Carbon is removed from the air by plants during Photosynthesis as they need this to make energy. Oppositely, respiration takes in O2 and releases CO2 to the atmosphere. CO2 is also released in to the atmosphere through decomposition. this process can occur fully in which all carbon containing compounds are broken down into CO2 or alternatively the carbon becomes trapped (fossilised) into coal or other fuels. this is then burnt which again releases the CO2 into the atmosphere to be once again taken up by plants.