Some 1,300 billion tons of carbon are believed to have accumulated in ground litter and soils. Terrestrial organisms, primarily plants, account for an estimated 550 billion tons of carbon. By far the largest carbon reservoirs are the deep oceans and fossil fuel deposits, which account for some 34,000 and 10,000 billion tons of carbon respectively. The global carbon cycle is out of balance, making rapid global climate change more likely. Carbon dioxide forms when the carbon in biomass oxidizes as it burns or decays.
Many biological processes set in motion by people release carbon dioxide.
Clearing vegetation cover from a forested hectare releases much of the carbon in the vegetation to the atmosphere, as well as some of the carbon lodged in the soil. Logging or sustainable fuelwood collection can also degrade vegetation cover and result in a net release of carbon. The release of CO 2 from deforestation is the forest's single most important contribution to climate change, but human activities in tropical forest zones produce several other greenhouse gases, too. Methane is released when combustion is incomplete or when biomass rots. When pastures or grasslands are regularly burned to maintain fertility and keep out woody plants, methane, carbon monoxide, nitrous oxide, and ozone are emitted.