The carbon oxygen cycle is the process by which plants use carbon dioxide for respiration during photosynthesis and produce oxygen. During this process, carbon dioxide becomes part of the plant, and when the plant dies in a carbon-rich state it is possible for it to become a fossil fuel.
Carbon is present in the atmosphere in the form of a gas called carbon dioxide. During photosynthesis, plants use carbon dioxide from air and water molecules alongside the sun's rays to produce energy in the form of glucose. This takes place in the leaves, but is also a process adopted by algae and phytoplankton. Oxygen is generated as a byproduct of this process, and it enters the atmosphere for other organisms to use. While there, carbon attaches to oxygen and carbon dioxide is formed, which means carbon dioxide enters the atmosphere again.
During photosynthesis, some carbon locks into the plant. When people use plants that have been dead for hundreds of years as fossil fuels, they interrupt the natural carbon oxygen cycle that makes some carbon become stationary while allowing others to remain in the atmosphere. When people burn fossil fuels, excess carbon is released into the atmosphere. As a result, levels of carbon dioxide in the atmosphere are much higher than they used to be.