Held in place by gravity the atmosphere is thicker at the equator than at the poles and rapidly becomes less dense with increasing distance from the Earth's surface with no clearly defined upper boundary. A generally accepted boundary between the atmosphere and outer space is an altitude of approximately 62 miles. Near the equator, about 99% of the atmospheric gases (by mass) are below an altitude of 19 miles(1). To visualize how thin the atmosphere really is, it helps to remember that a commercial jetliner typically flies above 75% of the atmosphere.

The atmosphere is composed of a mixture of gases; primarily nitrogen (78%) and oxygen (21%). Carbon dioxide is comprises only about 0.038% of the atmosphere. Collectively we call this mixture of gases "air"; and it is this thin layer that makes life on earth possible, providing the oxygen that we require and protecting us from much of the ultraviolet solar radiation.

The atmosphere/earth system reflects about 30% of the incoming solar radiation, the remainder is absorbed and warms the earth, water and atmosphere. All objects radiate energy, with the wavelengths becoming shorter as the temperature of the object increases. As the sun is extremely hot, much of its radiation is in the shorter wavelengths, such as visible light. These shorter wavelengths pass readily through the atmosphere and reach the surface of the earth. The major atmospheric gases, nitrogen, oxygen and argon, are also quite transparent to the long wavelength, or heat, radiation. However, due to their molecular structure some of the gases present in the atmosphere in small quantities, primarily water vapor, carbon dioxide and ozone do absorb the long wave, heat radiated by the earth's surface. These gases that absorb the radiation emitted by the earth are the so-called "greenhouse gases", and are the primary factor in global climate change.

Greenhouse gases do not act as a reflector, but rather absorb long-wave radiation, warming the individual molecules which re-radiating energy, in the form of heat, back to Earth and also to outer space. This cyclic process is illustrated in Figure 1.⁽²⁾ which shows the incoming energy from the sun as the yellow arrows, the energy radiated by the Earth as the brown arrows and the atmospheric radiation as white.

Figure 1. The Greenhouse Effect.

Note that in this figure, all of the numbers add up; The Earth receives 235 units of energy from the sun, and re-radiates 235 ( 195 + 30) back to outer space, while the surface of Earth absorbs 492 (168 + 324) units of energy, and re-radiates 492 units. Thus Earth is neither gaining or losing energy, and under these conditions the temperature of the land and water is not changing. While all energy that reaches the surface of the earth ultimately originates from the sun, on average about 65% of the heat that is absorbed by the earth's surface is energy recycled by the greenhouse effect.

Water vapor (not clouds, which technically are not a gas) is by far the major greenhouse gas, and the only greenhouse gas that occurs naturally to any extent. The concentrations of the remaining greenhouse gases, carbon dioxide, methane, nitric oxide and some of the fluoride gases are heavily influenced by man's activities and thus called the "anthropogenic" greenhouse gases, and make up about 1% of the atmosphere.

The recycling of energy by the greenhouse gases stabilizes the temperature of the Earths surface between night and day. Indeed, if it were not for the greenhouse gases and their effect, the planet Earth would have a hostile environment, not capable of supporting life due to high temperature variability and extremely cold nighttime temperatures. Since the beginning of the Industrial Revolution the burning of fossil fuels has resulted in the increase of atmospheric carbon dioxide and thus an increased energy load on our planet's surface; which is the source of the global climate crises.

1.) Lutgens, Frederick K. and Edward J. Tarbuck (2007) The Atmosphere, Prentice Hall, 10th ed. 544p.

2.) This figure is from Robert Rohde's amazing collection of Global Warming Art and has a reference link of http://www.globalwarmingart.com/wiki/Image:Greenhouse_Effect_png . The numeric values are approximate annual average energy flux (flow), in watts per square meter."Watts" are a measure of energy flow (joules/second). The values for this figure have been derived from the "solar constant", the solar energy received by a surface perpendicular to the sun and at the Earth's average distance from the sun.