what gases are in the earth's atmosphere today
By volume , dry air contains 78.09 % nitrogen , 20.95 % oxygen , 0.93 % argon , 0.04 % carbon dioxide , and small amounts of other gases . Air also contains a variable amount of water vapor , on average around 1 % at sea level , and 0.4 % over the entire atmosphere . Air content and atmospheric pressure vary at different layers , and air suitable for use in photosynthesis by terrestrial plants and breathing of terrestrial animals is found only in Earth 's troposphere and in artificial atmospheres .