Three kinds of “saturation”
- Saturation of solution (relevant to water vapour but not CO2)
- Saturation of energy level (not relevant at all in the atmosphere)
- Excess of absorptive capacity (basis of the Angstrom argument)
Saturation of Solution: eg salt added to water dissolves until the water is saturated – after which further salt does not dissolve. Similarly, air exposed to water absorbs water vapour until either saturated or moved by wind to a different location. But the saturation level depends on temperature so if the air moves to a cooler location then some of the vapour condenses out in the form of rain and if the air subsequently gets re-heated it may be less than saturated until it comes back in contact with water so in a constantly moving atmosphere the actual average humidity may be less than 100%. In fact, on average the relative humidity of the atmosphere is about 70% and for any extra water vapour that enters the atmosphere the same amount is quickly precipitated out so as to restore the balance. Since the major aspects of global airflow depend only very slightly on climate there is no reason to expect this to change with temperature. And since warmer air can hold more moisture, when the temperature rises this fixed relative humidity corresponds to a higher absolute humidity which by greenhouse effect may further raise the temperature. This positive feedback effect is limited but it does amplify the effect of any other source of global warming. So it is relevant but not as a way of debunking the CO2 greenhouse effect.
Saturation of energy level: When molecules absorb energy from radiation, they do so by moving to a different quantum state which requires a photon of a specific frequency and once a molecule has been excited by that frequency it will not absorb more of it until it has fallen back to its ground state – either by re-emitting an equivalent photon or by increasing the kinetic energy of the gas as the result of collision with another molecule. When all the molecules are in the excited state the energy level is said to be saturated since there is no way to absorb more of that particular wavelength. Such saturated states are deliberately created in the operation of a laser but they are hard to achieve and generally unstable both because of rapid de-excitation by collisions with other molecules and due to the stimulated emission process by which the laser operates. In the atmosphere this re-distribution of absorbed energy happens almost instantly so this kind of saturation has nothing to do with atmospheric physics (and if it did it would increase rather than decrease the effect of adding more CO2) but it is sometimes mistakenly referred to by people wanting to debunk the CO2 greenhouse effect who have heard of but misunderstood the Angstrom argument (which is based on the excess of absorptive capacity – described below).
Excess of Absorptive Capacity: (This is actually the exact opposite of saturation but it is often referred to as “saturation” by the ignorant and misinformed.) All of the gasses in the atmosphere are transparent to infrared radiation except in specific limited frequency ranges. The Angstrom argument was that once these have been totally blocked there is no further warming effect and that there is already many times enough CO2 in the atmosphere to accomplish that blocking so neither removing some nor adding more should make any difference. The flaw in that argument is the fact that an atmospheric layer that is almost totally opaque to the absorbed frequencies will not in fact completely eliminate them from the Earth’s emitted radiation. This is because when heated up by absorbing that radiation the layer in question radiates a full thermal spectrum from both its top and bottom surfaces and that from the top includes a portion that could be absorbed by a second such layer – and so on. So the number of such layers matters and increasing CO2 does lead to a slightly increased surface temperature (but the effect of successive layers diminishes so that the overall expected warming is only (approximately) logarithmic, with successive doublings of the CO2 concentration each giving just about the same small temperature increase). But even just a “small” increase can have a big environmental effect – especially if it is unusually rapid in geological terms and relative to the capacity of species (and economies) to adapt.