One reader commented on my post about geomagnetic reversal theory of global warming (http://robertkyriakides.wordpress.com/2008/02/27/geomagnetic-reversal-theory-of-global-warming-an-alternative-view/#comments) and although I answered his comment, I felt it best to expand my answer here. The reader, Mr Bob Wexelbaum, rightly referred to elementary physics. I cannot do paraphrase his point and do justice to it, so I set it out in full:-
Remember your high school science classes? We learned that heat energy can only be transferred by conduction, convection or radiation. Heat energy can only travel to the Earth from the Sun through space by radiation. We also learned that heated air rises above cold air, since it is less dense than cold air. You cannot heat the Earth very efficiently by trapped heated air in the atmosphere or above, as air being trapped by alleged greenhouse effect must be. You cannot heat a house efficiently by heating its attic. Is the trapped hot air the cause or the effect of Global Warming? It may well be an insignificant side effect. I’d rather bet that there is a lot of electro magnetics involved that may prove to be related to cyclic solar radiation variation which may not be solely related to measured photon transfer or radiant spectral power density.
Mr Wexelbaum makes an interesting point, one which goes to the heart of the theory anthropogenic climate change. Of course, school classes simplify the science. And the process of warming the planet is a bit more complicated that it appears.
A good example is of the top of a mountain, which is much colder than its base. This is not because the heat starts from the centre of the earth and heats the earth up (which it does to a very limited extent) but because of convection and solar radiation. Solar radiation penetrates to the earth’s surface, heats it (by molecular vibration) and the heat rises by convection spontaneously. Convection works with liquids and gases and the significant fact here is that so much of the earth is covered with water; water is evaporating all the time and convection transport water vapour into the atmosphere all the time.
If the atmosphere were a closed system – like water in a pot – the top would get hottest. What happens in the atmosphere is that as particles of warm air rise they expand – become less dense. And the expansion requires the expenditure of thermal energy, so the air particles start warm at the bottom of the mountain but are colder by the time they reach the top.
So it is radiation that heats the surface and convection that heats the atmosphere. The solar radiation is fairly constant. Some of it bounces out of the atmosphere but if you increase the greenhouse effect of the atmosphere by injecting more greenhouse gas into it (so the theory goes) less heat escapes and there is more convection in the atmosphere, and so the planet slowly warms up by convection. If you make the seas slightly warmer more water vapour will be created because radiation will be working on seas which require less energy to create more evaporation – starting say half a degree warmer will inevitably create more evaporation with the same amount of radiation.
Evaporation creates more water vapour in the atmosphere, and water vapour is a greenhouse gas too.
Thus the planet will become slowly but surely warmer, until, presumably, the atmosphere reaches its absolute humidity, so the story goes.
If that is right then gradually the planet will warm several degrees. I very much doubt if this process will provide us with a dangerous climate for humans for at least fifty or sixty years, possibly longer. That is the risk and the ultimate reason for changing our behaviour to emit less greenhouse gases is to preserve the planet for humanity.