The Role of Our Atmosphere
If the earth had no atmosphere, life would not exist here. The insulating blanket of air surrounding us maintains a temperature range favorable for life because the sun's radiant energy, which is primarily in the visible wavelengths, is absorbed by and warms the ground, which in turn reradiates energy in the infrared region of the electromagnetic spectrum. (The reradiated energy is in the infrared because the warming of the surface by sunlight maintains it at a temperature of not quite 300 K.) The reradiated infrared photons' passage outward into space, however, is restricted by carbon dioxide and water-vapor molecules in the atmosphere; they absorb the energy and reradiate much of it back to the surface of the earth. This is called the greenhouse effect, after the similarity of the action to that of the glass in a greenhouse, which prevents heat radiation produced inside the greenhouse from escaping.
Without its atmosphere the earth's average temperature would be about 20° to 30° lower than its present value of 15°C (288 K). Since water would be frozen at that temperature, it could not effect the development and maintenance of life as it does in a liquid form. Worldwide circulation of the atmosphere also transports thermal energy and helps to moderate the extremes in temperature that would otherwise exist.
Moreover, the upper atmosphere is important for our survival. It protects us from harmful ultraviolet and X-ray radiation from the sun, vaporizes meteoroids entering the atmosphere, and absorbs most of the incoming highly energetic subatomic particles that we call cosmic rays. Finally the atmosphere creates the soft blue appearance of the sky: atmospheric gases scatter the photons of sunlight in the blue region much more efficiently than they do photons of longer wavelengths.
In all, the earth's atmosphere plays a very vital role beyond the obvious one of providing the oxygen that we breath. One of mankind's most important challenges is in understanding all aspects of our atmosphere and preserving it; for our continued existence depends upon that knowledge.
