains: The Carbon Cycle
- NASA Science for Kids, What is the carbon cycle?
- UC Berkeley Understanding Global Change, Carbon cycle
- Woods Hole Oceanographic Institution, Carbon cycle
- IPCC AR6 Working Group I, Summary for Policymakers (2021)
- IPCC AR6 Working Group III, Summary for Policymakers (2022)
- USGS Open-File Report 2001-374, A guide to potential soil carbon sequestration
- USGS Fact Sheet 26-03, Geologic Sequestration of Carbon Dioxide
- NOAA Global Monitoring Laboratory, Trends in Atmospheric Carbon Dioxide
- Encyclopaedia Britannica, Carbon cycle
- Thomas & Martin (2012), Carbon Content of Tree Tissues, Forests 3:332