MadSci Network: Earth Sciences

Re: what about the CO2 molecule allows it to trap heat?

Date: Mon Nov 27 02:07:21 2000
Posted By: Peter Thejll, Staff, Solar-Terrestrial Physics, Danish Meteorological Institute
Area of science: Earth Sciences
ID: 974951408.Es

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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