One of the problems I faced when writing my futuristic novels was how to describe the environment, and the most difficult of all was what to do about global warming. What is really going to happen? At first sight, this looks easy: there are models that make various predictions. Unfortunately, all of these have serious faults, although many of these will cancel each other out so the predictions might even come about. So, what is causing this? One of the things I have noted is that much of what you see in the media is either wrong or at best, misleading. Even the name is misleading; a greenhouse largely works because it allows the inside to get warm while it stops outside cold air from getting in. How can the general public come to grips with this, when there are so many seemingly contradictory messages? I am going to try to sort this out in a series of blogs that hopefully will be comprehensible to the non-expert.
So, what is the effect? We start with the ground, which has become warm through the day. All things on the ground emit infrared radiation. This is usually treated as black body radiation, but in such radiation, all frequencies are available, if there is the energy to power them. The higher the temperature of a body, the more high frequency radiation is emitted. If it gets hot enough, light is emitted, and the body glows, at first red, and as it gets even hotter, whiter. Most bodies cannot do this, and are best described as grey bodies. Why, then, are they treated as black ones? Simply because black ones obey the Planck radiation law, from which you can calculate the energy intensity and frequency distribution. The radiation emitted from grey ones depend on exactly what they are made of so it is nigh on impossible to deal with non-uniform grey ones over the planet. It might be easier to calculate based on black bodies, but doing that automatically inserts errors. However, provided the planet is not changing, this problem can, with some difficulty, be corrected for. Unfortunately the planet is changing.
Accordingly, the ground emits continually infrared radiation as a distribution of radiation frequencies heading towards space, the ground slowly cools down during the night, and in a vacuum, that is the only way it can cool. What the so-called greenhouse gases do is they absorb certain frequencies. Molecules vibrate, and if they absorb infrared radiation, they vibrate faster, reaching what are called “excited states”. There are three important features about these excited states:
- Absorption of radiation only occurs if there is a movement of electric charge in the molecule while doing so (a change of electric moment). Symmetrical gases such as nitrogen and oxygen cannot do that. One of the vibrational states of carbon dioxide also cannot do that.
- Each vibrational state can only absorb a very narrow band of frequencies.
- When the radiation is absorbed, and this will surprise most readers, that in itself does not provide any heat to the gas. Heat is random kinetic energy and such vibrational energy is ordered.
So, what happens next? The excited states only last a very short period of time, and the radiation is re-emitted, but in a random direction. Accordingly, something approaching half of it goes back towards the ground, and half continues upwards, where it may strike more greenhouse gas, whereupon the same happens. Since half of that comes backwards, quite a bit comes back to the ground.
What happens to that which returns to the ground? Recall, the ground is full of vibrations that are emitting infrared radiation. When the radiation is returned to the ground it excites these radiators, and in turn they further emit. The difference, of course, is that before these radiators got their energy from heat in the ground. Now they get it from the back radiation. Accordingly, the air does not heat the ground, as is sometimes stated (that would violate the second law of thermodynamics: a colder body cannot heat a warmer one) BUT it does slow down the cooling by recharging the ground radiators where before heat was required. Water is a very good “greenhouse gas” and this is why, on overcast winter nights, you get milder or no frosts. The clouds do not heat anything, but they slow down cooling. I like to think of it as a blanket effect. You put blankets on the bed to keep warm. The blankets provide no heat, but they greatly slow the loss of heat from you, and since you (unlike the ground) generate heat, you maintain a constant temperature with the right number of blankets.
The gases that act in this way are gases with a strong change of electric moment when they vibrate, as this increases the probability of absorbing photons at a given frequency, and those with a number of different ways of vibrating, because they absorb at a number of different frequencies. Carbon dioxide is a linear molecule O – C – O and it has three ways of vibrating. Both oxygen atoms can go to or from the oxygen at the same time, and since this is symmetric, there is no dipole change so that vibration does not absorb. The carbon can vibrate between the oxygen atoms, and the molecule can bend, so there are two not especially strong relatively narrow bands of absorption. Water is a much stronger absorber, partly because it is bent so all possible vibrations absorb, and all involve a good change of electric moment. Further, water molecules tend to stick together (hydrogen bonding) and when this happens, there are changes to the frequencies of absorption, so the net result is, the bands are much broader, and more radiation is absorbed. Finally, there are a number of other gases such as methane, nitrous oxide, etc, and their problem is, they absorb frequencies that carbon dioxide and water do not, so the problem gets worse from the different gases. Methane, in particular, has a number of vibrations, and is considered to be about 25 times worse than carbon dioxide.
That is some of the bad news. More bad news next week. Meanwhile, is this comprehensible? If you do not understand something, please comment.