MODTRAN: The official greenhouse effect calculator

As a follow-up to the model of CO2's action based on traditional physics, I've been taking a look at MODTRAN, the official piece of computer software for estimating the effect of greenhouse gases.  This is one of the fundamental components of the computer models of the climate so oft-quoted by the IPCC.

The University of Chicago has an online version of MODTRAN where you can experiment with different scenarios and see what happens. It covers a wide range of conditions and various greenhouse gases. 

I was mainly interested in what would happen if CO2 were the only greenhouse gas- would the results match the predictions of my own model? Therefore I ran a series of calculations with all other gases set to zero. Below are the outputs for CO2 levels stating at 5 parts per million.

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As you can see, the 'notch' caused by CO2 is well established by 20ppm, and doesn't significantly change above that level, all the way up to the present day level of 400ppm.

This is the most important point. The greenhouse effect of CO2 is established at quite low levels, and adding more, even up to several times the present level, doesn't make very much difference.

I think it's worth repeating here that these results are from the official progam used by climate scientists.

Drawn to a logarithmic scale, the greenhouse effect increases by an equal amount for each doubling of concentration. 

This matches the results of my own computed model to a satisfactory degree. Which in turn matches quite well the predictions of Arrhenius' Equation for CO2 infrared effects.

The important point from a question of looming climate catastrophe, is just how little difference in greenhouse effect there is between the pre-industrial 270ppm and the current 400ppm. It's hard to see how that could cause the planet to explode.

If we go beyond present day CO2 levels, even up to 1600ppm which would be equivalent to around 10 times the present level of fossil fuel usage, there is only a 10% increase in greenhouse effect over present levels. The resultant warming effect will be an even lower percentage than the increase in forcing, since CO2 is not the only greenhouse gas. Thus it's unlikely that even this massive increase would have any drastic effects on the climate.

Spot the difference. Can this really cause the planet to explode? Just to see what happens, let's we take it to some crazy level of CO2, say 4%, which is about the most humans can tolerate without breathing difficulty and disorientation. This arises not through CO2 being poisonous (it isn't) but due to a simple inability to exhale the CO2 produced by body when there's more of it outside than inside. At this level in the atmosphere we do see some additional greenhouse effects appearing, notably the absorbtion notches near the peak of the earth's emission range.  At 10% these become even more prominent.

What this tells us, is that experiments using a jar or tube filled with concentrated carbon dioxide -such as have often been put forward as proof of the danger posed by human CO2 emissions- are not a valid way to measure the effect of 400 parts per million of the gas. The behaviour of CO2 under such extreme conditions is not an analog of its behaviour at lower concentrations. Likewise, no inference can be drawn regarding the Earth, from the fact that Venus has a nearly pure CO2 atmosphere and an extremely hot surface. At these huge concentrations, the rules of the ballgame change.

Other analyses done with this program indicate that water vapour's greenhouse effect renders insignificant  the effects of carbon dioxide at relatively high humidity levels.

A further question which can be answered by MODTRAN, is whether the activists' claim that methane is 'An extremely powerful greenhouse gas' holds-up under examination. In terms of effect per quantity it is more powerful than CO2, but at the very low levels present it has little effect. It would take an increase in methane concentration of 100 times the present level to have approximately the same effect as an 8-fold increase in CO2.

The perhaps surprising thing about the online MODTRAN used here, is that it was developed by a climate activist, David C. Archibald, who predicts climate catastrophe. So, I can hardly be accused of allowing myself to be misled by false data posted by a so-called 'Denier' in order to deceive people. If the doomsayers' own data doesn't seem to support the predictions of doom... where does that leave us?