The carbon cycle is the way carbon is stored and replaced on Earth. Some of the main events take hundreds of millions of years, others happen annually.

The main ways that carbon gets into the carbon cycle are volcanoes, and the burning of fossil fuels like coal and gas. Through most of history, volcanoes were the biggest source of carbon to the carbon cycle, but in the last hundred years, people burning fossil fuels have been adding about a hundred times more CO₂ to the air than volcanoes. That is, for every ton of CO₂ added to the air by volcanoes, about 100 tons of CO₂ have been added to the air by people through combustion.