|MadSci Network: Earth Sciences|
It is true that there are several important greenhouse gasses. CO2 is one of them, but methane (CH4) and water vapor are also important. CO2 is the one that human activity may influence the most and therefore gets a lot of attention in the climate debate, while water vapor changes due to natural phenomena - and as a consequence of possible manmade heating. The thing that makes these molecules important for the greenhouse effect is their ability to absorb infrared light. When sunlight enters the atmosphere it is turned into heat in the surfaces it strikes and these surfaces re-radiate the energy as infrared light. Therefore, if the atmsophere becomes less transparent to infrared light, the heat cannot get out into space as easily as before and temperatures will rise in order to reestablish the temperature gradient needed to drive the infrared heat out past the obstacles. You may think of a mountain stream where water flows downhill. If you push a boulder into the stream you will cause the water to rise behind the boulder, but after a while the same amount of water will flow past the boulder and downstream. Same thing with more IR absorbtion in the atmosphere. It will get hotter behind the obstacle (the IR-absorbing molecules in the air) but eventually a balance will be found where the same amount of energy flows into space as comes down from there. In terms of the molecules themselves, it is the electronic structure that causes IR light to be absorbed. Atoms and molecules absorb light at specific wavelengths. Most molecules absorb light in broad bands or wavelength ranges, while atoms tend to absorb light in narrow regions or lines. The molecules therefore absorb over a wider range of frequencies and block more light this way. The details as to why the light is absorbed at infrared wavelengths and not in the X-ray range or vissible light, is complex to explain. It has to do with the structure of the molecules - the strengths of the bonds inside the molecule and the masses of the atoms inthe molecule. Masses on springs tend to oscillate with frequencies that are determined by how stiff the spring is and by how large a mass is attached. The same goes for the bonds (or springs) inside molecules - the vibrational frequency of molecules is determined by the spring strengths and the masses attached to the springs. Because of the many electrons interacting in complex molecules you get many possible modes of vibration and therefore many possible wavelengths at which light can be absorbed. This gives you the light-absorbing bands typical for molecular spectra. By the way, the phrase 'greenhouse' effect is missused for molecules, because the effect of the glass on a greenhouse is not chiefly to stop infrared light getting out, but to stop warm air from blowing away from the inside of the house.
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