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Carbon dioxide, water vapor, and methane are the most common Greenhouse Gases.  The question is why?  The answer has to do with the shape and vibration of the molecules.

The Molecular Shape

The outermost electrons of Greenhouse Gases repel each other.  Their most stable arrangement is as far away from each other as possible.  Water Vapor for example takes on the shape of a tetrahedron—a four cornered figure with four equal triangular sides.  In this shape the outermost electrons are the furthest distance away from each other. Electron repulsion creates this shape that is the most common among the Greenhouse Gases, although other shapes do occur.

The Vibrations

The molecular shapes of these molecules cause an important phenomenon of how these molecules interact with infrared radiation.  When a molecules interact with infrared radiation, it absorbs a photon of energy.  When a photon of UV radiation is absorbed it can cause the molecule to break, but when infrared radiation is absorbed it enhances the vibrations of the molecule.  This is the action that creates the Greenhouse Effect.  The bonds of the molecule can be thought of as springs that allow the atoms of the molecule to move back and forth.  Depending on molecular structure, only certain vibrations are permitted.  This means that different molecules absorb infrared radiation at different wavelengths, thus, vibrate differently.  Any molecule that can vibrate in response to the absorption of infrared radiation is potential Greenhouse Gas.

Other Greenhouse Gases

Recent research suggests that approximately 50% of global warming may be caused by compounds other than carbon dioxide.  Methane is approximately 30 times more effective in absorbing infrared radiation than carbon dioxide.  Though the atmospheric concentration of methane is very low, its current level is about 2 parts per million, double what it was during the Industrial Revolution.  Data since 1979 shows that methane has been increasing annually by about 1%.  Methane comes from a variety of sources.  While most are natural, they have been magnified by human activity.  Examples that have caused this include: landfills, exploitation of natural resources, and agriculture.  Nitrous oxide is also on the rise.  Coming from mainly from fertilizers.  Nitrous oxide also contributes to ozone depletion, not to be confused with global warming.  It is important to note that not all Greenhouse Gases are equally effective in absorbing infrared radiation.  One CFC molecule has the same warming effect as 25,000 carbon dioxide molecules.  Fortunately, the amounts of the molecules that absorb the most infrared radiation are very low, and the use of this molecule has been suspended indefinitely by many nations.