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Re: FTIR, Carbon Dioxide, and Global Warming

Very interesting Janice.I would also like to see some
comments from those who have some knowledge in this field. At are some relevant notes and references.
PS The above gives three large absorption bands for CO2 in the infra red at around 2.7,4.3,and15 microns.

Re: FTIR, Carbon Dioxide, and Global Warming


You are quite right about gases absorbing energy directly from radiation passing through it occurring only in specified wavelength bands. However this is not the only way that a gas warms up, you have to take into consideration convection & conduction.

It may help to run through a simple descriptive "sanity check" of the situation as it pertains to global warming remembering, of course, that any attempt to get quantitative or even qualitative numbers has to be far more subtle.

Its safe, to a descriptive first approximation at least, to assume that all atmospheric gas absorbtion bands are saturated so no thermal radiation from the surface can escape through them. The relevant parameters are then direct thermal radiation from the atmosphere and band restricted thermal radiation from the surface where all absorbtion bands are assumed to have zero transmission.
The earth and atmosphere system is in radiation balance with outgoing thermal radiation equalling incoming solar radiation. Hence, in principle, it is possible to calculate what the surface and atmospheric temperatures need to be to achieve balance. The atmosphere is warmed both by direct energy absorbtion within the appropriate bands and by conduction/convection mechanisms scrubbing heat energy direct from the surface.

Given that the the overall system can be considered, over the short term at least, to be in a stable state and that the absorbtion bands are saturated changes in atmospheric temperature require corresponding changes in the ability of the atmosphere to take up heat directly. This corresponds to changes in the partial pressures of the various gases involved whether relative from composition variation or absolute from quantity changes.

It is, of course, self evident that any global warming effect must include a rise in atmospheric temperature. Reference to the bulk properties of gases and a little thought will show that the large changes predicted by the scary wing of the GW religion require impossibly large changes in atmospheric composition to generate the predicted rise.



Re: Re: FTIR, Carbon Dioxide, and Global Warming

Not sure I quite understood your penultimate paragraph there, Clive. If there are more CO2 molecules in the atmosphere than IR quanta to be absorbed, i.e. the atmosphere is opaque to that waveband it is surely the opposite of saturation.
Given that the atmosphere is opaque to those bands, for the heat to be in balance we need to know how it is radiated. Presumably the excited molecules re-emit the energy at a lower frequency (longer wavelength) which can penetrate the atmosphere.
In any case, surely Janice must be right if she says that with an atmosphere totally opaque to the CO2 absorption bands it is irrelevant how much the CO2 concentration increases above the threshold necessary to absorb all the energy in those wavebands. Rather like Charles II, though, I'm not sure of the premise.

Re: Re: Re: FTIR, Carbon Dioxide, and Global Warming


Please accept my apologies for not being completely clear, especially as I was trying to clarify matters! Another case of where knowing exactly what you mean to say makes it impossible to see how what you have written appears to others.

As Janice and yourself say transmission in the CO2 absorption bands falls to essentially zero once sufficient CO2 has been traversed. Whether its a short path at high concentrations of CO2 or a long one at low concentrations does not matter. All the IR energy in the bands will have been absorbed and maximum radiative energy transfer from the ground to the atmospheric CO2 achieved.

However CO2, along with all the other gases in the atmosphere, can also pick up heat by the contact, conduction and convection method. Hence if the quantity of CO2 in the atmosphere changes the capacity of the atmosphere to take up heat from the ground by these mechanisms will also change and hence the absolute amounts and relative balance of radiation from ground and atmosphere will also change to maintain overall radiation equilibrium with incoming solar radiation.

Assuming increased CO2 does not displace any other components it simply adds to the quantity of gas in the atmosphere so increasing the thermal capacity and displacing the ground/sea/atmosphere temperature equilibrium.

Obviously what really happens is fearsomely complicated but, as I understand things, extra atmospheric heat capacity leads to higher surface temperatures but nowhere near that predicted by the standard global warming scenario.

FTIR, Carbon Dioxide, and Global Warming

In the case of the primary carbon dioxide absorption bands in the IR spectra, there does not seem to be any heat being emitted back into space from those particular bands, thus they are saturated. The FTIR spectroscopist that pointed this out to me, has indicated that the saturation appears to occur within about 40 to 50 feet of the black body that is emitting those IR spectra (in this case the earth itself).

For radiation in other than the IR bands, collisions are causing heat, and in that case the composition of the atmosphere is irrelevant. Collisions do not care what is colliding. As I have been able to understand it, in non-IR radiation bands, heat capacity is not relevant.

However, the real problem comes when the data shows that only 0.28% of the energy that gets to the ground is eventually radiated back into space. We are already absorbing 99.72% of the total energy as either kinetic energy (which is where heat comes from), or potential energy (which is mostly stored as work done in evaporation, or as part of living things). Let us say that we CAN change how much heat we can absorb by increasing the carbon dioxide. There is only another 0.28% that can be absorbed. If we assume that space is at zero Kelvin (it is a little above that), and that earth is at 300 Kelvin, then changing that temperature difference by 0.28% is only going to raise the temperature by 0.84 Kelvin. And only part of that is going to be kinetic energy, because some will go into potential energy.

Re: FTIR, Carbon Dioxide, and Global Warming


I'm a bit worried by the figures you give.

40 to 50 ft saturation distance for atmospheric absorption bands is far, far too short. I spent a good deal of time over a 10 year period dealing with what might be called conspicuousity problems in long range thermal imaging. Much of the work was on getting good agreement between theoretical predictions, laboratory measurements and field trials results. It was never safe to assume saturation of any absorption band. For accurate results we had to factor in variations due to weather conditions and often resorted to direct measurement using a 4W frequency agile CO2 laser. These days the safety officer would have had conniptions, in those days we just didn't bother to tell him WTHWGO.
Its a long time since I actively used it I think the LOWTRAN model, which we used in those days and appeared pretty accurate, indicates that something like 50 to 100 KM horizontal sea level path is needed for saturation in the major bands under clear air conditions. The IR Handbook or Handbook of Infra Red Technology should have useful plots. (This trollop left his copies behind on redundancy!)

Only 0.28% of energy reaching the ground returning to space simply cannot be right. Radiation balance requires that the incoming and outgoing thermal and reflected radiation sums be equal. Non thermal absorption will be pretty small with, I imagine, biological photosynthesis being the main contributor and even here most of the energy appears, to this atomic physicist manqué, to end up as heat. Biologists and chemist may know different.

I'd have thought that 0.28% of the energy returning to space would have made the earth a very dim body. Certainly not the bright, beautiful blue planet seen in photographs from space vehicles. My practical experience in this area is very limited but, when working with 3 to 5 micron thermal imaging systems on moderate temperature targets we ran into major problems of target to background contrast reversal between day and night time conditions. I eventually managed to nail down this problem and sort out a sensible calculation model taking good enough account the spectral characteristics of background and target that the effect could be exploited rather than filtered out. Again I no longer have access to the figures but the reflectivity's would have been in the order of 20 to 70 % in the useable parts of the 3 to 5 micron spectrum depending on source and wavelength. As you know there is effectively only one major absorption band in the 3 to 5 region, the atmosphere is very transparent elsewhere.


Re: FTIR, Carbon Dioxide, and Global Warming

The OP is essentially correct that our atmosphere is opaque to radiation in the infrared part of the spectrum. But wrong to assume this means
"even if you went from ppm of carbon dioxide to 1 or 2 percent of carbon dioxide, it would make no difference to the amount of heat being absorbed by our atmosphere"

I'm not at all qualified to be trying to explain this, but i'll give it a shot. I think the wikipedia article at explains it fairly well.

Very little infrared radiation directly from the sun makes it to the surface. Infrared present at the surface is mostly black body radiation from objects heated by other parts of the spectrum, some of which will be emitted in the absorption bands of h2o and co2.
What the concentration of these gasses determines is
1. How close to the surface this infrared is re-absorbed
2. How close to the edge of the atmosphere you have to be before any infrared can be emitted back into space.

If you increase the concentration of co2 you reduce the amount of the atmosphere that can radiate back into space, thus less radiation from earth, thus temperature must increase, as radiation is the only means of heat transfer in a vaccum.
Increase the concentration of greenhouse gasses far enough and you end up with a planet like venus.

I'd be interested to know where you got this

"However, the real problem comes when the data shows that only 0.28% of the energy that gets to the ground is eventually radiated back into space."

If the earth is to remain at a (relatively) stable temperature then 100% of energy that gets to the ground must logically be radiated back into space. This seems like a misreading of some sort. Maybe only 0.28% is directly radiated back into space?

Re: Re: FTIR, Carbon Dioxide, and Global Warming

Everyone here has to be familiar with

The real inconvenient truth.

He has a few good diagrams of the heat balance a little way down the page.

My shortened analysis.

Global warming is a good thing, it makes it nice and confortable here on the planet. Without the "Greenhouse" effect, we would be living on Hoth. The greenhouse gasses are largely created naturally. If we (the US) were to magically stop creating C02, the carbon cycle wouldn't even hiccup. The greenhouse system wouldn't even know we had dissappeared. The planet would be exactly as it was (and chances are it would be worse, law of unintended consequences being what it is).

Janice's observations are vital to understanding the demagoguery surrounding this subject.


Exxon, please send me my check... I need the money. My address is ....

People like us are supposed to be getting lots of money from the Anti-anti-global warming crowd aren't we.