Carbon dioxide is the most significant greenhouse gas released by human activities, primarily through the burning of fossil fuels. It is the main contributor to climate change. Methane is produced when vegetation is burned, digested or rotted without the presence of oxygen. Large amounts of methane are released by garbage dumps, rice paddies and grazing livestock. Nitrous oxide occurs naturally in the environment but human activities are increasing the quantities.
Nitrous oxide is released when chemical fertilizers and manure are used in agriculture.