Scientists believe that the Earth was formed approximately 4.5 billion years ago. Gases produced by volcanoes, such as carbon dioxide, water vapour, hydrogen and methane, are believed to have formed the early atmosphere. The early atmosphere was made from water vapour, ammonia and methane, and little or no oxygen.
For the first billion years of Earth's existence, there was intense volcanic activity. As the Earth began to cool down, most of the water vapour in the atmosphere condensed to form the major oceans, and sedimentation occurred. As water vapour condensed into water, the water started to dissolve gases such as ammonia , removing them from the atmosphere, creating ammonia compounds, amines, and other nitrogen-containing compounds suitable for life.
As plants and algae began to evolve in the ocean, organisms conducting photosynthesis, the concentration of carbon dioxide in the atmosphere decreased and the concentration of oxygen in the atmosphere increased as photosynthesis occurred, the plants and algae acting as a carbon sink: Carbon Dioxide + Water -> Oxygen + Glucose 6CO2 + 6H20 -> 6O2 + C6H12O6
Other animals evolved on land and began to respire, increasing the amount of carbon dioxide in the atmosphere: Oxygen + Glucose -> Water + Carbon Dioxide 6O2 + C6H12O6 -> 6H20 +6CO2 However, other processes decreased the concentration of carbon dioxide in the atmosphere, such as the production of sedimentary rocks (such as limestone), the production of fossil fuels through the partial decomposition of dead remains, and dissolving in the oceans.
The composition of the current atmosphere is a contrast to the composition of the previous atmosphere: Nitrogen: 78% Oxygen: 21% Carbon Dioxide: 0.03%
Today, human beings are burning fossil fuels, such as oil, gas and coal, in order to generate electricity, finite, non-renewable resources. Since combustion released Carbon Dioxide into the atmosphere, burning fossil fuels increases the concentration of Carbon Dioxide in the atmosphere. Agricultural activities such as cattle ranching also increases the amount of methane in the atmosphere, and flooded rice paddies produce methane. Overall, humans have raised the amount of greenhouse gases in the atmosphere to an unnatural level.