Animation showing the composition of the atmosphere of Earth at sea level. The atmosphere comprises a number of gases, which are bound to Earth by gravity. The majority is unreactive nitrogen (N2), which makes up some 78%. Around 20& is oxygen (O2), the active component that is essential for animal life. 0.9% is the unreactive noble gas argon (Ar). Some 0.04% is carbon dioxide (CO2), which is taken up by plants during photosynthesis, during which they produce the oxygen in the air. At sea level the atmosphere also contains a highly variable amount of water vapour (H2O). This ranges from 0-5%, with an average around 1%. The changing levels of water vapour in the air play a critical role in the Earth's weather and climate.