The burning of fossil fuels such as coal, natural gas and petroleum releases carbon dioxide into the atmosphere. Deforestation also contributes to a rise in carbon dioxide in the atmosphere, as there are fewer trees to soak it up. Intensive farming of livestock, particularly sheep and cattle, produces an excess of methane in the atmosphere, which is also a greenhouse gas, and farming fertilisers release nitrous oxide, another greenhouse gas. All of this contributes to global warming.