Re: What is the GreenHouse Effect?

Question: "What is the greenhouse effect?"

The greenhouse effect is the trapping of the sun's warmth in the atmosphere by gases which are more transparent to the incoming solar radiation than to outgoing infrared. Now let me explain what that means.

Before I go on you need to know what a couple of scientific terms mean. The terms I am going to use are shortwave radiation and longwave radiation. They are important for understanding how the greenhouse effect works.

Shortwave radiation is another name given to light you can see, like sunlight, or the light from a lightbulb. Longwave radiation is another kind of light, but your eyes can't see it. It is also sometimes called infrared light. Everything around you radiates at long wavelengths (infrared remember), but only very hot things such as the sun, fire, a lamp, or even a heating element on a stove radiate shortwave radiation (visible light).

When shortwave radiation (from the sun) reaches the Earth it passes through the atmosphere (just as it does through the glass of a greenhouse) and warms the surface below. The warm surface now reradiates (not reflect, it is not a mirror) this energy but because it is not very hot it emits longwave radiation instead of visible light. Now we get to the greenhouse effect. The gases in the atmosphere cannot allow all of the longwave radiation to pass, so some is absorbed and acts to warm the atmosphere up a little bit. Finally, the atmosphere reradiates the energy, again at long wavelengths, to outer space and back to the surface of the Earth. In this way, the surface of the Earth is actually warmed by two sources of energy. The sun is the first and the most important. The second is the atmosphere itself, which acts to (in the case of the Earth) make the surface a little more pleasant. It is estimated that the average surface temperature of the Earth would be about -18 C without the beneficial natural greenhouse effect. With the natural absorbers present in the atmosphere, the average temperature near the surface is about 15 C. A 33 degree difference! The diagram below will help, I hope, to make this more clear.

sunlight (shortwave) 
  \
   \                                        /\ (longwave)
    \                                       |
     \                                      |
      \                                     |
-------\------------------------------------------------ Atmosphere
        \                     /\(longwave)  |
         \                    /             | 
          \                  /              |
           \                /               |
            \              /                \/ (longwave)
            \/            /
######################################################## Earth
########################################################

Now we come to what I think is what you are really asking. What is meant when the words greenhouse effect are mentioned in the newspaper or on TV. In this instance, the term that should usually be used is enhanced greenhouse effect. This is because, as I tried to indicate above, the greenhouse effect is natural and important to the survival of life on Earth. The problem is that some of the same gases that cause the natural greenhouse effect are also produced by the activities of human beings. These extra gases make the greenhouse effect stronger and in theory will warm the surface even more. Other gases that don't occur naturally, but are strong greenhouse gases, are also added to the mix. The most commonly mentioned greenhouse gas is probably carbon dioxide (CO2). CO2 is a good absorber of longwave radiation, so adding more of it to the atmopshere is probably a bad idea. This is because it will act to increase the surface temperature of the Earth. This is the case on the planet Venus. There a dense atmosphere rich in CO2 helps raise the surface temperature an estimated 500 C more than it would be without the presence of absorbers!

Some other important greenhouse gases are:
Water Vapour 		(yes it's true, water vapour is a 
                         very important greenhouse gas)
Carbon Dioxide		(CO2)
Methane 		(CH4)
Nitrous Oxide 		(N2O)
CFCs			(also very bad for ozone!)