RespirationCombustionDecayWeatheringThe major human processes that release carbon dioxide are the burning of carbon fuels (wood, coal, oil, gas) and the cellular respiration of animals and plants. CO2 is also released by the weathering of carbonate rocks, especially by acid rain.Cellular respiration is mainly balanced by the removal of carbon by photosynthesis, which releases oxygen. This is essentially the reverse process, removing the energy stored by forming carbohydrates.In terms of what process releases the most carbon dioxide into our atmosphere it is decaying organic matter. Total natural decay accounts for over 80% of all CO2 released annually. Natural processes in total account for 93% of all CO2 releases.