The Earth’s atmosphere is composed of about 77 percent nitrogen, 21 percent oxygen, and traces of argon, carbon dioxide, water, and other compounds and elements. It is interesting that the Earth maintains free oxygen, as it is a very reactive gas. Under most circumstances, it combines readily with other elements. But our atmosphere’s oxygen is pro- 17 duced because of biological processes. Without life on Earth, there would be no free oxygen in our atmosphere.
When the Earth formed, it is believed that the atmosphere contained a much larger amount of carbon dioxide-perhaps as much as 80 percent-but this diminished to about 20 to 30 percent over the next 2.5 billion years. Since that time, the gas has been incorporated into carbonate rocks, and to a lesser extent, dissolved into the oceans and consumed by living organisms, especially plants. Today, the movement of the continental plates, the exchange of gas between the atmosphere and the ocean’s surface, and biological processes (such as plant respiration) help continue the complex carbon dioxide flow, keeping the amount of carbon dioxide in balance.