Carbon dioxide is important because it is used in photosynthesis, a process that is necessary for the survival of life on Earth. Carbon dioxide is also a vital greenhouse gas that helps trap heat in the atmosphere, and it plays a key role in Earth's carbon cycle.
Plants and some microbes use a biochemical process called photosynthesis to make food by using up carbon dioxide. These organisms combine carbon dioxide and water to produce carbohydrates while giving off oxygen. The rise of photosynthetic organisms that consequently reduced carbon dioxide levels led to increased oxygen levels and the development of oxygen-breathing organisms.
Carbon dioxide contributes to the greenhouse effect by interfering with the return of energy from Earth's surface to space by infrared radiation. Greenhouse gases absorb infrared radiation. Without carbon dioxide in the atmosphere, Earth's climate would be much cooler.
Carbon dioxide acts as a climate buffer, because feedback in the carbon cycle helps to maintain global temperatures so Earth's climate does not get too hot or cold to support life. Outgassing from the Earth's interior at active volcanoes, volcanic arcs and mid-ocean ridges is the primary source of carbon dioxide. Some outgassed carbon dioxide remains in the atmosphere, but some is dissolved in the oceans or stored as biomass in living or decaying organisms.