Greenhouse effect

The reason that glass is such a valuable material is that it exhibits a very low absorption of electromagnetic radiation in the visible part of the spectrum, which is a wordy way of saying that it is transparent. It is not, however, transparent either side of the visible range (ultra violet and infra red).

When matter in general interacts with radiation it can absorb, reflect or transmit it. One of the basic principles in the understanding of energy is that it always degrades. The high grade (i.e. most usable) energy the comes into your home via the electricity mains always ends up as low grade energy or heat, via intermediate forms such as light, sound or mechanical motion. Likewise the light impinging on the earth that is not reflected back into space also ends up as heat. All matter re-radiates heat, mainly in the infrared part of the spectrum. Glass, however, is relatively opaque to infrared. Thus the greenhouse glass acts as a one-way energy valve.

In the atmosphere water vapour is a major component (1%-4%) and it has similar optical properties to glass. The water molecule is in the form of a shallow V with two hydrogen atoms attached to the heavier central oxygen atom. It absorbs radiation by the various modes of vibration of this system and by rotation (think of it as two light balls attached to a heavy one by springs). Pure liquid water is actually blue, as it absorbs slightly in the red, though heavy water is colourless because the heavier atoms do not vibrate so readily at optical frequencies. Like the greenhouse glass, the water vapour acts as a one-way energy valve. Thus not only does water keep the planet warm, but it also maintains a fairly constant temperature. It is this greenhouse effect that makes life on earth possible.

The other and major mechanism in the greenhouse is the inhibition of convection. This is not ideal, as there is heat loss from the roof by convection. The Earth, in contrast, loses no heat by convection, but only by radiation. Convection in the oceans and atmosphere merely serves to redistribute the heat.

In order to understand fully the warming effect of the atmosphere it is necessary to appreciate the nature of radiation. The ideal radiator is known as a black body, which is one that absorbs all frequencies completely. It also radiates all frequencies equally, so the distribution of energy in the radiation is a function of the distribution of  energy across the spectrum. This is determined by Wien's laws (later modified by Planck) and is a function of temperature only. Real bodies are not black.

The so-called greenhouse effect is simply explained as a three body problem. The three bodies are one that is very small but very hot, the Sun, one that is very large and very cold, outer space, and the earth. The first two may be considered approximately to act as black bodies, so they emit according to Wien's laws. The energy transfer is governed by the area under the black body spectrum, which depends only on the temperature. The blue planet, however, is not a black body. Its absorption/emission spectrum contains gaps, due mainly to the presence of water vapour in the atmosphere. As it is in thermal equilibrium with the other two bodies, it has to radiate as much as it absorbs. In order for the area under the spectrum to be sufficient, the equilibrium temperature must therefore be higher than if it were a black body.

There are other potential greenhouse gases, such as carbon dioxide and methane, but their atmospheric concentrations are so low that they may be ignored (CO2 at 0.033% and CH4 at 0.0002%).

 JEB September/October 2002

Reference: Global warming, a closer look at the numbers.

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