Is Greenhouse Gas the Main Cause of Global Warming? Experiments and Evidence

[sophiecentaur]

"The effective temperature of a planet can be calculated by equating the power received by the planet with the power emitted by a blackbody of temperature T."

That statement is true, as far as it goes, but it only describes the temperature which corresponds to the em spectrum as re-radiated into space. It doesn't consider what could be going on on the surface (part of which could be covered with a vast Dewar vessel, which would allow the surface to reach almost any temperature you wanted). I does make me wonder about the certainty 'they' seem to have about suitability of all those planets out there for Earth-type life - on the surface.

 

[JDoolin]

And it becomes more likely to be wrong.
Here is an aspect of reality that is not tracked properly:

The Antarctic emission spectra shows an increased heat emission due to green house gases. That does not fit to the results of simple experiments as suggested above because they do not include the influence of the temperature gradient in the atmosphere. Without a temperature gradient there would be no green house effect. Global warming works with a negative temperature gradient only. I'm not aware of simple experiment which demonstrate this effect correctly.

Carbon Dioxide is opaque right around the 667/cm line. And apparently the top surface of the Carbon Dioxide "cloud" is emitting into space at about 220 Kelvin. It's fairly consistent, though the top of the cloud is quite a bit colder in the Antarctic Region than it is in the Sahara region or the Mediterranean... But it seems to suggest that the top layer of the Carbon Dioxide is in sort of a thermal equilibrium near 220 Kelvin.

To me, I think the simple word for "Global Warming" is "insulation" so it makes sense that the top surface of this insulation is all near the same temperature, even though what's below it is colder in some places, and warmer in others.

The atmosphere is heated from the bottom by Infrared light from the Earth's surface, and it's heated from the top by solar wind and ultraviolet radiation. The insulation from ultraviolet light reaches a minimum temperature at the top of the exosphere. It basically 100% stops the ultraviolet light from coming in, because that bottom layer of gas isn't nearly hot enough to emit in the ultraviolet spectrum. On the other hand the top layer of Carbon Dioxide gas in the troposphere IS hot enough to emit in the infrared spectrum.

 

[JDoolin]

I think the question of Greenhouse Gas effects is 100% answered. Consider the Earth and the moon. Both experience the same solar energy flow (1.36 kW/m^2 of cross sectional area). The moon is a less reflective surface than our "blue marble". It reflects back about 12% of the sunlight that strikes it. The Earth surface AVERAGE reflects back about 30% of the sunlight that strikes it.

Now both bodies are in thermal steady state. The moon must re-radiate the remaining 88% of the sunlight it has landing on it, and the Earth must re-radiate the remaining 70% of the sunlight it has landing on it. Yet the Earth is MUCH warmer than the moon. The reason is that the Earth has an atmosphere. The atmosphere interferes with that re-radiation, by re-capturing part of that emission. The Earth atmosphere is the reason why the Earth (average temperature) is warmer than the moon (average temperature), even with less solar energy absorbed.

You might still question how significant the changes in CO2 concentration are, in the net effect of the atmospheric blanket.

Well, yes, I do question how significant the changes in CO2 concentrations are... But I think I can make that question more explicit:

From http://www.skepticalscience.com/saturated-co2-effect-advanced.htm

What does it mean when it says "if we recall that the absorption coefficient is gaussian"? Why is it that the "absorption dip ... becomes wider"

 

[sophiecentaur]

To me, I think the simple word for "Global Warming" is "insulation"

I like that a lot. The term is still user friendly and could have sold just as well.

 

[JDoolin]

I like that a lot. The term is still user friendly and could have sold just as well.

Really? To me calling it "insulation" is sort of like opening up a pandora's box of questions.

I can imagine three different kinds of insulation

Conductive insulation (prevents heat from passing between two touching particles)
Radiative Insulation (prevents heat from passing via electromagnetic waves)
Convection Insulation (prevents the bulk movement of material)

When an object or gas is opaque in some wavelengths, and transparent in others, it behaves as a bandwidth dependent radiative insulator.

But there seem to be other situations that defy categorization: In the sun, there is a region called the "radiation zone" where the primary means of heat tranport is via photons instead of direct exchange of momentum of the atoms. So even though the particles are close together, there's no actual "conduction" of heat. All the heat exchange is accomplished through radiation. Yet within that radiation zone there is a significant drop in temperature.

We have objects orbiting in the thermosphere that lose energy by radiation, but gaining energy through conduction. The thermosphere doesn't provide radiative insulation in the infrared. But the walls of the spacecraft s do provide convective insulation against the thermosphere.

 

[JDoolin]

It not a question of defending it more to do with trying to understand if any of the models we have are correct.
I can't see how such a small percentage of the greenhouse gas Co2 in the atmosphere can play such a major role in global warming when it's compared to the far larger amount of water vapour.Which is considered a more powerfull retainer of heat by comparison given an equal volume of both.

.

Two reasons I can think of.
#1 we aren't pumping additional water vapour into the atmosphere that wasn't there before.
#2 Search google images for "infrared emission spectrum of water vs carbon dioxide". You'll see that while water vapor does absorb infrared light, it doesn't absorb the same wavelengths of infrared light that Carbon Dioxide does.

 

[sophiecentaur]

Really?

Well - I got a response.!
Yes, any single term is a pandora's box; "greenhouse" is, as we've seen, a massive one, too. People will always grasp onto a single idea an try to fit it to a complex situation. Heat, mechanics and electricity are full of such examples. I just thought that 'insulation' was a bit less specific than 'greenhouse'. But we can't put the toothpaste back in the tube, on this one.

 

[JDoolin]

Well - I got a response.!
Yes, any single term is a pandora's box; "greenhouse" is, as we've seen, a massive one, too. People will always grasp onto a single idea an try to fit it to a complex situation. Heat, mechanics and electricity are full of such examples. I just thought that 'insulation' was a bit less specific than 'greenhouse'. But we can't put the toothpaste back in the tube, on this one.

Okay, rather than "insulation" which is catchy but ambiguous, let's call it "bandwidth dependent radiation insulation"... Not quite as catchy, but more specific than "greenhouse".

I think classifying the different types of insulation, into radiative insulation, convective insulation, and conductive insulation may be helpful in understanding how heat moves via convection, conduction, and radiation. Then you can understand, to some degree, the analogy with the green house... In the greenhouse, the glass is a convective insulator because it is made of a solid substance, and it is a bandwidth dependent radiative insulator because it passes light in the visible spectrum, but stops light in the infrared.

In the Earth's atmosphere, Carbon Dioxide is a bandwidth dependent radiative insulator because it passes light in the visible spectrum, but blocks light in the infrared. However, does it provide any convective insulation? Actually, it's Earth's gravity that provides convective insulation, preventing gasses from escaping into space. Also, somewhere in the stratosphere the magnetic fields and stratosphere provide convective insulation against the solar wind, and bandwidth dependent radiative insulation against Ultraviolet and X-Ray radiation. which is why the thermosphere is like, 1200 Kelvin.

 

[votingmachine]

Well, yes, I do question how significant the changes in CO2 concentrations are... But I think I can make that question more explicit:

From http://www.skepticalscience.com/saturated-co2-effect-advanced.htm

What does it mean when it says "if we recall that the absorption coefficient is gaussian"? Why is it that the "absorption dip ... becomes wider"

I can comment on the "guassian" part.

First, I would never say an absorption coefficient is Gaussian. An absorption coefficient is by definition at a specified wavelength. Any absorption of radiated energy is best modeled as a single quanta of energy, that raises an internal system to an allowed/stable excited energy level. The easy ones to understand are the hydrogen atom with it single electron moving up and down between the orbitals. Infrared is energy that corresponds to the molecular bonds between atoms.

If the molecules were perfectly the same in every state, then the absorption would be exactly at a single wavelength. But the reality is that there are always slight differences. Consider two excited CO2 molecules that emit exactly the same photon of energy. But one is traveling toward you, and the photon is slightly blue shifted, and one is traveling away, and the photon is slightly red shifted. The average would then be at the non-moving, but there would be guassian distribution fom velocity around that. Consider that molecules are colliding. There can be a deformation of the energy states of each molecule by that collision. Say that the bond length of one molecule is momentarily shorter and the other is bent to momentarily longer. The absorption energy levels are then changed by that tiny amount. It will again be guassian around the mean. If the molecules are constantly colliding in a gas, and they are constantly vibrating as a result, then there can be quite a bit of spread to what might otherwise be a very perfectly defined energy gap. We just can't measure absorption in perfect isolation, so we end up with every spectroscopic peak being wider than a single wavelength.

Your link kept crashing on me so I can't fully comment on its arguments. But ...
I'm not sure I follow the argument about CO2 saturation. Anytime you work with a spectrometer, you have a beam of light. You can always stick a piece of cardboard in, and block the light. A second piece of cardboard doesn't see any light. Once you've blocked it all, you've blocked it all. There is always a goal in measuring absorption of a chemical to use the lowest possible concentration. The Beer-Lambert law applies. I've followed chemical reactions using spectroscopy, and it is important to know the extinction coefficient for the wavelength you are using, and to not have the particular chemical outside of the concentration window for which that applies. In a spectrometer, you use a defined path length ... for the atmosphere, it could be quite a bit longer.

Simply saying that CO2 shows saturation of absorption is sort of obvious. Everything does. The bottom of the ocean is quite dark, even though a meter of seawater is pretty transmitting of light. Go far enough, and it adds up. The same is surely true with anything. Longer path-lengths and higher concentrations will deviate from Beer's law.

Look at the spectra collected from space shown in post 771. Maybe there is an argument for CO2 absorption already being maximized at the central wavelength. I'm not convinced that the peak broadening is insignificant though.

 

[Buzz Bloom]

Hi JDoolin:

The atmosphere is heated from the bottom by Infrared light from the Earth's surface

The quote above is not quite right. The atmosphere is heated from the bottom by conduction, not radiation. Then there is a convection effect: the heated air rises and is replaced by cooler air which is then heated by conduction.

The insulation from ultraviolet light reaches a minimum temperature at the top of the exosphere. It basically 100% stops the ultraviolet light from coming in

This quote is correct, but raises an interesting, but still currenty, a controversial issue.
See http://www.science.uwaterloo.ca/~qblu/qblu_website/Research_files/QingBinLu.pdf
especially Figure 3 on page 5. Lu's argument is that during the unusual very rapid rise in global temperature between 1975-2005, CO₂ and the greenhouse gas effect were the usual suspects, but NOT the culprits. The cause was CFCs. During this period, CFCs built up which caused the ozone hole which allowed UV radiation to reach the Earth which had previously been blocked by ozone. This extra UV radiation hitting the Earth cause this particular 30 year unusually high rate of rise in Earth's temperature.

In 1989 the Montreal Protocol which banned almost all uses of CFCs went into effect. Gradually over the next 15 years the ozone layer was substantially was returned to normal. Then the excess heat that had built up, and which had been mostly stored in the oceans, began to re-radiate this excess, thereby decreasing the Earth's temperature toward what it woul have been if the CFCs had never been used.

See https://en.wikipedia.org/wiki/Montreal_Protocol .

Regards,
Buzz

 

[JDoolin]

Hi JDoolin:
The quote above is not quite right. The atmosphere is heated from the bottom by conduction, not radiation. Then there is a convection effect: the heated air rises and is replaced by cooler air which is then heated by conduction.

I totally agree with you that the bottom of the atmosphere is heated by conduction, and I left that out. But there is also heat escaping from the ground by radiation. Some of that radiation travels a few meters, and some of it travels a few kilometers before being absorbed.

Earlier I referenced the radiation zone in the sun, where, supposedly the transfer of heat from the core of the star out to the convection zone. They call it the radiation zone because convection and conduction are not the primary modes of heat transfer. But I think both modes of heat transfer are present.

This quote is correct, but raises an interesting, but still currenty, a controversial issue.
See http://www.science.uwaterloo.ca/~qblu/qblu_website/Research_files/QingBinLu.pdf
especially Figure 3 on page 5. Lu's argument is that during the unusual very rapid rise in global temperature between 1975-2005, CO₂ and the greenhouse gas effect were the usual suspects, but NOT the culprits. The cause was CFCs. During this period, CFCs built up which caused the ozone hole which allowed UV radiation to reach the Earth which had previously been blocked by ozone. This extra UV radiation hitting the Earth cause this particular 30 year unusually high rate of rise in Earth's temperature.

In 1989 the Montreal Protocol which banned almost all uses of CFCs went into effect. Gradually over the next 15 years the ozone layer was substantially was returned to normal. Then the excess heat that had built up, and which had been mostly stored in the oceans, began to re-radiate this excess, thereby decreasing the Earth's temperature toward what it woul have been if the CFCs had never been used.

See https://en.wikipedia.org/wiki/Montreal_Protocol .

Regards,
Buzz

I see the title of Lu's paper is "What is the Major Culprit for Global Warming: CFCs or CO2?"

My impression was hat the Ozone Hole was a much MORE major culprit than the carbon dioxide. So we stopped using aerosols, and sealed up our Freon air-conditioners, and the problem was pretty well solved by adjusting our technology.

Meanwhile Carbon Dioxide is a LESS major culprit, and the methods of adjusting our technology are much more difficult.

 

[JDoolin]

I can comment on the "guassian" part.

First, I would never say an absorption coefficient is Gaussian. An absorption coefficient is by definition at a specified wavelength. Any absorption of radiated energy is best modeled as a single quanta of energy, that raises an internal system to an allowed/stable excited energy level. The easy ones to understand are the hydrogen atom with it single electron moving up and down between the orbitals. Infrared is energy that corresponds to the molecular bonds between atoms.

If the molecules were perfectly the same in every state, then the absorption would be exactly at a single wavelength. But the reality is that there are always slight differences. Consider two excited CO2 molecules that emit exactly the same photon of energy. But one is traveling toward you, and the photon is slightly blue shifted, and one is traveling away, and the photon is slightly red shifted. The average would then be at the non-moving, but there would be guassian distribution fom velocity around that. Consider that molecules are colliding. There can be a deformation of the energy states of each molecule by that collision. Say that the bond length of one molecule is momentarily shorter and the other is bent to momentarily longer. The absorption energy levels are then changed by that tiny amount. It will again be guassian around the mean. If the molecules are constantly colliding in a gas, and they are constantly vibrating as a result, then there can be quite a bit of spread to what might otherwise be a very perfectly defined energy gap. We just can't measure absorption in perfect isolation, so we end up with every spectroscopic peak being wider than a single wavelength.

Thank you for that. That gives me something that I can play with in my head; to see how it works out with pressures and the Gaussian distribution of velocities. I guess I still have the trouble with how increasing the concentration of Carbon Dioxide molecules will necessarily increase the width of the Gaussian distribution of velocities. I don't think it would affect the overall pressure of the gas by much...

Your link kept crashing on me so I can't fully comment on its arguments. But ...
I'm not sure I follow the argument about CO2 saturation. Anytime you work with a spectrometer, you have a beam of light. You can always stick a piece of cardboard in, and block the light. A second piece of cardboard doesn't see any light. Once you've blocked it all, you've blocked it all. There is always a goal in measuring absorption of a chemical to use the lowest possible concentration. The Beer-Lambert law applies. I've followed chemical reactions using spectroscopy, and it is important to know the extinction coefficient for the wavelength you are using, and to not have the particular chemical outside of the concentration window for which that applies. In a spectrometer, you use a defined path length ... for the atmosphere, it could be quite a bit longer.

Come to think of it, I found their argument on CO2 saturation difficult to follow as well.

I think the trouble is, calling it an argument is a little bit confrontational. Let's call it a conjecture. The conjecture is that the Carbon Dioxide is already absorbing all of the light it can. So adding more Carbon Dioxide won't affect the situation significantly.

For an analogy, think of an ordinary greenhouse, made with glass ceiling. How much would it affect the temperature inside if you added another pane of glass? If the greenhouse effect is already saturated, then the light that is blocked by the second pane was already blocked by the first, and the light that was transparent to the first pane is also transparent to the second pane.

If the greenhouse effect is NOT already saturated, then some of the light was partially blocked by the first pane, and it will become MORE blocked by the second pane.

Simply saying that CO2 shows saturation of absorption is sort of obvious. Everything does. The bottom of the ocean is quite dark, even though a meter of seawater is pretty transmitting of light. Go far enough, and it adds up. The same is surely true with anything. Longer path-lengths and higher concentrations will deviate from Beer's law.

Look at the spectra collected from space shown in post 771. Maybe there is an argument for CO2 absorption already being maximized at the central wavelength. I'm not convinced that the peak broadening is insignificant though.

One meter below the sea, the "seawater effect" is not saturated. If you have enough depth, then it becomes saturated.

(Finally, I think you only gave a partial reference to post 771. I'm not sure what website you're referring to.)

 

[DrStupid]

The Beer-Lambert law applies.

Not for heat radiation in the atmosphere. The Beer-Lambert law is a special solution of the equation of radiative transfer for negligible emission. This condition is not fulfilled in the atmosphere with the result that CO2 saturation is impossible. There will always be a significant emission within the absorption bands of the green house gases - no matter how much of them is in the atmosphere. The intensity depends on the temperature of the atmosphere at the altitude of emission.

 

[Buzz Bloom]

Some of that radiation travels a few meters, and some of it travels a few kilometers before being absorbed.

Hi JDoolin:

Please take a look at my post #28. There may be some kinds of air molecules that behave differently, but I think the mechanism I described in post #28 for CO₂ is the much more usual behavior when an IR photon is absorbed. It doesn't heat the air molecule because the molecule re-radiates an equivalent photon much more quickly than the time it takes for the molecule to interact with another molecule. The molecule-molecule interaction is necesary for a photon's energy to result in an increase in the kinetic energy of a molecule, thereby raising the temperature of the air. Therefore, no matter how far the radiation travels before being absorbed, it doesn't heat the air.

 

[JDoolin]

Not for heat radiation in the atmosphere. Beer-Lambert law is a special solution of the equation of radiative transfer for negligible emission. This condition is not fulfilled in the atmosphere with the result that CO2 saturation is impossible. There will always be a significant emission within the absorption bands of the green house gases - no matter how much of them is in the atmosphere. The intensity depends on the temperature of the atmosphere at the altitude of emission.

Do we have a way of defining the boundary between "negligible emission/absorption" and "significant emission/absorption"

Clearly CO2 has significant emission/absorption in the 667 cm line. I think that we can say the CO2 spectrum is "saturated" there; It's already absorbing 100% of that light. Doubling or tripling the concentration should keep it at 100%--no additional heat absorbed. (Then, it is still possible in that case that if the gaussian distribution of velocities of Carbon Dioxide were widened, you could still get more heat absorption.)

And then there's regions where there is "insignificant emission/absorption". Maybe it absorbs .01% of the light. In those areas, the gas is transparent. Insignificant means essentially zero, and doubling, tripling the concentration of the gas wouldn't change it's transparency. It would go up to .02% or .03% but it would still be insignificant.

But is there something in between--bandwidths where it's currently absorbing 20% of the light, and doubling or tripling the concentration would make it absorb 40% or 60% of the light?

 

[Buzz Bloom]

Meanwhile Carbon Dioxide is a LESS major culprit, and the methods of adjusting our technology are much more difficult.

Hi JDoolin:

I belief that Lu's research shows that for 30 years CFCs did more heating of the Earth than CO₂, and would likely have continued to do so without the Montreal Protocol. However, the banning of CFCs was not because of their Earth warning effects, because that understanding is even now controversial. On the other hand, CO₂ has been since industrial revolution, and continues to be now the major culprit for global warming, with methane in second place. Water vapor also is a major greenhouse gas, but that's more as a feedback mechanism which exacerbates the effects of CO₂ and methane.

Regards,
Buzz

 

[JDoolin]

Hi JDoolin:

Please take a look at my post #28. There may be some kinds of air molecules that behave differently, but I think the mechanism I described in post #28 for CO₂ is the much more usual behavior when an IR photon is absorbed. It doesn't heat the air molecule because the molecule re-radiates an equivalent photon much more quickly than the time it takes for the molecule to interact with another molecule. The molecule-molecule interaction is necesary for a photon's energy to result in an increase in the kinetic energy of a molecule, thereby raising the temperature of the air. Therefore, no matter how far the radiation travels before being absorbed, it doesn't heat the air.

Let me see if I can figure out what you're trying to say.

There are some kinds of molecules such as N₂ and O₂ that behave differently from greenhouse gasses. These gasses are essentially transparent in the infrared. If there were no Carbon Dioxide or water vapor or in the atmosphere, radiation would be completely unhindered in leaving the dark-side of the planet.

Then the only mechanism by which heat transferred through the atmosphere would be conduction.

I'm not entirely sure how to model that environment, but I think that the atmosphere of the planet would simply not radiate any spectrum at all. If the gas is too cold to activate it's emission modes, you would just get the spectrum of the planet's surface.

But because we have infrared radiation absorbed by Carbon Dioxide and water vapor, they also absorb infrared light which means, they are catching energy from the ground that would otherwise have radiated freely into space. They are then transferring that energy via conduction to the Nitrogen and the Oxygen in the system.

So what do you think? Does the Carbon Dioxide and Water Vapor hold onto the radiation energy for long enough to transfer it to the Nitrogen and Oxygen? Or does it kick out the infrared photon so quickly that it doesn't affect the temperature of the gas?

 

[votingmachine]

Not for heat radiation in the atmosphere. The Beer-Lambert law is a special solution of the equation of radiative transfer for negligible emission. This condition is not fulfilled in the atmosphere with the result that CO2 saturation is impossible. There will always be a significant emission within the absorption bands of the green house gases - no matter how much of them is in the atmosphere. The intensity depends on the temperature of the atmosphere at the altitude of emission.

I agree. I thought that was what I was saying. It must not have been clear. There are conditions for which you can use an extinction coefficient for absorption vs concentration and conditions when you cannot.

 

[votingmachine]

Thank you for that. That gives me something that I can play with in my head; to see how it works out with pressures and the Gaussian distribution of velocities. I guess I still have the trouble with how increasing the concentration of Carbon Dioxide molecules will necessarily increase the width of the Gaussian distribution of velocities. I don't think it would affect the overall pressure of the gas by much...
Come to think of it, I found their argument on CO2 saturation difficult to follow as well.

I think the trouble is, calling it an argument is a little bit confrontational. Let's call it a conjecture. The conjecture is that the Carbon Dioxide is already absorbing all of the light it can. So adding more Carbon Dioxide won't affect the situation significantly.

For an analogy, think of an ordinary greenhouse, made with glass ceiling. How much would it affect the temperature inside if you added another pane of glass? If the greenhouse effect is already saturated, then the light that is blocked by the second pane was already blocked by the first, and the light that was transparent to the first pane is also transparent to the second pane.

If the greenhouse effect is NOT already saturated, then some of the light was partially blocked by the first pane, and it will become MORE blocked by the second pane.

One meter below the sea, the "seawater effect" is not saturated. If you have enough depth, then it becomes saturated.

(Finally, I think you only gave a partial reference to post 771. I'm not sure what website you're referring to.)

My bad ... I wanted to refer to post #24 in this thread, with the space measurement of emitted spectra.

I think we are saying the same thing about path length. Whether it is the cumulative effect of many panes of glass, or many meters of water, there is a point where the absorption is "saturated". The bottom of the ocean would be no darker with another meter of water above it.

 

[DrStupid]

The molecule-molecule interaction is necesary for a photon's energy to result in an increase in the kinetic energy of a molecule, thereby raising the temperature of the air. Therefore, no matter how far the radiation travels before being absorbed, it doesn't heat the air.

As air is not an ideal gas it's temperature does not only depend on translational kinetic energy but also on the energy of vibration and rotation of the molecules.

It's already absorbing 100% of that light. Doubling or tripling the concentration should keep it at 100%--no additional heat absorbed.

That's wrong or misleading. You forgot to mention that the IR radiation is not only absorbed but also emitted in the same order of magnitude (which is clearly not negligible). The spectra in my post #24 show that there is around 25 % of the original IR intensity left in the Sahara region and almost 50 % in the Mediterranean spectra (which is clearly significant). The reduction of the IR intensity will be further increased by additional CO2 but it will never reach 100 %. There is no such thing like CO2 saturation.

 

[Buzz Bloom]

As air is not an ideal gas it's temperature does not only depend on translational kinetic energy but also on the energy of vibration and rotation of the molecules.

Hi DrStupid:

I think that for the concept of temperature to be meaningful, the stuff for which temperture is to be measured must be in a steady state. I accept that the vibrational and rotational energy of a gas (ideal or otherwise) may be included in averaging the energy of a mass of stuff, including a non-ideal gas like air. But how exactly is such a measurement made? A measuring device must take some time to complete the measurement. The amount of time this will take is much longer than the time for any excited molecule to spontaneously re-emit a photon which will carry away the energy of the excited state.

Can you cite any reference that shows that the temperature of a gas, ideal or otherwise, can be measured as higher when the gas is irradiated with IR?

Regards,
Buzz

 

[Buzz Bloom]

They are then transferring that energy via conduction to the Nitrogen and the Oxygen in the system.

Hi JDoolin:

This quote is incorrect. In my post # 28 I described the behavior of an excited CO₂ molecule in some detail. It very quickly gets rid of the energy of its excited state by spontaneous re-radiating it away as a IR photon. No conduction of this energy is possible to any other molecules because conduction takes longer.

Regards,
Buzz

 

[votingmachine]

Hi DrStupid:

I think that for the concept of temperature to be meaningful, the stuff for which temperture is to be measured must be in a steady state. I accept that the vibrational and rotational energy of a gas (ideal or otherwise) may be included in averaging the energy of a mass of stuff, including a non-ideal gas like air. But how exactly is such a measurement made? A measuring device must take some time to complete the measurement. The amount of time this will take is much longer than the time for any excited molecule to spontaneously re-emit a photon which will carry away the energy of the excited state.

Can you cite any reference that shows that the temperature of a gas, ideal or otherwise, can be measured as higher when the gas is irradiated with IR?

Regards,
Buzz

Surely you have seen or played with an infrared lamp? It warms things. Air is difficult to experience that warming, as it has such low heat capacity. But it does warm. If there is something in the air with higher IR absorption, then the infrared lamp will warm it more.

Hi JDoolin:

This quote is incorrect. In my post # 28 I described the behavior of an excited CO₂ molecule in some detail. It very quickly gets rid of the energy of its excited state by spontaneous re-radiating it away as a IR photon. No conduction of this energy is possible to any other molecules because conduction takes longer.

Regards,
Buzz

Critically though, the infrared that is of interest is emitted from the Earth outward, and the infrared emitted by the CO2 is in a random direction. The infrared from the Sun headed at the Earth is also intermediated by the same process.

 

[JDoolin]

The total internal energy of the gas is based on translational, rotational, and vibrational energy of the gas, but the temperature is based entirely on the translational portion of the kinetic energy.

Here's an equation from my notes.

$$U=n N_A \frac{df}{2}(\frac 1 2 m \overline{v^2})=\frac{df}{2}nRT=n C_v T = \frac{df}{2}PV$$

See the n R T is multiplied by the degrees of freedom... df=3 for an ideal monatomic gas (Argon), df=5 for an ideal diatomic gas (Oxygen, Nitrogen).

That's wrong or misleading. You forgot to mention that the IR radiation is not only absorbed but also emitted in the same order of magnitude (which is clearly not negligible). The spectra in my post #24 show that there is around 25 % of the original IR intensity left in the Sahara region and almost 50 % in the Mediterranean spectra (which is clearly significant). The reduction of the IR intensity will be further increased by additional CO2 but it will never reach 100 %. There is no such thing like CO2 saturation.

(Sorry... there are a lot of things to remember all at once. Let me repeat the question, with that in mind. I really don't think it changes the central question here, though.)

Do we have a way of defining the boundary between "negligible emission/absorption" and "significant emission/absorption"

Clearly CO2 has significant emission/absorption in the 667 cm line. I think that we can say the CO2 spectrum is "saturated" there; It's already absorbing 100% of that light (on the bottom surface) Doubling or tripling the concentration should keep it at 100%--no additional heat absorbed. (Then, it is still possible in that case that if the gaussian distribution of velocities of Carbon Dioxide were widened, you could still get more heat absorption.) (Also, if the top surface of the gas were for some reason significantly colder than 220 Kelvin it would re-emit less than 25% of the original IR intensity)

And then there's regions where there is "insignificant emission/absorption". Maybe it absorbs .01% of the light. In those areas, the gas is transparent. Insignificant means essentially zero, and doubling, tripling the concentration of the gas wouldn't change it's transparency. It would go up to .02% or .03% but it would still be insignificant.

But is there something in between--bandwidths where it's currently absorbing 20% of the light, and doubling or tripling the concentration would make it absorb 40% or 60% of the light?

 

[JDoolin]

Surely you have seen or played with an infrared lamp? It warms things. Air is difficult to experience that warming, as it has such low heat capacity. But it does warm. If there is something in the air with higher IR absorption, then the infrared lamp will warm it more.

Critically though, the infrared that is of interest is emitted from the Earth outward, and the infrared emitted by the CO2 is in a random direction. The infrared from the Sun headed at the Earth is also intermediated by the same process.

I think this sort of answers what I asked in post 47... This is what my intuition says, as well. (I shouldn't say "intuition" but rather my "working model" of greenhouse gasses) If there are no greenhouse gasses present in the air, then the air won't warm up. i.e. if it is entirely made of Nitrogen, Oxygen, and Argon. But if there are greenhouse gasses in there, the greenhouse gasses would catch the infrared light, and heat up the gas.

But then again, I've not done the experiment in detail...

I know my SKIN warms up if I aim an infrared lamp at it, but I don't know for sure whether the AIR between the lamp and my skin warms up.

 

[Buzz Bloom]

Hi votingmachine:

Surely you have seen or played with an infrared lamp? It warms things.

Of course IR radiation warms "things" - SOLID and LIQUID things, not GASSES.

Critically though, the infrared that is of interest is emitted from the Earth outward, and the infrared emitted by the CO2 is in a random direction.

As I explained in detail in post #28, HALF of the IR absorbed by CO2 (and other greenhouse gases) is re-radiated back to the Earth's surface.

The infrared from the Sun headed at the Earth is also intermediated by the same process.

This is true, but the IR from the sun is a very small fraction of the radiant energy from the sun.

Regards,
Buzz

 

[Buzz Bloom]

The total internal energy of the gas is based on translational, rotational, and vibrational energy of the gas, but the temperature is based entirely on the translational portion of the kinetic energy.

Hi JDoolin:

Thanks for correcting my mistake regarding temperature. I had thought I had remembered correctly (from ages ago) that it only included translational kinetic energy, but I wasn't sure. And even if it did include rotational and vibrational energy, that didn't affect the argument because of the speed of re-radiation.

Regards,
Buzz

 

[Buzz Bloom]

Clearly CO2 has significant emission/absorption in the 667 cm line. I think that we can say the CO2 spectrum is "saturated" there
Hi JDoolin:

I tried to find online some information about the CO2 absorbsion spectrum, but my search seems to result in a saturation by stuff put out by skeptics. Somewhere in my files I have this information, but right now it is "lost". "667 cm" cannot be right. That's 6.67 meters. That's a radio wave length. Probably a typo. For the purpose of this post, I am going to designate the wavelength you intended as w.

I am not sure what "saturated" means here. I have seen this term used in two ways:
1. All of the IR from Earth at wavelength w is currently 100% absorbed.
2. The current CO2 in the atmosphere has absorbed all of the IR radiation at wavelength w from the Earth that this CO2 can hold.

(1) has been used by greenhouse effect skeptics to argue that more CO2 cannot have any effect. I belief this argument has been thoroughly discredited. I will try to find some references and cite them in a separate post.

(2) may have been demonstrated, but I am not sure. It should be a testable proposition. In any case, I think the concept needs a clearer definition.
If (2) is true, it would strongly support the concept that more CO2 will cause more IR photons at wavelenth w from Earth to be absorbed, and thereby more heating of the Earth.
If (2) is false, then this would mean that the current level of IR radiation at wavelength w from the Earth (and also from the sun?) is insufficient to saturate the current amount of CO2. This could be interpreted as not all CO2 molecules are in an excited state at the same time, which seems quite plausible. It might also be interpreted in other ways.
Each IR photon from the Earth at wavelength w has a chance of being absorbed by a CO2 molecule if the photon passes within the molecule's absorbsion cross section area, and the molecule is not already excited. I understand that this cross section depends on several variables which I do not understand very well. In any case, whatever the IR radiance is from Earth, and what ever the relative density of CO2 is in the air, it is plausible that some (perhaps a small) fraction of absorbable photons at wavelength w will escape into space without being absorbed. That means, at least in some reasonable sense, that more CO2 is still likely to absorb more such photons.

ADDED
I looked up the paper by Hanel & Conrath, and your typo is that "cm" should be "cm^-1". The value 667 is a frequency in this uncomfortable unit defined as the inverse of wavelenth. This means that the wavelength you intended is 15 μm.

Regards,
Buzz

 

[votingmachine]

Hi votingmachine:Of course IR radiation warms "things" - SOLID and LIQUID things, not GASSES.

I understand that the proper answer is that the energy is promptly re-radiated, and that has been well stated here. The problem with an infrared lamp is there is always some convective heating. So my bad. As I gave the answer I was puzzling over how you could eliminate the surface effects, and measure with an IR thermometer shielded from the light bulb. I was thinking that it would register temperature rise ...

Nope, I don't think I could get around the convective heat. Never mind. My mistake. The convection from the various surfaces makes me miss the re-emittance.

 

[Buzz Bloom]

I decided that a very brief history of the changes of educated opinions about the "greenhouse effect" might be of some interest in this thread.

In about 1902 (I am not sure of the exact date) a graduate student of Knut Angstrom (son of Anders Angstrom after whom the Angstrom unit was named) demonstrated that CO2 is NOT warmed by being irradiated by IR. This experiment was later reproduced by others. This result (and other work by Knut Angstrom) caused a complete reversal in educated acceptance of the "greenhouse effect" that had developed since early in the 19th century. (The term was first used in this way by Nils Gustaf Ekholm in 1901.) At the time there was no alternative mechanism known that could explain how the greenhouse effect might work.

A half century later, in 1955, Gilbert Plass published a paper titled "The Carbon Dioxide Theory of Climatic Change".
http://onlinelibrary.wiley.com/doi/10.1111/j.2153-3490.1956.tb01206.x/pdf
Besides refuting the conclusions about the greenhouse effect from Knut Angstrom's work, this paper includes an excellent history summary regarding the greenhouse effect. Plass's paper un-reversed the educated opinions against the greenhouse effect caused by Angstrom.