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 time more CO to the air than volcanoes. That is, for every ton of CO added to the air by volcanoes, about 100 tons of CO have been added to the air by people.