There has been a great debate during the past week on the What’s Up With That blog, about a new global warming theory. I no longer believe the new theory to be valid, but taking part in the debate had the advantage that it clarified for me some of the issues.
If you consider this as a standard problem in heat transfer, at equilibrium the outgoing energy must balance the incoming energy. We know how much radiant energy the sun inputs, and we seem to agree on its fate – X reflected into space, Y adsorbed by the globe. So the question is how does Y (plus any input from geothermal and man-made energy) leave the globe? Ultimately it can only leave by radiation, because convection and conduction can play no part in space. And indeed, viewed from space we can see all the atmospheric absorbers playing their part, just as we wondered about the sun’s spectrum a century or so ago.
Interestingly, the only gases that play a role in our radiation budget are those that are active in the infrared, which are known as the greenhouse gases. Oxygen, nitrogen, argon and other monatomic and symmetrical diatomic gases in the atmosphere are to all intents and purposes totally transparent to the outgoing radiation. But the greenhouse gases play a role in both incoming and outgoing radiation. They heat the atmosphere by adsorbing ultraviolet from the incoming radiation, classically in the case of ozone, which undergoes photolysis with photons of less than 330nm to produce an oxygen molecule and an active oxygen atom. Nitrous oxide also absorbs strongly in the ultraviolet, but only photolyses with photons of 240nm and less, which is one reason why it is relatively long-lived in the atmosphere. Carbon dioxide and water do not absorb ultraviolet significantly at wavelengths in the atmosphere, so play no part in warming the atmosphere by uv absorption.
Greenhouse gases absorb relatively short wavelength infrared and re-emit slightly longer wavelength infrared from both incoming and outgoing radiation. They therefore heat the atmosphere by absorbing the incoming radiation and so reduce the energy available at the surface.
However, when the surface re-radiates, the greenhouse gases absorb the outgoing short wavelength infrared, which also heats the atmosphere. It is an interesting question whether the reduction in energy reaching the surface from incoming sunlight is greater than the energy trapped by absorption from the outgoing radiation. But key is the fact that the greenhouse gases warm the atmosphere by an estimated ~33 deg K. The more greenhouse gases there are, the greater the energy they contribute (whence the concern about global warming).
We know this from looking at the spectra. The figures below show the spectra over the Arctic – chosen because there is much less water vapour in the way, and because water vapour is such a strong absorber, it dominates the same spectrum over the tropics. In the upper curve, we are looking down at Earth, and can see that the outgoing radiation is characterized by:
1. The Arctic is basically radiating at about 268oK
2. There is an absorption ‘bite’ at around 15 µm due to both H2O and CO2
3. There is a further ‘bite’ at around 9-10 µm due to CO2
4. There is a big ‘bite’ at less than 8 µm due to H2O