Carbon is the backbone of all life on Earth. It's in every cell of your body, in every plant, in the air you breathe, and in the rock beneath your feet. But carbon doesn't sit still — it moves constantly between the living world, the atmosphere, the oceans, and the ground. This movement is the carbon cycle.
Carbon in the air
The atmosphere contains carbon in the form of carbon dioxide (CO₂) — a gas made of one carbon atom bonded to two oxygen atoms. Plants absorb CO₂ from the air during photosynthesis and use the carbon to build their leaves, stems, and roots. In doing so, they lock atmospheric carbon into solid matter. This is called carbon fixation.
Carbon through the food chain
When animals eat plants, they take in that carbon. When they breathe, some of it is released back as CO₂. When animals and plants die, decomposers (bacteria and fungi) break down their bodies, releasing carbon back into the soil and air. Some carbon gets buried in sediment and, over millions of years, is compressed into coal, oil, and natural gas — what we call fossil fuels.
Imagine carbon as money in an economy. It flows from the bank (atmosphere) to shops (plants), from shops to customers (animals), and customers eventually deposit it back at the bank (by breathing, decomposing, or dying). For millions of years, the total amount in circulation stayed roughly the same. Burning fossil fuels is like finding an enormous buried treasure chest full of old money and spending it all at once — suddenly flooding the economy with far more carbon than it's used to handling.
The ocean's role
The oceans absorb roughly a quarter of all the CO₂ humans emit — acting as a massive carbon sink. This has slowed climate change, but it comes at a cost: as seawater absorbs more CO₂, it becomes more acidic (a process called ocean acidification). This threatens shellfish, coral reefs, and the entire marine food chain that depends on them.
How humans are changing it
For most of human history, the carbon cycle was roughly in balance. Then we started burning fossil fuels — releasing carbon that had been locked underground for hundreds of millions of years, all within the space of about 200 years. The cycle can't absorb this extra carbon fast enough. It accumulates in the atmosphere as CO₂, trapping heat and causing the climate to warm. The carbon cycle isn't broken — but we are definitely stressing it in ways it has never been stressed before.
Carbon is found in everything on Earth. It is in every cell of your body. It is in every plant and in the air. It is even in the rocks under your feet. But carbon does not stay in one place. It moves around between living things, the air, the oceans, and the ground. This movement is called the carbon cycle.
Carbon in the air
The air contains carbon in the form of carbon dioxide (CO₂). CO₂ is a gas made of one carbon atom joined to two oxygen atoms. Plants soak up CO₂ from the air. They do this during a process called photosynthesis. They use the carbon to grow their leaves, stems, and roots. The carbon gets locked inside the plant as a solid. Scientists call this carbon fixation.
Carbon through the food chain
When animals eat plants, they take in that carbon. When animals breathe out, some carbon goes back into the air as CO₂. When plants and animals die, tiny living things like bacteria and fungi break down the bodies. This sends carbon back into the soil and air. Sometimes carbon gets buried deep underground. Over millions of years it gets squeezed into coal, oil, and natural gas. We call these fossil fuels.
Think of carbon like pocket money being passed around. The air is like a piggy bank. Plants are like a school tuck shop that takes money from the piggy bank. Animals are like children who spend money at the tuck shop. Eventually the money finds its way back to the piggy bank through breathing and decomposing. For millions of years the same amount of money kept going around. Burning fossil fuels is like finding a huge tin of very old coins buried in the garden and spending it all in one week. Suddenly there is far more money going around than anyone is used to.
The ocean's role
The oceans soak up about a quarter of all the CO₂ that humans produce. The oceans act like a giant sponge for carbon. This has helped slow down climate change. But there is a problem. When seawater soaks up more CO₂ it becomes more acidic. This is called ocean acidification. It is harmful to shellfish and coral reefs. It also threatens all the sea creatures that depend on them for food and shelter.
How humans are changing it
For most of human history, the carbon cycle stayed roughly in balance. Then humans started burning fossil fuels. Those fuels had been locked underground for hundreds of millions of years. We have released huge amounts of that carbon in just about 200 years. The carbon cycle cannot soak up all this extra carbon quickly enough. The extra CO₂ builds up in the air. It traps heat around the Earth and makes the climate warmer. The carbon cycle is not completely broken. But humans are putting it under more pressure than it has ever faced before.