CO2 Saturation

This is a favourite old argument from the skeptics, and if you read the believers blogs, you will note they always put a strong emphasis on "favourite old" to make us think skeptics are several decades behind in their reasoning here. The fact as I see them are: CO2 is a greenhouse gas and absorbs infrared radiation in a certain narrow bandwidth which coincides with the infrared emissions of the earth. The absorption spectrum of CO2 does not cover the entire earths emission spectrum, and the in the bandwidths it is covering, it is doing a pretty good job already - so much so that it is blocking 50% of the radiation escaping to space in these bandwidths. It turns out that the atmosphere can only block half the radiation that the surface emits, as every photon it absorbs invariably results in a new photon being re-emitted.  Half go up towards space, and half go back down to the surface. The result being that as the greenhouse gas becomes saturated it simply becomes a conduit for passing radiation up and down through the atmosphere

Earths Infra-red spectrum

(from )

The above simplified graph unfortunately has no scale, but does depict nicely the way different greenhouse gases may be currently affecting the earths infrared emission spectrum.

It is now imperative to cut to the proponents argument. I make no claim that these arguments are new and have not been considered by climate scientists. Spencer Weart makes a good defense of the position that CO2 bandwidth saturation is no obstacle to global warming. He reminds us that the simple assumotions I make assume the atmosphere acting like a simple barrier to escaping infrared rather than the complex multi-layered system responsible for all our weather.


The main crux of the argument against CO2 saturation is that at the upper levels of the atmosphere the atmosphere is drier, so water vapour levels are low. The atmosphere is not just acting as a shield to stop the passing of radiation, but is in itself re-emitting infrared light from every vibrating gas particle. According to Spencer, the earth is warming from the top up. Spencer draws the analogy of a dam wall flooding a valley - raising the dam wall will raise the water level in the reservoir, just as increasing CO2 in the atmosphere raises the energy potential (ie temperature).

No doubt I have poorly summed up a complex argument, yet, I still find myself falling back to the position that no matter how complex the system is, the potential to raise the earth's temperature by further blocking infrared in the CO2 absorption bandwidth is entirely limited to blocking the small trace of infrared currently escaping the earth in this bandwidth zone. Half of this (ie a doubling of CO2) would currently account for maybe 1% of the total radiation escaping the earth, and corresponds to a temperature rise at the surface of about 1.5 degrees.

To use the dam analogy in a more accurate manner, I would say the atmosphere is like a dam wall made of layers of clay and sand. The water escapes the damn by seeping through the porous pile of earth.
The clay layers represent part of the atmospheres absorption spectrum where it is difficult for radiaation to escape through, while the sand layers are like the gaps and shoulders between the different absorption bandwidths of the greenhouse gases absorption peaks. An engineer looking at the seepage would probably note that 99% of the water escaping the dam is going through layers of porous sand, and only one percent is seeping through clay. The engineer would surely recommend focusing efforts on reinforcing the sandy layers rather than the clay layers, however if he were to cement up the face of the clay perfectly, this would only divert more water through the porous sand layers and force the level of the reservoir up a non-staggering 1%. It would not really matter how complex the seepage process was inside the dam wall, stopping the seepage through the clay will not block any more seepage than what it is currently letting through?

Here I leave this argument, no doubt those who believe in human induced climate warming would say that the secondary effect of warming is to increase the level of water vapour in the atmosphere, which further increases the overall effect. A topic I will cover in an upcoming post.