The primary human source of carbon dioxide (CO2) in the atmosphere is from the burning of fossil fuels for energy production and transport. Changes in land use and deforestation also contribute significantly. Trees, for example, are natural 'carbon sinks' - they absorb carbon dioxide while alive and when they are destroyed, carbon dioxide is released into the atmosphere. Once in the atmosphere, most of the carbon dioxide stays there for 50 to 200 years, and some of it stays there indefinitely.
