Science

The Earth’s atmosphere has changed over billions of years, but for the past 200 million years it has been much as it is today. We are, however, causing our atmosphere to change by human activity. Burning fossil fuels and deforestation are two examples of human effect on the environment.
Composition of the Earth's atmosphere

The composition of air
You need to know the proportions of the main gases in the atmosphere.
The Earth's atmosphere has remained much the same for the past 200 million years. The pie chart shows the proportions of the main gases in the atmosphere.

It is clear that the main gas is nitrogen. Oxygen - the gas that allows animals and plants to respire, and fuels to burn - is the next most abundant gas. These two gases are both elements and account for about 99% of the gases in the atmosphere.
The remaining gases, such as carbon dioxide, water vapour and noble gases such as argon, are found in much smaller proportions.

Scientists believe that the Earth was formed about 4.5 billion years ago. Its early atmosphere was probably formed from the gases given out by volcanoes. It is believed that there was intense volcanic activity for the first billion years of the Earth's existence.
The early atmosphere was probably mostly carbon dioxide, with little or no oxygen. There were smaller proportions of water vapour, ammonia and methane. As the Earth cooled down, most of the water vapour condensed and formed the oceans.
It is thought that the atmospheres of Mars and Venus today, which contain mostly carbon dioxide, are similar to the early atmosphere of the Earth.