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

[Let'sthink]

As I understand and I think correctly, that Green house effect is in essence conversion of lower wavelengths in the Sun's energy to higher wavelength near infrared regions which can heat up the medium more efficiently. So if we have a closed transparent box containing some amount of green house gas under normal pressures and room temperature, and keep in the sun, the temperature of the box goes on increasing. But to pin point the cause of heating up due to green house gas effect only and not due to stoppage of convection currents with the surroundin we must have two such identical containers one with green house gas like CO2 and other filled with say nitrogen, We should then check up whether the container with CO2 gets heated up more. have such experiments been done?

 

[DaveC426913]

You seem to be asking if CO₂ and other greenhouse gases are known contributors to the greenhouse effect.

They are.

https://en.wikipedia.org/wiki/Greenhouse_gas#Greenhouse_gases

 

[davenn]

So if we have a closed transparent box containing some amount of green house gas under normal pressures and room temperature, and keep in the sun, the temperature of the box goes on increasing.

Your idea has some flaws
in a closed system like that... the temperature is going to rise quickly anyway regardless of what gas is inside
consider how hot it gets in your closed up car on a sunny day
Dave

 

[Buckleymanor]

Your idea has some flaws
in a closed system like that... the temperature is going to rise quickly anyway regardless of what gas is inside
consider how hot it gets in your closed up car on a sunny day
Dave

You would not expect the same temperature rise regardless of what gas was inside .With some gases the temperature should rise more quickly and remain hotter longer depending on there thermal capacity.
Some double glazing units have gas injected because of the gases thermal capacity but they don't use Co2 it's way down the list.
Yes sure the temperature will rise quickly regardless of what gas is used but more to the point some will be better at it than others.

 

[DaveC426913]

The point is, it's not a valid test.
A valid test could be designed, but we'd need clarification on exactly what it is we're trying to test here.

Greenhouse gasses do not work the way the OP suggests they do. They work on solar energy reflecting off the surface of the Earth, preventing it from exiting the atmosphere.
You'l want to examine this process carefully before
a] doubting it, and
b] trying to test it.

 

[davenn]

Greenhouse gasses do not work the way the OP suggests they do. They work on solar energy reflecting off the surface of the Earth, preventing it from exiting the atmosphere.

yes exactly ... that was going to be my response
thanks Dave

 

[sophiecentaur]

They work on solar energy reflecting off the surface of the Earth, preventing it from exiting the atmosphere.

You could refine that statement. In a steady state situation, all the incident energy gets radiated and it is not "prevented" from leaving. What in fact happens (basic simple model) is that the effect of the 'greenhouse gases' is to raise the surface temperature of the planet to a value which allows the radiated energy to equal the incident power. They present a 'high resistance path' (electrical analogy) to the IR radiation because they absorb and re radiate down as well as upwards. There is a constant (average) level of received power at the surface, dominated by higher frequency radiation, which is more or less independent of the presence of the greenhouse gases, which is analogous to a constant current electrical source and the higher path resistance causes the voltage to be higher, to enable the current to pass.
I wouldn't mind betting there is a Water Analogy of that Electrical Analogy too!

 

[DaveC426913]

You could refine that statement.

Yes, that was my point: it's much more complex than the OP's setup. Gotta do some readin' before doin' some questionin'...

 

[sophiecentaur]

Yes, that was my point: it's much more complex than the OP's setup. Gotta do some readin' before doin' some questionin'...

When someone asks a question in the "homework" section, they are expected to show that they have actually tried. Perhaps the same demand should be made for OPs on all forums.
That could take some of the fun out of things, tho'.

 

[Let'sthink]

I agree with sophiecentaur and admit my mistake in presenting the opening thread. You mean to say that whatever experiments we need to do to confirm experimentally the cause of Green House effect have to be done on open systems. I would very much like to know about such experiments. Also my asking this question many others who have this misconception that our closed car kept in sun explodes due to green house effect only will be clarified.

 

[sophiecentaur]

our closed car kept in sun explodes due to green house effect

That is a 'real' greenhouse effect, in as far as there is a direct equivalence between the car windows and the greenhouse glass. The difference between that and the effect due to the atmosphere is that the heat loss is reduced in a greenhouse by the fact that the glass just stops exchange of the air with the outside air (convection). It has nothing (very little) to do with the absorption and re-radiation of EM by the glass. That mechanism is very sophisticated for non-scientific minds to grasp, I think.
The greenhouse effect is a handy term to use in describing global heating but it doesn't imply the same mechanism - just the result. Bad journalism, perhaps, as the term is used by people who have no idea about (or choose to ignore) the mechanisms at work in the atmosphere. (The blessed Lord Lawson, for one - along with a tame Scientist he seems to have trawled the World to find!)

 

[Buckleymanor]

That is a 'real' greenhouse effect, in as far as there is a direct equivalence between the car windows and the greenhouse glass. The difference between that and the effect due to the atmosphere is that the heat loss is reduced in a greenhouse by the fact that the glass just stops exchange of the air with the outside air (convection). It has nothing (very little) to do with the absorption and re-radiation of EM by the glass. That mechanism is very sophisticated for non-scientific minds to grasp, I think.
The greenhouse effect is a handy term to use in describing global heating but it doesn't imply the same mechanism - just the result. Bad journalism, perhaps, as the term is used by people who have no idea about (or choose to ignore) the mechanisms at work in the atmosphere. (The blessed Lord Lawson, for one - along with a tame Scientist he seems to have trawled the World to find!)

Well could you not simplify it.If the atmosphere was totally glazed in (one big bubble) and more greenhouse gases were present at any given time than another would the temperature remain higher and for longer when the gases were present than when they were not.
Which is basically what the op is asking.
I can't see exactly why the negative no can do replies to an honest question.

 

[sophiecentaur]

I can't see exactly why the negative no can do replies to an honest question.

It depends on what the questioner really wants to know. I can't see any "no can do" in my reply. I am just pointing out that there is a much better model than a glass envelope to explain the global warming effect. And it's not that difficult to understand as it can be communicated without any of the dreaded Maths - although there's a lot of Maths involved in a proper quantitative approach.
The word 'greenhouse' is good enough on an elementary level but, if you want to get a fair 'Physics Explanatio'n, it doesn't describe the mechanism although the result is broadly the same. Why would one prefer an inadequate model just on the basis that it is easier to understand?
The atmosphere is one big bubble. The gases are kept in place by gravity, rather than a membrane. If more greenhouse gases were introduced then the surface temperature would go up - unless there were some other mechanism which the simple model doesn't include. This proposed "big bubble" would need to be specified more than just with a name. What would its characteristics be? Would it absorb or reflect IR wavelengths? Would it have a vacuum outside it or something else? What would its thermal conductivity be? I guess it would be possible to make a model of the Earth with an atmosphere of non-greenhouse gases and surround it with a suitable membrane which would produce the same surface temperature as we get on an ideal model with greenhouse gases. I would take some convincing that it would be a better approach than what's used at present; there are too many loose ends and undefined variables. Assuming you got the membrane to work OK in one particular set of circumstances then could you be sure it would track reality as circumstances changed?
Otoh, it is not hard to construct experiments to establish the absorption of IR by greenhouse gases. This is what's already been done and it gives fair results (given all the other variables at work in a real atmosphere. Why take a step backwards?

 

[Buckleymanor]

It depends on what the questioner really wants to know. I can't see any "no can do" in my reply. I am just pointing out that there is a much better model than a glass envelope to explain the global warming effect. And it's not that difficult to understand as it can be communicated without any of the dreaded Maths - although there's a lot of Maths involved in a proper quantitative approach.
The word 'greenhouse' is good enough on an elementary level but, if you want to get a fair 'Physics Explanatio'n, it doesn't describe the mechanism although the result is broadly the same. Why would one prefer an inadequate model just on the basis that it is easier to understand?
The atmosphere is one big bubble. The gases are kept in place by gravity, rather than a membrane. If more greenhouse gases were introduced then the surface temperature would go up - unless there were some other mechanism which the simple model doesn't include. This proposed "big bubble" would need to be specified more than just with a name. What would its characteristics be? Would it absorb or reflect IR wavelengths? Would it have a vacuum outside it or something else? What would its thermal conductivity be? I guess it would be possible to make a model of the Earth with an atmosphere of non-greenhouse gases and surround it with a suitable membrane which would produce the same surface temperature as we get on an ideal model with greenhouse gases. I would take some convincing that it would be a better approach than what's used at present; there are too many loose ends and undefined variables. Assuming you got the membrane to work OK in one particular set of circumstances then could you be sure it would track reality as circumstances changed?
Otoh, it is not hard to construct experiments to establish the absorption of IR by greenhouse gases. This is what's already been done and it gives fair results (given all the other variables at work in a real atmosphere. Why take a step backwards?

I doubt that it would be a better approach than what is used at present though a model which is easier to understand does have benefits.
By simplifying a subject it becomes more likely to be understood and if reality is not tracked properly as circumstances changed then either the glass envelope model is wrong or something else.You learn more sometimes by being wrong than right.

 

[sophiecentaur]

You seem to be defending an inferior model. I wonder why. The glass reduces convection losses. The gases reduce radiation losses. Whic has more relevance in space?
Isn't this about the 'learning' you refer to? Why not move on?

 

[JDoolin]

Aren't the mechanisms behind glass, and the mechanism behind greenhouse gasses the same in principle? The glass is transparent to visible light... It let's the visible light penetrate to the ground, which absorbs part of that light as heat. Then the ground re-emits the light (largely) in the infrared spectrum. The glass is opaque to the infrared spectrum so this heat cannot escape.

Heat can still escape through the glass, via conduction. But it can no longer escape via radiation.

Conceptually, I think this model is great, and almost perfectly analogous to greenhouse gasses. For greenhouse gasses, they are transparent to the incoming light, and then they are opaque to the outgoing light. Once they absorb 100% of that outgoing light, you're concerned about their heat-conductivity properties instead of their band-pass properties.

However the reason that the OP's experiment wouldn't work, if I'm right, is that whatever transparent container holds the gas will also interfere with the transparency and the conductivity of the gas. But in principle, if you had a perfectly transparent container material that wouldn't block any light at all in any wavelength... e.g. an "ideal glass" we could at least approach the idea "gedanken"-style... Am I right?

 

[sophiecentaur]

The glass is opaque to the infrared spectrum so this heat cannot escape.

Question: If you replaced the greenhoouse glass with a set of louvres, which allowed convection but still intercepted the IR, would the greenhouse stay the same temperature?
Evidence: As soon as the Sun gets too strong, greenhouse operators open the roof windows ( 20% of the area, max) and the temperature drops considerably. Is that not experimental evidence that the effect on the IR by the glass is much less compared with its action in suppressing convection losses?
That's my problem with accepting the 'greenhouse' explanation for what goes on in our atmosphere. It's just not thought out enough.

 

[Alastair McD]

As I understand and I think correctly, that Green house effect is in essence conversion of lower wavelengths in the Sun's energy to higher wavelength near infrared regions which can heat up the medium more efficiently. So if we have a closed transparent box containing some amount of green house gas under normal pressures and room temperature, and keep in the sun, the temperature of the box goes on increasing. But to pin point the cause of heating up due to green house gas effect only and not due to stoppage of convection currents with the surroundin we must have two such identical containers one with green house gas like CO2 and other filled with say nitrogen, We should then check up whether the container with CO2 gets heated up more. have such experiments been done?

The experiment you suggest has been performed. See:
http://www.tufts.edu/~rtobin/Wagoner%20AJP%202010.pdf
with result similar to those predicted above.

It has been known since Professor Wood published his short paper http://scienceblogs.com/stoat/2011/01/07/r-w-wood-note-on-the-theory-of/ that the greenhouse gases do not behave in the same way as a greenhouse, which works by preenting convection. But it is too late now to change their name. Greenhouse gases work by absorption.

 

[JDoolin]

Here's a graph from http://www.skepticalscience.com/saturated-co2-effect-advanced.htm
http://ishmael.altervista.org/fig1.jpg

The part of the curve marked 300 Kelvin represents the transparent part of the gas. This is light from the surface of the planet at 300 Kelvin. The part of the curve marked 220 Kelvin represents the opaque part of the gas. This is light from the top part of the gas, well above the surface, which follows the blackbody curve of a 220 Kelvin surface.

If you increase the depth of that layer, it would also increase the distance over which convection operates, and hence lower the amount of escaping light in the 600-700 /cm bandwidth.

There seem to be two different concerns about increased Carbon Dioxide in the atmosphere. One is that the bandwidth of infrared opacity of the atmosphere will widen (in the graph above). But also that the Carbon-Dioxide layer of the atmosphere will thicken, which would increase the insulative properties and make the temperature of the outer layer drop (in the graph above), even if no additional light bandwidth of infrared was absorbed.

Now, according to the article which I linked above, the bandwidth increases according to the absorption coefficient of the atmosphere in a "very crude model". So I think that's where I am... If I saw either the argument, or the experiments which showed quantitatively and qualitatively how this bandwidth expanded (and/or how the depth of CO2 layer increased), then I would no longer have to tell my students "Most climate scientists believe that CO2 induced global warming is happening." I could say "Here, let me show you."

 

[votingmachine]

Aren't the mechanisms behind glass, and the mechanism behind greenhouse gasses the same in principle? The glass is transparent to visible light... It let's the visible light penetrate to the ground, which absorbs part of that light as heat. Then the ground re-emits the light (largely) in the infrared spectrum. The glass is opaque to the infrared spectrum so this heat cannot escape.

Heat can still escape through the glass, via conduction. But it can no longer escape via radiation.

Conceptually, I think this model is great, and almost perfectly analogous to greenhouse gasses. For greenhouse gasses, they are transparent to the incoming light, and then they are opaque to the outgoing light. Once they absorb 100% of that outgoing light, you're concerned about their heat-conductivity properties instead of their band-pass properties.

However the reason that the OP's experiment wouldn't work, if I'm right, is that whatever transparent container holds the gas will also interfere with the transparency and the conductivity of the gas. But in principle, if you had a perfectly transparent container material that wouldn't block any light at all in any wavelength... e.g. an "ideal glass" we could at least approach the idea "gedanken"-style... Am I right?

Greenhouses primary reason for heat trapping is that they stop convection circulation. Ordinarily, hot air rises and is replaced by cooler air. The glass enclosure stops that. So it is a bit wrong to say the mechanism is based on transmission limits in the infrared by the glass.

As I understand and I think correctly, that Green house effect is in essence conversion of lower wavelengths in the Sun's energy to higher wavelength near infrared regions which can heat up the medium more efficiently.

That is almost right. The sunlight that hits the Earth is in a broad spectrum. The Earth re-radiates energy as black-body radiation. That black body radiation is SCATTERED and ABSORBED and RE-RADIATED. Those mechanisms trap some of the infrared energy. The black-body temperature of the Earth must be at a steady state, exactly re-radiating the Sun's incident energy back into space. If the Earth had no atmospheric blanket, the black-body temperature that balances the Sun's "solar constant" would be about 0-degrees F (IIRC). The Earth's average surface temperature is about 60-degrees-F. Changes to the Earth's atmospheric blanket will change the amount of energy that gets retained. That moves the black-body temperature higher, which moves the radiation profile (this is a physics board, someone probably even knows the formula off the top of their head, I use google). A new steady state is arrived at with a slightly higher temperature.

The experiment you describe is simply absorbance spectroscopy. Different gases have different absorption spectra. You could also fill two bottles, one with black paint and one with white paint, and put them in the same sun. One would have a higher black-body temperature (if I am using the words correctly) ... it would be the black one. It is somewhat trivial to deduce that absorption spectrum changes the steady state temperature. But just as you could mix one drop of the black paint into the white paint and see a slight change in temperature, if you mix a gas that has absorption in the spectrum passing thru that space into a gas that has less, there is a slight temperature increase.

I'm not sure if that precise experiment has been done. I considered something similar as a simple bit of work to propose to a group of kids as a science fair project.

But the step from absorption spectrum to energy re-distribution should be one which is strong theoretically.

The sun radiates as a body at a very high temperature. That is the spectrum hitting the Earth surface. The Earth radiates as a body at a much lower temperature. That is the spectrum leaving the Earth surface. In between, there is an atmosphere.

That bit of science is remarkably simple. There are a LOT of other things that matter though. The Earth's albedo ... how much sunlight is reflected directly back into space. Heat capacity and phase changes matter in the energy flow. The Sun's solar constant matters. Heat energy from the Earth's core matters. Some people choose to regard the changes to the Earth's atmospheric blanket as trivial, when compared to other factors. I think that while the system may be complex, the atmospheric factor is large and important. There are feedback loops that are both positive and negative. In particular the heat capacity if the oceans, ocean currents and temperature mixing, the phase changes of water, and the changes in net albedo ... those all can create a lot of noise in the data.

I am always a bit confused by the use of averages in these systems. If you look at the moon, which has no atmosphere, and is generally (barring eclipses) sunlit on one side and dark on the other, the sunlit side is WAY warmer than the dark side. Is it truly accurate to speak of the AVERAGE temperature as the black body radiation profile of re-emitted radiation? Is it equivalent energy outflo? EG, if right now it is 70-degrees-F at noon for me, and on the other side of the earth, at midnight, it might be 50-degrees-F. Is the energy flow from those two points the same as if they were two points at the AVERAGE temperature of 60-degrees-F?

And I know ... I could do the math myself. But I'm being lazy.

 

[JDoolin]

Question: If you replaced the greenhoouse glass with a set of louvres, which allowed convection but still intercepted the IR, would the greenhouse stay the same temperature?
Evidence: As soon as the Sun gets too strong, greenhouse operators open the roof windows ( 20% of the area, max) and the temperature drops considerably. Is that not experimental evidence that the effect on the IR by the glass is much less compared with its action in suppressing convection losses?
That's my problem with accepting the 'greenhouse' explanation for what goes on in our atmosphere. It's just not thought out enough.

I don't understand your question, but I think what you're trying to do here is to make an argument that doesn't make sense in order to provide an example of the sort of argument that is presented for CO2 invoked global warming. If so, I can give you an even better one-- This video appeared in "An Inconvenient Truth" and it really was the only explanation given, in that particular movie, for how greenhouse gasses work.

The problem with this video isn't that it's silly--it's that it has exactly the same sunbeam leaving the planet as it has going in. If Mr. sunbeam came out in the same form as he was when he went in, the greenhouse gasses are not going to bother him. It's only if Mr. Sunbeam "really gets into his work" and transfers his entire energy into the earth; then its his "kids" that are going to be beaten up by the greenhouse gasses.

 

[JDoolin]

That moves the black-body temperature higher, which moves the radiation profile (this is a physics board, someone probably even knows the formula off the top of their head, I use google).

https://en.wikipedia.org/wiki/Effective_temperature#Surface_temperature_of_a_planet

For a "rapidly spinning planet"
T=\left(\frac{L(1-\alpha)}{16 \pi \sigma D^2}\right)^{1/4}

I think this can be further simplified to

T=(1-\alpha)^{1/4}\frac{278}{\sqrt{D}} (where D is measured in astronomical units, and T in Kelvin).

 

[JDoolin]

You could refine that statement. In a steady state situation, all the incident energy gets radiated and it is not "prevented" from leaving. What in fact happens (basic simple model) is that the effect of the 'greenhouse gases' is to raise the surface temperature of the planet to a value which allows the radiated energy to equal the incident power. They present a 'high resistance path' (electrical analogy) to the IR radiation because they absorb and re radiate down as well as upwards. There is a constant (average) level of received power at the surface, dominated by higher frequency radiation, which is more or less independent of the presence of the greenhouse gases, which is analogous to a constant current electrical source and the higher path resistance causes the voltage to be higher, to enable the current to pass.
I wouldn't mind betting there is a Water Analogy of that Electrical Analogy too!

I just linked to: https://en.wikipedia.org/wiki/Effective_temperature and gave an equation that is derived from "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."

However, is this the greenhouse effect? Well, it's a start in the right direction, but this is a model the surface temperature of a rapidly spinning body directly exposed to space. In fact it is the DIFFERENCE between this temperature and the temperature on the ground which represents the greenhouse effect. Because the atmosphere insulates the earth, the top of the atmosphere is colder, it releases LESS heat back into space, And the difference between the heat loss at the surface of the planet, and the heat loss at the surface of the atmosphere represents the greenhouse effect.

On Venus, for instance, the atmospheric pressure and temperature at about 50 km to 65 km above the surface of the planet is nearly the same as that of the Earth, the top of the atmosphere there is quite cool. The surface temperature, though is hot enough to melt lead.

We have about four examples in the solar system which we could test basic models for the greenhouse effect: Titan, Venus, Earth, and Mars. I think I've gone through calculations for all four, in the past, and found Mars barely has any greenhouse effect at all, Titan's methane driven atmosphere has something close to earth's, and Venus traps something crazy like 99% of its incoming radiation. If you're fond of incredibly boring and unscripted youtube videos of me doing math, you can see that here:

 

[DrStupid]

By simplifying a subject it becomes more likely to be understood

And it becomes more likely to be wrong.

and if reality is not tracked properly as circumstances changed then either the glass envelope model is wrong or something else.

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.

 

[Buckleymanor]

You seem to be defending an inferior model. I wonder why. The glass reduces convection losses. The gases reduce radiation losses. Whic has more relevance in space?
Isn't this about the 'learning' you refer to? Why not move on?

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.

.

 

[votingmachine]

I agree with sophiecentaur and admit my mistake in presenting the opening thread. You mean to say that whatever experiments we need to do to confirm experimentally the cause of Green House effect have to be done on open systems. I would very much like to know about such experiments. Also my asking this question many others who have this misconception that our closed car kept in sun explodes due to green house effect only will be clarified.

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. But no one should question that the atmosphere captures heat. The reality is that attempting to sum all of the climate contributions of all of the things changing on the Earth is a difficult thing to do. And it is difficult to tease out exactly what the full effect of "greenhouse gases" are. The issue tends not to be the basic science of black body radiation, and the basic science of atmospheric heat capture, but rather what other things might be important. Many people who reject a greenhouse gas model think that other factors are more important. Many are simply uninformed on the basic science, but many also have alternate climate models.

An experiment to show that the introduction of a greenhouse gas interferes with heat escape would not be compelling to anyone who thinks the greenhouse gases are not the dominant factor in climate cycles. It would be simpler to point out that the moon is colder than the earth, even though it is less reflective. And the reason is that the Earth has an atmospheric blanket.

FWIW, I regard greenhouse gases as a significant threat to humans.

 

[Buckleymanor]

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. But no one should question that the atmosphere captures heat. The reality is that attempting to sum all of the climate contributions of all of the things changing on the Earth is a difficult thing to do. And it is difficult to tease out exactly what the full effect of "greenhouse gases" are. The issue tends not to be the basic science of black body radiation, and the basic science of atmospheric heat capture, but rather what other things might be important. Many people who reject a greenhouse gas model think that other factors are more important. Many are simply uninformed on the basic science, but many also have alternate climate models.

An experiment to show that the introduction of a greenhouse gas interferes with heat escape would not be compelling to anyone who thinks the greenhouse gases are not the dominant factor in climate cycles. It would be simpler to point out that the moon is colder than the earth, even though it is less reflective. And the reason is that the Earth has an atmospheric blanket.

FWIW, I regard greenhouse gases as a significant threat to humans.

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. But no one should question that the atmosphere captures heat. The reality is that attempting to sum all of the climate contributions of all of the things changing on the Earth is a difficult thing to do. And it is difficult to tease out exactly what the full effect of "greenhouse gases" are. The issue tends not to be the basic science of black body radiation, and the basic science of atmospheric heat capture, but rather what other things might be important. Many people who reject a greenhouse gas model think that other factors are more important. Many are simply uninformed on the basic science, but many also have alternate climate models.

An experiment to show that the introduction of a greenhouse gas interferes with heat escape would not be compelling to anyone who thinks the greenhouse gases are not the dominant factor in climate cycles. It would be simpler to point out that the moon is colder than the earth, even though it is less reflective. And the reason is that the Earth has an atmospheric blanket.

FWIW, I regard greenhouse gases as a significant threat to humans.

I think the question of Greenhouse Gas effects is 100% answered. Consider the Earth and the moon.
Problems occur when you consider the Earth and Mars.
Mars has a predominant atmosphere of the Greenhouse gas carbon dioxide but it's still cold and thick enough to use a parachute for landings.

 

[Buzz Bloom]

it is not hard to construct experiments to establish the absorption of IR by greenhouse gases.

Hi: sophie: I very much like all of your posts in this thread. This one particular quote, however I found to be a little bit careless, in that it is likely to mislead most non-expert readers. I think many of these readers will misinterpret the quote as saying that "greenhouse gases", e.g., CO₂, become heated by their absorption of IR photons.

I am sure you know that when a CO₂ molecule absorbs an IR photon, an equivalent photon is re-radiated by spontaneous emission much more quickly than the absorbed energy can be converted to the heating of the atmosphere by the excited CO₂ molecule interacting with another molecule. Since about a half of the re-radiated photons are downward towards the Earth's surface, the energy of these downward photons is eventually re-absorbed by the Earth's surface.

If the annual CO₂ fraction of the atmosphere is stable, the average annual surface (equilibrium) temperature will be (on the average) more-or-less-stable. The net extra downward IR radiation from additional CO₂ molecules will result in a higher equilibrium temperature.

The above summary is, of course, oversimplified a little. A downward IR photon may not hit the Earth directly. A fraction may hit another CO₂ molecule first, and half of such molecule's re-radiated photons will be downward. To compensate for this, the photons originally re-radiated upward will also on the average hit about the same fraction of other CO₂ molecules, and these will send about half of its re-radiated photons downward. And, of course, there might well occasionally a larger number of intermediate steps in a path starting from a CO₂ molecule which originally absorbed a photon from the Earth's surface before a resulting photon is re-absorbed by the Earth's surface. In any case, about half of the original absorptions of an IR photon by a CO₂ molecule will result in an IR photon being reabsorbed by the Earth's surface.

Regards,
Buzz

 

[Buzz Bloom]

I would very much like to know about such experiments.

Hi @Let'sthink:

The following link may be what you are looking for, or at least may help get you started:

http://dailysciencejournal.com/researchers-measure-green-house-effect-for-the-first-time/21420/​

The following quote is from the last paragraph of this news article"

It was found that CO2 has caused a significant uptick in radiative forcing at both locations. Thermal infrared energy which travels from atmosphere to the surface of Earth was also measured by the researchers by using state-of-the-art high precision spectroscopic instruments. The instruments detect CO2 by its unique spectral signature. Researchers observed that thermal radiation was prevented from escaping into space by atmospheric carbon dioxide which led to more solar heat absorption by Earth. They suggest that the concentrations of carbon are associated with the burning of fossil fuels. Feldman said “We measured radiation in the form of infrared energy. Then we controlled for other factors that would impact our measurements, such as a weather system moving through the area.”​

Regards,
Buzz

 

[sophiecentaur]

I am sure you know that when a CO2 molecule absorbs an IR photon, an equivalent photon is re-radiated by spontaneous emission much more quickly than the absorbed energy can be converted to the heating of the atmosphere by the excited CO2 molecule interacting with another molecule. Since about a half of the re-radiated photons are downward towards the Earth's surface, the energy of these downward photons is eventually re-absorbed by the Earth's surface.

Interesting (in a nice way! ). So you are saying that the actual temperature of the atmosphere is not what my rough summary is implying but that the chain of IR energy is not simply 'thermal? That makes sense. I would also be interested to know how many C02 molecules would actually be involved in the path of the non-black body IR radiation as it finally escapes into space.
I see that you are totally right about this. I looked at the many graphs that Google threw up, showing the temperatures in the atmosphere at various heights and, of course, they are much lower than the surface temperature - so it is not a source of thermal radiation which could keep the surface warm. So the greenhouse effect is even more narrrow-band radiation based than I appreciated. The glass envelope appears an even more flawed model than I originally thought.
Considering the effect of clouds on the surface temperature (frosts on clear nights and no frosts on cloudy nights) would that also be a narrow-band radiation effect? The effect on the surface temperature is very marked.
Our atmosphere really is a tricky devil to understand.

 

[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.