Nitrous oxide and other nitrogen oxides are produced because fuel combustion causes nitrogen in the fuel or air to combine with oxygen in the air. Extracting, processing, transporting, and distributing fossil fuels also releases greenhouse gases. These releases can be deliberate, as when natural gas is flared or vented from oil wells, emitting mostly carbon dioxide and methane, respectively. They can also result from accidents, poor maintenance, and small leaks in well heads, pipe fittings, and pipelines. Hydrocarbons enter the atmosphere as a result of oil spills from tanker ships or small losses during the routine fueling of motor vehicles.
Deforestation is the second largest source of carbon dioxide.
When forests are cleared for agriculture or development, most of the carbon in the burned or decomposing trees escapes to the atmosphere. However, when new forests are planted the growing trees absorb carbon dioxide, removing it from the atmosphere. Recent net deforestation has occurred mainly in the tropics. There is a great deal of scientific uncertainty about emissions from deforestation, but it is estimated that from 600 million to 2.6 billion tonnes of carbon are released globally every year. Like the CO 2 emitted from fossil fuels, the carbon dioxide released during cement production is derived from limestone and is thus of fossil origin, primarily sea shells and other biomass buried in ancient ocean sediments.