What is the carbon cycle?

Just as the "water cycle" describes the transfer of water molecules between rivers, oceans, land and the atmosphere, the "carbon cycle" refers to the flow of carbon through different parts of the Earth system – including the air and the bodies of plants and animals.
The carbon cycle is a natural and integral part of life on Earth. A single carbon atom stored in a blade of grass may shift to the body of an animal that eats the grass. When the animal dies, its body might rot, and the carbon atom could join with oxygen to form CO₂ in the air. From there it might be taken in by a tree in the process of photosynthesis and used as a building block in a branch or trunk, or absorbed by the ocean. And so on.
Although the basic flows of the carbon cycle haven't significantly changed, in the last century or so humans have increased the amount of CO₂ in the air by taking carbon that has been locked up in the ground for millions of years – in the form of oil, coal and gas – and releasing it into the atmosphere by burning those fuels. There is strong evidence that this has led to global warming.