Background Information: Evolution of the Atmosphere
The earth's atmosphere plays a crucial role in shaping the weather, climate, and life-supporting systems. However, the ocean and atmosphere are the earth's fluid outer layers and are closely connected. Before focusing upon the ocean and its processes, it would be appropriate to examine the atmosphere and the processes that control its movements. In fact, most human beings spend their lives at the base of the huge mass of air that forms the outer layers of our planet. Air is a mixture of gases. The various gases which surround the earth are bound to it by the gravitational force of attraction.
Like the earth's ecological and biological systems, the atmosphere has evolved over time, changing in properties and composition. The earth's atmosphere was created from the inside. It is believed that dissolved gases escaped from the earth's interior as the planet cooled and the crust solidified. The gravitational pull of the planet trapped the gases close to the earth.
Degassing from the earth's interior continues today in less spectacular processes such as active volcanoes, and provides clues as to the composition of the early atmosphere. Volcanic emissions include nitrogen, sulfur dioxide, carbon dioxide, and trace gases such as argon. Although oxygen, the life sustaining gas required by many living things, is absent from the list of volcanic gases, nevertheless, evidence from geological records shows the presence of oxygen in the atmosphere from an early stage, and oxygen now comprises one-fifth of the atmosphere.
