Definition
Carbon is never really created or destroyed on Earth. It just changes where it sits. Carbon dioxide in the air enters a plant, then passes through animals, soil, the ocean, and rock, and eventually returns to the air. The whole flow of carbon moving around the planet like this is called the carbon cycle.
The Fast Route Through Breathing and Growing
On a sunny day, a leaf pulls carbon dioxide out of the air. Using the energy of sunlight, it turns carbon dioxide and water into glucose. This process is called photosynthesis. A tree's trunk and roots are built from the carbon it has caught and piled up this way.
That carbon soon passes to other living things. It enters an animal that ate the grass, and when the animal breathes out, it leaves as carbon dioxide and goes back into the air. Fallen leaves and dead animals are broken into tiny pieces by decomposers such as fungi and bacteria, which scatter the carbon into the air and soil.
This route is fast enough to see. In summer, when leaves are thick across the Northern Hemisphere, carbon dioxide in the air drops. In winter, when the leaves fall, it rises again. The level goes up and down each year as if the Earth were taking one big breath in and out.
The Slow Route Held by Oceans and Rocks
Not every path brings carbon back in days or years. Carbon dioxide in the air also dissolves into seawater. The ocean holds far more carbon in its water than the air does, even now. Colder water dissolves carbon dioxide better, so its power to hold on to it grows too.
Clams and corals in the sea use the dissolved carbon to build their shells and skeletons. When they die and sink to the bottom and are pressed for a very long time, they become rock such as limestone. Remains of living things buried underground can also turn into coal or oil after long exposure to heat and pressure.
Carbon locked away this way rarely comes back soon. It returns to the air only after millions of years, riding out on gases from volcanoes. In the other direction, rain wearing away rock pulls carbon dioxide out of the air and sends it to the sea. So the carbon cycle has a route that takes a few years and a route that takes millions of years running side by side.
To Be a Little More Precise
To be a little more precise, the carbon cycle was not a perfectly balanced scale before humans came along. Still, the amount coming in and the amount going out stayed roughly matched for a very long time. What has changed now is that we are digging up carbon that was buried underground and burning it in a short span of time.
Coal and oil are the storehouse of the slow route. When we open that storehouse and burn what's inside, the carbon becomes carbon dioxide and pours into the fast route. Oceans and forests take in some of it, but they can't keep up with how quickly it arrives.
As a result, the carbon dioxide level in the air rises, and seawater tips little by little toward acidic. The key question is not whether carbon disappears, but where it stays and how much. Once you see how the cycle is built, it's easier to understand why protecting forests and preserving tidal flats are talked about together.
Common misconceptions
Carbon is a harmful substance, so the less of it on Earth, the better.
Carbon is a basic building block of life, found in our bodies, in trees, and even in microbes. The problem isn't carbon itself. It's the imbalance that appears when carbon locked underground for a long time rushes into the air in a short period.
If you plant a tree, the carbon you emitted is erased by that amount.
A tree holds on to carbon while it grows, but when it dies or burns, a large part returns to the air. It's closer to looking after the carbon for a while inside the fast route.
Where you meet it
When you eat an apple and breathe out, the carbon that was in the apple goes back into the air as carbon dioxide.
The white cliffs of Dover in England (chalk) and a lot of limestone are carbon that was once in the shells of ancient sea creatures, now hardened into rock.
In one sentence
It's the flow that carries carbon once around the Earth, along a fast route between air and living things and a slow route where it is locked in oceans and rocks before returning.