Carbon dioxide is a gas that we breathe out and a gas that plants use to make sugars and oxygen. It has been a small part of the air around us for a very long time. It is made up of one carbon atom and two oxygen atoms. While the earth has a balance of animals and plants, the level of carbon dioxide is constant. The carbon dioxide we breathe out is used by the plants we eat and there is no change! The problem with carbon dioxide comes when lots of extra carbon dioxide suddenly appears in the atmosphere: exactly what burning fossil fuels has done.