Changes to the atmosphere

The atmosphere on Earth has been continuously changing ever since the planet was formed. The eruption of volcanoes, and the processes which caused life to begin brought about the first major changes thousands of millions of years ago. As first plant life and then animal life developed, the concentration of oxygen in the atmosphere increased and that of carbon dioxide decreased and then reached a more or less steady state. The next major change was after the Industrial Revolution, when fossil fuels (mainly coal at first) began to be burned in large quantities to produce steam to power the new industries.

The composition of Earth's primitive atmosphere

When the Earth was formed over 4500 million years ago, the composition of the atmosphere was very different from that we experience today. At that time, the largest component of the primitive atmosphere was carbon dioxide, followed by nitrogen and hydrogen (which soon escaped from the planet) with almost no oxygen at all. This atmosphere resulted largely from the gases released as the molten planet cooled and during volcanic eruptions. There was virtually no oxygen except for that produced by the action of ultraviolet radiation from the Sun on water vapour in the atmosphere. The proportion of carbon dioxide decreased rapidly as the gas dissolved in the oceans and was removed as sedimentary rocks (see also 'The Carbon Cycle'). Later, when the first plant cells formed, photosynthesis gradually increased the oxygen content.

The composition of Earth's atmosphere today

Today the atmosphere contains mainly nitrogen (78.08%) and oxygen (20.95%), with the remainder consisting largely of argon (0.93%) and carbon dioxide (0.03%). This is deceptive, since it is the oxygen content upon which life on Earth depends, and the tiny percentage of carbon dioxide (together with some other gases) which ultimately controls the temperature at the Earth's surface. We are all conscious of pollutants around us because they affect us on a local scale, yet these make up a tiny fraction (less than a millionth of one percent) of the atmosphere.

Elements in Nature

Click on the small image to see a diagram which shows the proportions of different elements in the whole earth, the crust of the earth and the universe. Notice the difference between them...